Filtration in the surface installation of a geothermal doublet: from

Transcription

Confidential
Earth, Life and Social Sciences
Utrechtseweg 48
3704 HE ZEIST
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TNO report
TNO 2013 R11739 | 1
Filtration in the surface installation of a
geothermal doublet: from practice to better
practice to best practice
Date
31 March 2014
Author(s)
Robin van Leerdam
Wilfred Appelman
Copy no
No. of copies
Number of pages
Number of
appendices
Sponsor
Project name
Project number
70 (incl. appendices)
TNO Programma MKB Kennisoverdracht met inzet SMO in
samenwerking met Platform Geothermie en leden
TC scheidingstechnologie geothermie
052.04097
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© 2014 TNO
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TNO report | TNO 2013 R11739 | 1 CONFIDENTIAL
Samenvatting
Introductie
Dit rapport is geschreven in het kader van een TNO Technologiecluster project en
geeft een overzicht van de gebruikte filtratieprocessen en de ervaringen hiermee bij
oppervlakte-installaties van geothermische doubletten in Nederland. Het doel is om
na te gaan of kosten van het filtratieproces kunnen worden bespaard, hoe filtratie
efficiënter kan en om te adviseren hoe risico van putverstopping door geïnjecteerde
deeltjes kan worden verminderd.
Filters worden geplaatst in de boveninstallatie om het systeem (met name de
warmtewisselaar) te beschermen, maar vooral om te voorkomen dat de injectieput
verstopt door de injectie van deeltjes, waardoor injectiedruk toeneemt en meer
energie moet worden gebruikt.
De volgende geothermisch projecten waren onderdeel van de studie:
- Green Well Westland (Honselersdijk)
- Aardwarmte Den Haag (Den Haag)
- Aardwarmtekluster 1 KKP (IJsselmuiden)
- Duijvestijn Tomaten (Pijnacker)
- Ammerlaan, The green innovator (Pijnacker)
- A + G van den Bosch – Petuniaweg (Bleiswijk)
- A + G van den Bosch – Noordeindseweg (Berkel en Rodenrijs)
- Californië Wijnen Geothermie (Grubbenvorst)
- Floricultura (Heemskerk)
Met de filters in de oppervlakte-installatie worden deeltjes als zand, silt, klei, kalk en
ijzer verwijderd uit het geproduceerd water voordat het wordt geïnjecteerd. Kritische
factoren voor de selectie van een filtersysteem zijn:
- Debiet van de waterstroom
- Concentratie gesuspendeerde delen in het water (TSS)
- Deeltjesgrootteverdeling (psd) van de deeltjes in het water
- Temperatuur, zuurgraad van het water
- Eigenschappen van het injectiereservoir
Reservoireigenschappen
Permeabiliteit (doorlaatbaarheid) en poriegrootteverdeling van het reservoir bij de
injectiezone zijn belangrijke eigenschappen voor de evaluatie van het risico van
injecteren van deeltjes in de injectieput en voor de keuze van het micronage van de
filters in de oppervlakte-installatie. Deze eigenschappen bepalen de kritische
plugging (verstoppings) range van de deeltjes die geïnjecteerd worden. De kritische
ratio [grootte geïnjecteerd deeltje / poriegrootte in reservoir] ligt tussen 1/3 en 1/10.
Deeltjes binnen deze kritische range hebben een hoge potentie om poriën in het
reservoir te blokkeren. Deeltjes met een diameter kleiner dan 1/10 van de
porieopening zullen zich vrijelijk door de formatie begeven en als de ratio groter is
dan 1/3 is de blokkering substantieel lager bij deeltjes in de kritische plugging
range. Deeltjes in deze range moeten met filters worden verwijderd voorafgaand
aan injectie. Grotere deeltjes worden dan automatisch ook verwijderd.
Relevante reservoireigenschappen (korrelgrootteverdeling sediment, porositeit,
permeabiliteit bij injectiezone) van geothermische putten in Nederland zijn
momenteel gebaseerd op berekeningen en aannames. Die moeten nauwkeuriger
vastgesteld worden om de poriegrootteverdeling in het injectiereservoir vast te
kunnen stellen en daarmee de kritische plugging range van de geïnjecteerde
deeltjes.
Een betrouwbare bepaling van de poriegrootteverdeling kan alleen worden gedaan
met monsters van kernen, maar deze zijn niet beschikbaar van de geothermisch
projecten in Nederland.
2 / 70
De deeltjesgrootteverdeling van een cuttingmonster kan wel worden geanalyseerd
d.m.v. laser deeltjesanalyse of een zeefanalyse. Gebaseerd op de
korrelgrootteverdeling van representatieve cuttingmonsters kan de permeabiliteit en
poriegrootteverdeling nauwkeuriger worden uitgerekend en daarmee de kritische
plugging range.
Huidige filterpraktijk
In de beschouwde boveninstallaties van de geothermisch doubletten in Nederland
worden vooral zakkenfilters en kaarsfilters gebruikt (high flow of conventioneel) en
in één geval een automatisch filter. Uit deze studie is naar voren gekomen dat
momenteel 10 µm (nominale) filtratie (zakkenfilters), soms gevolgd door 10 µm
absoluutfiltratie (kaars), de standaard is. Filters worden vooral voor de
warmtewisselaar geïnstalleerd om naast de injectieput ook de warmtewisselaar te
beschermen tegen deeltjes.
Kosten van de filterinstallatie
De investeringskosten van een complete filterinstallatie kunnen variëren van minder
dan 10 k€ voor een schone productieput tot 50-80 k€ voor een put die grote
hoeveelheden deeltjes of olie produceert. De onderhoudskosten (arbeid, materiaal)
van een filterinstallatie kunnen oplopen tot 100-150 k€ per jaar in het eerste jaar.
Na een jaar opereren, kunnen de kosten sterk dalen tot circa 20-50 k€ per jaar. Dit
heeft te maken met de lagere hoeveelheid deeltjes in het water t.o.v. de opstartfase
na langere tijd opereren.
Aanbevelingen voor “best practice”
Opschalen van de filterinstallatie
- Een lagere aanstroomsnelheid verhoogt de levensduur van de filters
doordat het filter dan een grotere “dirt-holding-capacity” heeft. Verdubbelen
van het filteroppervlak of de filterdiepte leidt maximaal tot een kwadratische
levensduurverlenging. Daarom wordt aanbevolen om een grotere
filtercapaciteit te installeren dan strikt noodzakelijk voor de te behandelen
waterstroom. Een aanvankelijke investering in een extra filterhuis wordt
terugverdiend door een lager verbruik van zakken- en kaarsfilters.
Theoretisch nemen de kosten van het filterverbruik met een factor 1.5 tot 2
af als de capaciteit van de filterinstallatie wordt verdubbeld. Door grotere
filterzakken te gebruiken kan vaak het oppervlakte al eenvoudig worden
vergroot zonder de filterinstallatie verregaand aan te passen.
Gebruik van filters
- Er wordt aanbevolen om direct voor de injectieput een absoluut (kaars)filter
te plaatsen die alle deeltjes verwijdert in de kritische plugging range. In de
praktijk zal dit een absoluutfilter zijn met een micronage tussen 1 en 10 µm.
Dit filter reduceert het risico van verstopping in de injectiezone als gevolg
van deeltjesinjectie.
Deeltjesmetingen in het geproduceerde water en van het reservoirmateriaal
- Er wordt aanbevolen om één keer per jaar de deeltjesconcentratie (total
suspended
solids,
TSS),
deeltjesgrootteverdeling
(psd)
en
deeltjessamenstelling van het geproduceerde water te laten bepalen voor
en na het filtersysteem. Hiermee wordt de efficiëntie van het filtersysteem
en de kwaliteit van het geïnjecteerde water (wat betreft deeltjes)
vastgesteld.
- Er wordt aanbevolen de psd over een brede range van deeltjesgroottes te
bepalen (0.02 µm - 2000 µm) d.m.v. laser deeltjesanalyse. De kosten
hiervan zijn circa 200 euro per monster. Deeltjes tussen 1 en 10 µm zijn
vaak kritisch voor verstopping van de injectieput. De aantallen deeltjes in
deze range moeten nauwkeurig vastgesteld worden.
3 / 70
-
-
-
Er wordt aanbevolen om een vergelijkbare psd analyse te doen van
representatieve cuttingmonsters uit de injectiezone. Dit kan worden gedaan
als het cuttingmonster uit losse korrels bestaat.
Daarnaast kan een slijpplaatje worden gemaakt van het cuttingmonster
voor optische analyse met een microscoop. De korrelgrootte en afstanden
tussen de korrels kunnen worden gemeten, waarna psd en porositeit
kunnen worden geschat.
Karakterisatie en chemische analyse van deeltjes in het geproduceerde
water en van cuttingmonsters uit de injectiezone kan worden gedaan d.m.v.
stereo lichtmicroscopie, Scanning Electron Microscopy gecombineerd met
röntgen microanalyse en infrarood spectroscopie.
Wanneer de bovengenoemde metingen worden gedaan, kunnen betrouwbaarder
schattingen worden gedaan van de permeabiliteit, poriegrootteverdeling en kritische
plugging range in de injectiezone. Daarmee kan een beter onderbouwd advies
gegeven worden voor het micronage van de laatste filterstap voorafgaand aan
injectie.
4 / 70
Summary
Introduction
This report has been written in a “TNO Technologiecluster” project and reviews the
filter use and experiences in the surface installations of the geothermal doublets in
The Netherlands. The goal is to examine if costs for filtration can be saved, how
filtration can be done more efficiently and to recommend on how risks of clogging of
the injection well by particles can be reduced.
The purpose of the filters in the surface installation is to protect the system
equipment, mainly the heat exchanger, but mainly to protect the injection well from
clogging of particles, keep the injection pressure low and save energy.
The following geothermal projects were part of the study:
- Green Well Westland (Honselersdijk)
- Aardwarmte Den Haag (Den Haag)
- Aardwarmtekluster 1 KKP (IJsselmuiden)
- Duijvestijn Tomaten (Pijnacker)
- Ammerlaan, The green innovator (Pijnacker)
- A + G van den Bosch – Petuniaweg (Bleiswijk)
- A + G van den Bosch – Noordeindseweg (Berkel en Rodenrijs)
- Californië Wijnen Geothermie (Grubbenvorst)
- Floricultura (Heemskerk)
Filters in the surface installation of the geothermal doublet are aimed to remove the
produced particles like sand, clay particles, calcite and iron particles from the water
before it is injected. The most critical factors for the selection of a filter system are:
-
Water flow rate
Total suspended solids (TSS)
Particle size distribution (psd)
Temperature and pH of the water
Properties of the injection reservoir
Reservoir properties
Permeability and pore size distribution of the reservoir at the injection zone are
important parameters for the evaluation of the risk of injecting particles in the
injection well and for the choice of the filter rating of the filters in the surface
installation, because they determine the critical plugging range of particles that are
injected. The critical injected particle/pore size reservoir ratio is between 1/3 and
1/10. Injected particles within this critical plugging range have a high potential to
block the pores in the reservoir. When the particle diameter is smaller than 1/10 of
the pore throat, no blocking will occur and the particle can migrate freely through
the formation. When a particle has a diameter of more than 1/3 the size of the pore
throat, blocking can occur, but at a substantially lower rate than of the particles with
a particle/pore size ratio between 1/3 and 1/10. Particles in the critical plugging
range have to be removed from the water by filtration in the surface installation.
Bigger particles will be removed automatically at the same time.
Relevant reservoir properties like particle size distribution of the rock, porosity and
permeability in the reservoir at the injection zones of the geothermal wells in The
Netherlands are currently based on assumptions and calculations. They must be
known in more detail to determine the pore size distribution at the injection zone
more precisely and therewith the critical plugging range of injected particles.
A reliable pore size distribution can only be done with samples from the cores, but
they are not available from the wells of the geothermal doublets in The Netherlands.
However, the cuttings from the injection well can be analysed.
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6 / 70
A laser particle analysis and/or sieving analysis can be done to determine the grain
size distribution of the sediment within a certain interval. Based on the grain size
analysis of the sediment/cutting of the injection zone, the permeability and pore size
distribution can be estimated more precisely than the current estimations by using
existing relations. When the pore size distribution is known, the critical plugging
range can be calculated.
Current filter practice
In the surface installation of geothermal doublets in The Netherlands, mainly bag
filters and cartridge filters (high flow or conventional) are used. Only in one project
under consideration in this report, an automatic filter was used. From this study, it
follows that currently 10 µm filtration - nominal bag filtration, sometimes followed by
10 µm absolute filtration by cartridge filters - is the standard. Filters are in most
cases installed before the heat exchangers to protect both the heat exchangers and
the injection well against particles.
Costs of the filter installation
The investment costs of a filter installation in a geothermal doublet can vary
between < 10 k€ for a clean production well to about 50-80 k€ for a well that
produces high amounts of oil and solids.
The maintenance costs (labor + material) of the filter installation in the surface
installation are estimated on 100-150 k€ per year in the first year after the start-up,
3
3
based on flow rate between 100 m /h and 200 m /h. After more than a year of
operation and experience with filter usage, the costs will go down and are estimated
on about 50 k€ or even 20 k€ per year. This drop in filter costs has to do with the
lower solids load in the production water after a year of operation, compared to the
start-up period.
Recommendations for best practice
Upscaling of filter installation
- Decreasing the filter velocity will increase the lifetime of the filters. At lower
flow velocity, the filter has a higher dirt-holding-capacity. Doubling the
filtration area or the filter depth squares the life time (maximally) of the filter.
Therefore, it is recommended to install a higher filtering capacity then
strictly necessary for the water flow. An initial investment in extra filter
houses and material will be earned back by reduction in the use of filter
bags and cartridges. Theoretically, cost for filters will decrease by a factor
1.5 to 2 when the capacity of a filter installation is doubled. Using filter bags
with higher surface area is a cheap option to increase the total filtration
capacity.
Use of filters
- It is recommended to install a final absolute filter just before injection that
removes the particles in the critical plugging range. In practice this will be
and absolute (cartridge) filter with a filter rating between 1 and 10 µm. This
filter reduces greatly the risk of plugging in the injection zone.
Measurements of particles in produced water and reservoir material
- It is recommended to measure particle concentration (total suspended
solids, TSS), particle size distribution (psd) and particle composition in the
produced water once a year before and after the filter installation. With
these measurements the removal efficiency of the filter installation is
determined and the quality of the injected water (regarding particles).
- It is recommended to measure the psd over a wide range (0.02 µm - 2000
µm) by laser particle analysis. The costs will be about 200 euro per sample.
Particles between 1 and 10 µm are often critical for plugging. This size
range must be measured precisely.
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-
-
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It is recommended do a similar psd analysis for representative cuttings from
the reservoir at the injection zone. The analysis can be done if the cutting is
a sample of a fragile sediment with loose grains.
If the cutting cannot be destructed, an optical analysis by microscope can
be done. For that, a 2 by 2 cm slice or plaquette of a few mm thick is
prepared from mm-sized particles. By measuring the grain sizes and
distances between the grains under the microscope, estimations can be
done on the particle size distribution and porosity of the sample.
Characterisation and chemical analysis of the particles in the
produced/injected water and sediment/cuttings from the injection zone of
the injection well can be done by stereo light microscopy (SLM), Scanning
Electron Microscopy combined with energy dispersive X-ray microanalysis
(SEM / XRMA) and µ-Fourier Transformed Infrared spectroscopy (µ-FTIR).
When the above recommended measurements are done, more reliable estimations
can be made for the permeability, pore size distribution and critical plugging range
in the injection zone of the reservoir. With these data a more underpinned
recommendation can be given for the final filtration step before injection.
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Contents
Samenvatting ........................................................................................................... 2
Summary .................................................................................................................. 5
1
Introduction ............................................................................................................ 10
2
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
2.10
From current practice to better practice ............................................................. 12
Introduction .............................................................................................................. 12
Interviewed operators and suppliers........................................................................ 13
Reservoir type at injection zone and reservoir properties ....................................... 13
Downhole screen in the production and injection well............................................. 14
Coarse filter at the beginning of surface installation ................................................ 14
Composition of the production water ....................................................................... 15
Filter types and filter strategy in geothermal doublets in The Netherlands ............. 16
Removal of oil .......................................................................................................... 20
Experience abroad................................................................................................... 21
Costs of the filter installation .................................................................................... 22
3
3.1
3.2
3.3
3.4
3.5
Reservoir properties and risk of plugging of the injection well by particles .. 24
Introduction .............................................................................................................. 24
Plugging of particles in the injection well ................................................................. 24
Calculation of average pore diameter based on the permeability ........................... 28
Methods to determine sediment properties at the injection zone ............................ 29
Estimating permeability based on grain size ........................................................... 30
4
4.1
4.2
4.3
4.4
4.5
4.6
4.7
4.8
Filtration theory for geothermal doublets ........................................................... 32
Introduction .............................................................................................................. 32
Critical factors for filter choice ................................................................................. 32
Surface or depth filtration ........................................................................................ 33
Pressure drop .......................................................................................................... 34
Absolute rating and nominal rating for filters ........................................................... 36
Dirt holding capacity ................................................................................................ 37
Relation surface area/flow velocity and lifetime of a filter........................................ 37
Filterability of a liquid ............................................................................................... 38
5
Conclusions ........................................................................................................... 39
6
Recommendations for best practice ................................................................... 42
7
Literature ................................................................................................................ 46
8
Authentication ........................................................................................................ 48
9
Appendices ............................................................................................................ 49
9.1
9.2
9.3
Appendix 1: Estimating permeability based on grain size ....................................... 49
Appendix 2: Relation surface area/flow velocity and lifetime of a filter ................... 52
Appendix 3: Beschrijving diverse microscopisch analysetechnieken ...................... 54
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9.4
9.5
9 / 70
Appendix 4: Overview commercially available filter systems for geothermal doublets
................................................................................................................................. 56
Appendix 5: Summarizing table of nine geothermal doublets (data mid 2013) ....... 68
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1
10 / 70
Introduction
In the Netherlands, currently about ten geothermal doublets are in operation or in
far development for the production of geothermal heat for the heating of
greenhouses and other buildings. This number is growing fast.
Hot formation water is produced from reservoirs from a depth of about 2 to 3 km
and has a temperature of 60ºC-90ºC. The water from the producer well is pumped
through a heat exchanger in the surface installation of the geothermal doublet and
the cooled water (30-40 ºC) is injected via the injection well to the same formation
(Figure 2). The water flow through the doublet is normally in the range of 100-200
3
m /h to be economically feasible.
The injection process is not without risks. Sandstone reservoirs, rich in clay
minerals, can be susceptible for clogging as a result of migration of fine particles
and suspended solids in the formation water. This decreases the permeability and
higher injection pressures are needed for the same flow rate resulting in higher
energy use.
To prevent the production of sand and other (course) particles from the producer
well, often a wire wrapped screen is installed in the production well at reservoir
depth. In addition, in the surface installation of a geothermal doublet, often one or
more filter steps are applied for the removal of particles. In practice bag filters and
cartridge filters with separation sizes of e.g. 25 µm, 10 µm and 5 µm are used.
Goal
This report reviews the filter use and experiences in the surface installations of the
geothermal doublets in The Netherlands. The goal is to examine if costs for filtration
can be saved, how filtration can be done more efficiently and to recommend on how
risks of clogging of the injection well by particles can be reduced.
Recommendations are done for the use of filters and the configuration of the filter
installation in the surface installation of geothermal doublets.
Filters in the surface installation are installed to protect the surface installation and
the injection well from potential clogging in shorter or longer term. Dissolved
compounds are not removed by these filters. If oil is present in the production water,
sometimes extra oil filters need to be installed in addition to the oil gas separator to
remove the remainder of the oil before the production water is injected.
Information and practical experience of filter use in geothermal doublets was
obtained by interviewing Dutch operators of geothermal doublets and filter
suppliers, discussions between TNO’s water treatment specialists and geologists,
information from geothermal doublets abroad and by literature research.
Chapter 2 discusses the experiences of the operators, vendors, costs and improved
filter practice for geothermal doublets. Chapter 3 describes relevant reservoir
properties and the risk of plugging of injected particles in the injection well. In
Chapter 4 the critical factors that determine the choice of a filter system in the
surface installation of a geothermal doublet and general filtration theory are
discussed. In Chapter 5 the conclusions are listed and the recommendation (best
practices) are discussed in Chapter 6.
The following geothermal doublets are included:
-
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Green Well Westland
-
11 / 70
Aardwarmte Den Haag
Aardwarmtekluster 1 KKP
Duijvestijn Tomaten
Ammerlaan, The green innovator
A + G van den Bosch - Petuniaweg
A + G van den Bosch - Noordeindseweg
Californië Wijnen Geothermie
Floricultura
On request of the operators, the information in this report is not directly linked to the
individual geothermal doublets.
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2
From current practice to better practice
2.1
Introduction
12 / 70
In this chapter the current practice regarding filter use in the geothermal doublets is
described and it is discussed how this practice can be improved. The current
geothermal doublets in The Netherlands have total water flow rates of 90 to 240
3
m /h. The produced water temperature range from 60°C to 90°C and it is cooled till
25°C to 40°C. The doublets have total heat capacities of 5 to 12 MW. In Appendix
9.5 the current practice in the nine geothermal doublets under consideration is
summarized.
2.1.1
Set-up of the surface installation of a geothermal doublet
The surface installation of a geothermal doublet consists of several components. In
general, the main components are:
- Variable frequency inverter for the ESP (electric submersible pump) which
controls the flow rate of the production water
- Oil/gas separator for the removal of the major part of mineral oil and/or gas,
if present in the production water
- Heat exchanger for the transfer of the geothermal heat to the heat
distribution network
- Pumps for the circulation in the distribution network
- Filters to remove sand, fine particles, potential corrosive particles and oil if
present
- Reinjection pump
- Sometimes a combined heat and power plant, heat buffers and a heat
pump are present at the geothermal doublet.
2.1.2
Main challenges for filtration and parameters affecting the formation
Optimal filtration is filtration in such a way that the planned lifetime of an injection
well (normally 25-30 years) is reached with a minimal filtration effort. However,
formation impairment (worsening) at the injection zone does not only depend on the
filtration steps, but also on other factors, like:
 injection temperature,
 pressure and flow rates (geomechanical formation damage at high flows
and pressures),
 presence of dissolved minerals with a scaling tendency in the injection
water,
 swelling or agglomeration of clay particles at the injection zone,
 mineral compostion of the sediment/formation and aquifers,
 permeability and pore size distribution of the reservoir,
 trapped gas and oxygen contamination.
During injection, the downhole water quality may become worse, with higher solid
concentrations than at the wellhead (Saripalli et al., 1999). Thus, filtration can only
prevent part of the injection problems and formation damage.
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However, the damage originating from suspended solids is the most commonly
encountered in geothermal engineering, and regarded as the major challenge
(Ungemach, 2003).
The dilemma is that only afterwards (on the long term, maybe after years of
operation) it can be concluded with certainty if a certain filtration regime was
efficient, but at that time it might be too late and injection rates could already have
declined. The challenge is to predict the effect of filtration on the injectivity as good
as possible.
2.2
Interviewed operators and suppliers
Information and practical experience with regard to filter use in geothermal doublets
was obtained by interviewing Dutch operators of geothermal doublets and filter
suppliers, discussions between TNO’s water treatment specialists and geologists,
information from geothermal doublets abroad and by literature research.
The following current and future geothermal operators were interviewed:
 Ted Zwinkels (Greenwell Westland, Honselersdijk)
 Ad van Adrichem (Duijvestein Tomaten, Pijnacker)
 Leon Ammerlaan (Ammerlaan, The green innovator, Pijnacker)
 Wart van Zonneveld (Floricultura, Heemskerk)
 Pieter Wijnen (Californië Wijnen Geothermie, Grubbenvorst)
 Frank Schoof (Aardwarmte Den Haag)
 Rik van den Bosch (A + G van den Bosch, Bleiswijk and Berkel)
 Radboud Vorage (Aardwarmtekluster 1 KKP, IJsselmuiden)
 Saskia Hagedoorn & Floris Veeger (Hydreco)
The following filter suppliers were interviewed. They cooperated in the
“Technologiecluster”:
 Tony Dinsbach & Hennie de Oude (Hitma)
 Martin Kramer & Evert Jan Hoveling (Twin Filter, Zaandam)
2.3
Reservoir type at injection zone and reservoir properties
The injection zones of the geothermal projects under consideration in this report are
part of one or sometimes two of the formation types:
Delft sandstone
Alblasserdam sandstone
Rijswijk sandstone
Pijnacker sandstone
Berkel sandstone
Slochteren sandstone
Carboniferous limestone
Especially poorly cemented sandstone reservoirs, rich in clay minerals, are
sometimes susceptible for plugging as a results of migration of fine particles and of
plugging by suspended solids in the formation water (Raemakers et al., 2006).
The formations have various petrophysical properties. Relevant reservoir properties
like particle size distribution of the rock, porosity and permeability in the reservoir at
the injection zones of the geothermal wells are currently based on assumptions and
calculations.
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They must be known in more detail to determine the pore size distribution at the
injection zone more precisely and therewith the critical plugging range of injected
particles. By analyzing the cuttings (see paragraph 3.4) this properties can be
estimated in more detail.
The currently estimated critical plugging range for particles that are injected is
between 1 and 9 µm for the geothermal projects under consideration in this report.
2.4
Downhole screen in the production and injection well
At the production and injection zone of the wells, the tubes have vertical grooves
with a length of about 5-10 cm and a width of about 2-3 mm to be able to produce
or inject the water. In addition, eight of the nine geothermal doublets under
consideration in this report have wire wrapped screens in the production zone of the
production well and seven have a similar screen at the injection zone of the
injection well. These screens cover the tubes at the production and injection zone.
In all cases, HP Well Screens with a separation size of 300 µm were chosen quite
arbitrary to remove coarse sand and other particles in the production water.
The downhole screen can be considered as first filtration step of the produced
water. The screen reduces the particle loading on the filters in the surface
installation. Through the use of a downhole filter screen followed by one or more
filtration steps in the surface installation with decreasing filter rating, the surface
installation and injection well are effectively protected against the solids in the
produced water.
A downhole screen is an extra expenditure and can reduce the rate of the flow
when the meshes are blocked with coarse particles. Therefore, in cases when the
reservoir is a hard sediment, which is not expected to release much particles after
the well cleaning, a downhole filterscreen can be omitted. Then, the filter installation
in the surface installation is fully responsible for removing produced particles.
Sometimes operators choose to install a filter screen in the production zone of the
production well and omit the filter in the injection well. Advantage is that injection is
not hindered by a screen on which particle or scales might block or oil might
accumulate. Drawback is that the wells are not exchangeable, like in most of the
other doublets.
Some operators go one step further and do not have any tubing system in the
production and injection zone. These are doublets that produce and inject in a
reservoir that is a solid rock, like Carboniferous Limestone. This is not
recommendable for sandstone types of reservoir, because of the risk of collapse of
the well and the production of particles. In these reservoirs always a tubing system
covered with wire wrapped filter screen is recommended.
In should be noted that some operators choose to install the wire wrapped screens
only at the (production) zones with the highest permeability. In between, blind pipes
are installed.
2.5
Coarse filter at the beginning of surface installation
Some operators choose to install a coarse filter (e.g. 200 µm or 300 µm) at the
beginning of the surface installation, to remove the particles that might be formed in
the production tubing system and to protect the filter system in the surface
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installation from coarse particles. For warranty on advanced filtration equipment like
an automatic filter, the suppliers demand a minimum of pre filtration.
The selectivity of the coarse screen (300 um, finer or coarser) depends on the
following filtration steps. The consecutive filtration steps must be in proportion.
Such a screen at the beginning of the surface installation can be beneficial if it
extends the life time of the filters in the filter installation. However, such a screen
should not cost much maintenance/cleaning time and effort. It is the choice of the
operator if he wants to invest in the protectioin of the subsequent equipment in the
surface installation (cost/benefit analysis).
What should be known about this coarse prefilter is:
- How much total suspended solids (TSS) does it remove per time interval?
- What are the costs of cleaning and/or replacement of the coarse screen?
When these questions are answered, it can be estimated, using the dirt-holdingcapacity of the filters in the filter installation, how much the life time of the filters can
be extended and if costs can be saved by placing a coarse filter at the beginning of
the filter installation.
2.6
Composition of the production water
The produced waters of the nine geothermal doublets under consideration in this
report have high salinities, ranging from about 78 g/L to about 250 g/L, a produced
water temperature (at the beginning of the surface installation) ranging from 60 to
90ºC, and a pH between 5.1 and 6.8.
This report focusses on the removal of particles in the produced water. Formation of
salt precipitates due to changing process conditions (e.g. temperature, pressure) is
covered by another Technologiecluster project. In this Technologiecluster (Wasch,
2013) a list of potential scaling minerals is selected for the geothermal doublets
under consideration based on the supersaturation (precipitation potential)
calculated in the simulation program.
When these minerals are present as particles in the water of the surface installation
they will be removed by the existing filter installation if they are bigger than the
micron rating of the filters installed.
The main (inert) particles in the produced water are sand and clay particles (for
sandstone type reservoirs). Quartz and feldspar (tectosilicate minerals: K-Na-Ca
and an alumina silicate,
KAlSi3O8, NaAlSi3O8, CaAl2Si2O8) are the main
components in these categories.
In addition, Fe-Cr-Ni steels can be found, which can origin from the casing/tubing
system of the doublet. Furthermore, iron hydroxide is often part of the produced
particles. Depending on the calcium content of the reservoir, also calcite (CaCO3)
particles are present in the produced water. However, the other minerals can be
potential scale formers (Wasch, 2013).
The size range of these particles often ranges between 0.45 µm and 100-300 µm.
By definition, the smallest non-dissolved particles have a diameter of 0.45 µm. This
is based on a widely used convention that considers particulate matter to be larger
than 0.45 µm in diameter. Anything smaller is considered to be dissolved. This
boundary is not entirely valid because clay particles and silt can be smaller than
0.45 µm. For practical purposes, however, the boundary is convenient, not least
because standard membrane filters with 0.45 µm diameter pores can be used to
separate suspended particles from dissolved solids. Particles bigger than 300 µm
are not expected if a 300 µm filter screen has been installed at reservoir depth.
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During well tests, high amounts of mud and particles are produced. This is not
representative for the amounts of particles produced during stable doublet operation
after e.g. a year of operation. The operators experienced that the total suspended
solids content decreases during operational time in the first year of operation. This
is indicated by the life time of the filters in the surface installation.
Not much data are available about the total suspended solids (TSS) during well
tests. Values of 11, and 67 mg/L are reported during early operation of the doublet
or during the well test. This values can decrease after some weeks/month of
operation.
It can be concluded that not much information is available about the particle
concentration, size distribution and composition of the particles during stable
process operation of the geothermal doublet. These measurement are often done
during the well tests, but these measurements are not representative for a stable
doublet operation after e.g. half a year or a year.
2.7
Filter types and filter strategy in geothermal doublets in The Netherlands
2.7.1
Types of filters used
In the geothermal doublets under consideration in this report, mainly bag filters and
cartridge filters (high flow or conventional) are used. Only in one project, an
automatic filter was used. In every geothermal doublet, one of the following five
configurations is applied:
-
Course screen (40 µm or 200 µm)
Bag filters (10 µm)
Bag filters (10 µm) – bag filters (10 µm) (before and after heat exchanger)
Bag filters (10 µm) – (high flow) cartridge filters (10 µm)
Course screen (300 µm) – automatic filter (25 µm) – oil filter – cartridge
filter (2 µm)
What can be seen from this list is that currently 10 µm filtration - nominal bag
filtration, sometimes followed by 10 µm absolute filtration by cartridge filters - is the
standard.
According to Twin Filter, several years ago mainly 25 µm filters were used in the
geothermal doublets in The Netherlands based on trial and error. Currently, more
10 µm nominal filters are used. This resulted in an improved/restored injectivity.
This could be due to (partly) deplugging/removal of particle between 10 µm and 25
µm in the formation at the injection zone, e.g. after an increase of the injection flow
rate.
When more filter steps are applied, the filter installation set-up is always gradually:
first coarse filters are used, followed by finer filters. The general strategy is to first
remove the bulk of the solids with cheap nominal bag filters. In a second filtration
step, the remainder of the solids can be removed with an absolute filter with a finer
filter rating. The absolute filters are more expensive than the nominal filters (see
paragraph 2.10). The nominal filters protect the more expensive absolute filters and
extend their life time.
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Filter system configurations
Several configurations of the filter systems are applied in The Netherlands:
-
2 parallel filter lines
2 or 3 (optional) filter lines
3 parallel filter lines (Figure 1)
2 or 3 parallel lines, depending on filter step
Most geothermal doublets have 2 parallel filter lines. A filter line can consist of e.g.
bag filtration followed by a cartridge filtration. Filter lines can be combined, resulting
in equal pressure build up and life time of every filter line. Alternatively filter lines
can be physically separated from each other, resulting in unequal pressure build up
and life time of the filters.
Figure 1. Example of a filter system containing three parallel filter lines.
3
Most of the geothermal doublets in The Netherlands have a flow rate of 100 m /h to
3
200 m /h. Filter houses contain multiple bag or cartridge filters. The geothermal
doublet in The Netherlands have currently 4 to 8 bag filters per filter house (and
normally 2 filter houses operated parallelly). The amount of bag filters is not only
dependent on the flow rate of the doublet, but also on the filter rating and particle
loading (see paragraph 4.2).
Conventional cartridge filters have a much lower (about ten times) flow capacity
(see Appendix 9.4) than bag filters and therefore, filter houses with conventional
cartridge filters contain about ten times more filters than the houses with bag filters.
Some operators use high flow cartridge filters as a final filtration step. These filters
3
can deliver a flow rate of up to 100 m /h. When these high flow filters are used,
normally only two or three filters are operated parallelly.
2.7.3
Operational strategy during replacement of filters
Most of the operators use the full capacity of their filter system during winter time
when the highest heat capacity is requested. In summer time, when water flows are
lower, the filter system is normally not operated at its highest capacity.
Most of the geothermal doublets in The Netherlands have two parallel filter lines.
One operator has three filter lines, but only two are in use at the same time. During
a replacement of the filters in one filter lines several strategies are applied:
- Total installation is stopped or filters are bypassed
- Total flow is reduced by 50% and temporary only one of the two filter lines
are in use
- Total flow is let through one filter line, no reduction in total flow rate
- 2 of the available 3 filter lines are used alternately, assuring a constant flow
through the filters lines.
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Start and stop actions of the doublet should be avoided as much as possible. Startups cause an extra particle loading from the production well or from the surface
installation, as observed by the operators.
Bypassing the filter installation is a risk. Replacing filters in a filter house or in a
complete filter line normally takes about 30 to 60 minutes, depending on the amount
of filters that need to be replaced. During that period the injection well is
unprotected.
Reducing the flow by 50% in the geothermal doublet when filters are replaced in
one of the two filter lines, is a better option than completely stopping the whole
process. However, here also the flow rate is first decreased and after replacement
of the filters again increased, which can generate an extra particle loading in the
system.
A third option that is applied, is to direct the complete flow rate through one filter line
when one of the two filter lines need replacement. This is only possible when this
single filter line has enough capacity for the total flow rate. The advantage of this
option is that the flow rate through the geothermal doublet is not decreased.
However, to improve the life time of a filter, it is recommended to operate the filter
not at its maximal capacity but to use lower flow velocities, as discussed in
paragraph 4.7.
The fourth option has the preference. When three filter lines are available and two
filter lines have enough capacity to treat the complete water flow, there is always
one spare filter line (flexibility). This one can be used when in one of the other filter
lines filters need to be replaced. In this way the flow rate through the filter
installation is always constant. However, an extra investment has to be done for the
installation of an extra (third) filter line and the space must be available.
Instead of using a third filter line as spare filter line, it can be chosen to operate the
three filter lines continuously to lower the flow velocity in the filters and increase the
total filtration area. This will increase the filter life, as discussed in paragraph 4.7.
Only during replacement temporary two filter lines can be used.
If two filter lines are combined, their pressure build up is equal and they have to
replaced at the same time (or directly after each other). The second or the third
replacement option mentioned above is then normally applied, depending on the
pump capacity.
Only if the filter lines are physically separated from each other, resulting in unequal
pressure build up and life time of the filters in the parallel filter lines, the
replacement of the filters can take place at unequal time points.
2.7.4
Replacement time / life time of filters
The replacement time/life time of the filters depends on the total volume and flow
rate through the filter, the particle concentration in the water, the filter rating
(separation size) and the dirt-holding capacity of the filter. Based on filterability tests
(see paragraph 4.8) estimations can be made on the life time of a filter.
A general observation is that the filters need to be replaced more frequently after
the startup of the geothermal doublet (order: first several weeks) than during stable
process operation after a few months.
After the testing period of the wells, wells are not completely clean yet and particles
from the formation and also from the drilling process are still produced.
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The Dutch operators have different experiences with the time intervals between
replacement during a stable process operation. This ranges from 4-5 days for bag
filters to about 2 months for cartridge filters that are protected by the bag filters.
These time intervals are quite hollow numbers, because in addition to the factors
mentioned above, the life time depends also on the capacity of the filter installation.
When the capacity is increased (more filters used at the same time), the life time of
the filters will be longer.
The time it takes to physically replace the filters in a filter house (down time)
depends on the type of filter house and the amount of bag and cartridge filters. Twin
Filter estimates the average down time, measured between draining and restart, for
bag filters on 45 to 60 minutes per filter house and for cartridge filters between 30
and 45 minutes per filter house. Hitma notes that the replacement of a high flow
filter can be done in 5 minutes, where the replacement of 50 double open end filters
can take half an hour or more.
The down time of a filter line must be as short as possible, because during this
down time the second filter line is loaded more heavily (volumetric and particle
loading). This results in an disproportionate decrease of the life time of the filters.
2.7.5
Criteria for replacement filters / pressure drop
The optimal pressure difference for replacement of filters is normally mentioned in
the documentation of the filters. For example, cartridge filters can be operated till a
pressure drop of about 2.5 bar but it is advised to replace them at a pressure drop
of 1.5 bar. See paragraph 4.4 for a discussion on the pressure drop.
Operators indicate that they replace their filters at the following pressure drops:
-
1 bar over total filter line of bag filters and cartridge filters
0.3-0.4 bar (first bag filter step) and 0.4-0.5 bar (second bag filter step)
0.8 bar over bag filter house
1.5-2 bar over cartridge filters
This 0.3-0.4 bar and 0.4-0.5 bar seems rather early to replace the bag filters. It is
recommended to use a filter till the maximum advisable pressure as indicated by
the manufacturer to minimize the filter costs. When a filter line exists of two stages
(e.g. bag filter – cartridge filter) the first stage filters protect the second stage filters
and prolong their life time.
Ideally, the pressure drop of both stages is measured separately and not over the
total system, to have detailed information about the pressure build up in both single
filtration steps. If only the total overpressure over two filter stages is measured, and
from practical experience it is known that the first filter stage reaches its critical
overpressure e.g. 5 times as fast as the second filter stage, it is advisable to replace
only the first stage filters and to replace both filter stages together only at the fifth
replacement time of the first filter stage. However, separate measurements of
overpressure of both stages is more reliable.
Normally filter lines (e.g. two parallelly operated bag filter lines) are combined. Their
pressure build up is equal and they have to be replaced at the same time (or
directly after each other). The second or the third replacement option mentioned
paragraph 2.7.3 is then normally applied, depending on the pump capacity. Also
with combined filer lines, the single filter vessels should be equipped with their own
pressure meters, to monitor if the operation goes well.
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Place of the filters in the surface installation of the doublet
The purpose of the filters in the surface installation is to protect the system
equipment, mainly the heat exchanger, and secondly to protect the injection well
from clogging of particles, keep the injection pressure low and save energy. The
investment of an (extra) filter installation before the heat exchangers (HEX) should
be weighed against the lower maintenance and cleaning costs for the heat
exchangers.
Currently, in the geothermal installation under consideration, the filters are installed
on the following places (Figure 2):
- Bag filters and cartridge filters before the heat exchangers (3 times)
- Bag filters before heat exchanger (2 times)
- Bag filters before and after the heat exchanger (1 time)
- Course filter – HEX – automatic filter – oil filters – cartridge filters (1 time)
- Wire mesh screens before heat exchanger (2 times)
Advantage of filtration after the heat exchanger is that the temperature is lower. Not
all filter materials are compatible with the high temperature before the heat
exchanger.
Figure 2. Place of filters in the geothermal doublet. Downhole screens (2, 10), filters before heat
exchangers (6) and after heat exchangers (7), polishing filter directly before injection
(8).
2.8
Removal of oil
Five or six of the geothermal doublets under consideration in this report use or will
use an oil-water-gas separator to remove the bulk of the oil in the produced water at
the beginning of the surface installation. The other three or four do not degas and/or
remove oil in a separator.
For most of the geothermal doublets in The Netherlands, additional oil removal by
oil filters is not an issue. The mineral oil content after the degasser (if present) is
below 1000 µg/L at the geothermal doublets under consideration in this report and
in most cases it is only around 100 µg/L.
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In one case the mineral oil content (before degassing/deoiling) is 0.1%. This is the
only project that uses additional oil filters as a safety barrier.
Dispersed oil (droplets of oil) must be prevented in the injection water. These
droplets can accumulate in the pores at the injection zone causing a reduced
relative permeability to water. This is expected to result in reduced invasion depth
by injected solids, as well as the creation of a more effective filtercake on the
wellbore wall (Saripalli et al., 1999). Dissolved oily compounds like benzene,
toluene, ethlybenzene and xylene (BTEX) are not likely to have a negative effect on
the injectability.
When mineral oil is present in excessive amounts in the produced water (low
percentage range), the first step for the oil treatment is to optimize the oil gas
separator to remove the mineral oil as far as possible. The high salt content of the
production water (about 100-200 g/L) has a negative effect on the separation of oil
and water in the oil gas separator compared to separation at fresh water conditions.
Additional oil separation technology must be considered, like a settler,
hydrocyclone, chemical injection, before high amounts of oil filters are being
installed and used. Optimising the oil-gas-water separator or use other chemical or
mechanical measures to reduce the oil content from the produced water falls out of
the scope of this report.
It is carefully estimated that concentrations of dispersed oil (droplets of oil in water)
below 1 mg/L do not harm the filter installation in the surface installation. This is
based on observations at a geothermal installation with oil filter cloths inside the
regular filter bags. This would mean that below a concentration of 1 mg/L dispersed
oil, oil filters are not needed. However, more research must be done on this critical
dispersed oil concentration. The droplet size is an important parameter. Can the oil
droplets pass through the pores of the bag/cartridge filter? The bag filter can serve
as a coalescer (merging of droplets).
The risk is that a bag filter temporary collects the dispersed and free oil from the
produced water and that after a certain point the oil slips through and a substantial
amount of free oil will block the following cartridge filters or the injection well.
In addition, the mineral oil can affect the recovery of the heat transfer in the heat
exchangers and enhance fouling of the heat exchanger.
2.9
Experience abroad
Bag filters with a mesh size of 10 µm up to 20 µm at the production well and a 1 µm
bag filter system on the injection well (or a similar configuration) are widely used in
geothermal power plants in the North German basin (BWG, 2012).
In The Netherlands, currently 10 µm filtration - nominal bag filtration, sometimes
followed by 10 µm absolute filtration by cartridge filters - is the standard (paragraph
2.7.1).
According to Twin Filter, several years ago mainly 25 µm filters were used in the
geothermal doublets in The Netherlands based on trial and error. Currently, more
10 µm nominal filters are used.
According to Hitma, 10 µm absolute filtration can be considered as a general
standard in France.
In the oil production in the Middle East, injection wells were installed with 10 µm
absolute filters. Nowadays, this is sharpened to 5 µm nominal (information Twin
Filter).
It must be noticed that the numbers mentioned here are quite general numbers and
not set for specific situations. The choice for a filtration system remains case
specific. Until now, reservoir properties are not taken into account for the choice of
the filter system/filter micron rating.
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The investment costs of a filter installation in a geothermal doublet can vary
between < 10 k€ for a clean production well to about 50-80 k€ for a well that
produces high amounts of oil and solids. The investment costs for an automatic
filter are often > 10 k€. This depends much on the required capacity and the filter
rating. For a smaller filter rating, a bigger filter house and screen size is needed
(higher investment).
Hitma estimated the maintenance costs (labor + material) of the filter installation in
the surface installation of the geothermal doublet on 100-150 k€ per year in the first
year after the start-up. This was based on their experience with a geothermal
3
3
project (flow rate between 100 m /h and 200 m /h) that started in 2012.
This are relatively high expenses, compared to total installation costs of the surface
installation.
After more than a year of operation and experience with filter usage, the costs will
go down and are estimated on about 50 k€ or even 20 k€ per year. This drop in
filter costs has to do with the lower solids load in the production water after a year of
operation, compared to the start-up period. Costs of the filter installation will
decrease during the life time of the geothermal doublet and most of the geothermal
doublets in The Netherlands are still in their infancy.
Based on filter costs of current geothermal doublets and comparison of the water
quality parameters (mainly solid load) and flow capacity, an estimation can be made
of the costs of a filter installation for a new geothermal doublet.
The maintenance costs (labor + material) of the filter installation mainly depend on
dirt loading and flow rate. The current active geothermal doublets in The
3
3
Netherlands have flow rates between 90 m /h and 200 m /h.
Operators that started the geothermal doublet recently (less than a year ago) are
often not yet fully aware of the annual maintenance costs. Two operators that
operate their geothermal doublet for more than two years estimate their current
annual maintenance costs for the filter installation on 15 k€ to 20 k€ and 35 k€,
respectively. When only coarse screens are installed in the surface installation the
costs are negligible.
Filtering with a lower flow velocity is more effective, as discussed in paragraph 4.7.
At lower flow velocity, the filter has a higher dirt-holding-capacity.
As a rule of thumb, it can be stated that doubling the filtration area will increase the
life time of the filters by a factor 3 to 4. For depth filtration, as a rule of thumb, it can
be stated that doubling the depth results in an quadratic life time extension at the
most. Therefore, it can be cost saving to have a higher filtering capacity then strictly
necessary for the water flow. After an investment for extra filter houses and piping
system, the costs can be earned back by using less filter bags and cartridges.
A boundary conditions is that enough space must be available in the building of the
surface installation.
Theoretically, operational costs for filters will decrease by a factor 1.5 to 2 when the
capacity of a filter installation is doubled.
Example of cost reduction
In this example, the reduction in operational costs are estimated when the capacity
of a filter installation is doubled. In Table 1, assumed prices for filters are listed.
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Table 1. Price assumption for filters (based on information of Hitma and Twin Filter).
Filter type
Bag filter (nominal)
Wound cartridge
(nominal)*
Pleated cartridge
filter*
Oil block cartridge
High flow cartridge
filter (40 inch)
Assumed
average price (€)
7
14
22
35
350
Remark
Few euros per bag, depending on type
18-26 euro, depending on filter rating
Prince depends on filter rating and
surface. Price up to 575 euro per filter
2
with high surface (14 m )
* a conventional cartridge filter (wound, pleated) can range in price from 2-25 euro per 10 inch,
depending on type. 40 inch: 4 time higher price
Remark: a 10 inch conventional cartridge filter is on average 2-3 times more
expensive than a nominal bag filter. However, because the filter bag has about 5-10
times higher capacity than the conventional cartridge filter, 5-10 times more
cartridge filters are needed than bag filters, leading to a huge price difference for a
bag filtration and cartridge filtration step. Therefore, bag filters are normally used to
remove the bulk of the solids and cartridge filters are used for polishing to improve
their life time.
Assumed initial filter installation:
- 2 filter vessels each containing 6 nominal bag filters
- 1 filter vessel containing 2 high flow cartridge filters
Initial replacement time:
- Bag filters: 1 week
- High flow cartridge filter: 6 weeks
Costs of filters per year:
- 12 bag filters x 52 weeks x 7 euro = 4368 euro
- 2 high flow filters x 52/6 weeks x 350 euro = 6067 euro
- Total: 10435 euro/year
When the capacity of the filter installation is doubled, these filter costs are expected
to go down by a factor 1.5-2, as discussed before. In the example above, this
means a reduction in filter costs of 3478 – 5218 euros. In additions, costs of labor
will also decrease by a factor 1.5-2.
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3
Reservoir properties and risk of plugging of the
injection well by particles
3.1
Introduction
24 / 70
In this chapter, it is discussed how particles can plug the formation and what is their
cirtical size. Methods are described to determine sediment properties at the
injection zone.
3.2
Plugging of particles in the injection well
Both particles in the formation matrix and particles in the injected water can cause
pore throat plugging of the injection zone and therewith impairment (deterioration)
of the permeability (Figure 3). These external particles are partly removed from the
production water by the filter system in the surface installation of a geothermal
doublet. To what extent depends on the filter specifications and the specifications of
the particles (mainly the size).
Figure 3. Particles that can cause plugging of the injection zone (Vernoux et al. 1997).
The currently installed filters in the surface installations of geothermal doublets in
The Netherlands are in general meant for the mechanical removal of particles of
about 5 µm or bigger (see Chapter 2). When particles are injected, the formation
works as a filter (deep filtration). In Table 2 it can be seen that for particles in the
range 7-30 µm, spontaneous deplugging is unlikely, thus injection of these particles
must be prevented.
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Table 2. Classification of deep filtration types (Herzig et al., 1970).
Filtration type ........................ mechanical
Particle size ............................ 7-30 m
Retention
 sites .................................. constrictions,
crevices,
cavernes
 forces ................................ frictions
fluid,
pressure
physico-chemical
1-3 m
colloidal
<0.1 m
surface
surface
Van der Waals,
electro kinetic
Capture mechanism ...............sedimentation,
direct
interception
Deplugging:
 spontaneous ..................... unlikely
 provoked .......................... flow reversal
direct interception
Van der Waals,
electro kinetic,
chemical
bounding
direct interception,
diffusion
possible
increase in flow
rate
possible
increase in flow
rate
Four elementary mechanisms how solids particles can cause well and formation
damage are indicated in Figure 4. These mechanisms all cause a decrease of the
permeability at the injection zone, requiring higher pressures for injection. Figure 5
shows in more detail how particles can block the pores of the formation at the
injection zone. Several mechanisms can be distinguished: bridging, size exclusion,
aggregate formation.
Figure 4. Well and formation damage mechanisms caused by solid particles (Barkman and
Davidson, 1972).
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Figure 5. Mechanisms of particle capture in the pores of the formation (Vernoux et al. 1997).
To estimate the critical pore size and the critical plugging range for the injection
zone of the formation, a pragmatic approach can be used. Ideal spherical particles
are assumed with all the same size.
Table 3 shows the critical ratios between the particle diameter and the pore throat
diameter. When a particle has a diameter of more than 1/3 the size of the pore
throat, bridging or size exclusion can occur. When the particle diameter is smaller
than 1/10 of the pore throat, no blocking will occur and the particle can migrate
through the formation freely. Sometimes already free migration at 1/7 the size of the
pore throat is mentioned. This lower boundary is influenced by the inflow velocity at
the wellbore (Figure 6). A particle with a diameter between 1/10 and 1/3 the pore
throat will invade the formation and block the pore by bridging deeper in the
formation. Particles between 1/3 and 1/10 the size of the pore throat can be
considered as the most critical ones for plugging the pore channels (Figure 6).
Laboratory investigations on core samples have shown that external filter cake
build-up (no pore invasion) predominates when the particles of suspended matter
are larger than 1/3 of the median pore size of the formation (Smit, without year).
Deep bed invasion with little internal filter cake build-up far away from the wellbore
occurs when these particles are smaller than 1/10 of the median pore size. For
water containing particles smaller than 1/3 of the pore throat size, mixed filtration
occurs with significant internal filter cake build-up.
A key reservoir property which determines the rate of plugging is permeability; low
permeability reservoirs tend to plug easier than high permeability or fractured
reservoirs. Particle size relative to pore throat size determines the severity of this
plugging.
The rate of impairment (damaging) of the injection zone by particles in the injection
water is influenced by the flow velocity at the wellbore. A lower velocity decreases
the risk of clogging by injected particles (Figure 6). This means that in the winter
period, when higher flow are applied than in summer, the risk of damaging the
injection well is higher than in the summer period.
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Table 3. Critical ratios between particle diameter and pore throat diameter (GPC IP, 2013).
Particle diameter / pore throat
diameter
> 1/3
< 1/10
1/3 – 1/10
Entrainment
Process
Bridging or size exclusion
Entrainment
Formation invasion and deep bridging
of pore constrictions
Formation invasion, deep bridging
Bridging, size exclusion
of pore constrictions
1/10
1/3
Ratio particle diameter / pore throat ->
Figure 6. Rate of impairment (damaging) of the injection zone at varying particle/pore size ratios.
Laboratory study by Van Velzen and Leerlooijer (1992).
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Calculation of average pore diameter based on the permeability
Based on an empirical formula of Alen Hazen, the permeability of granular rock
such as sandstone and the average pore diameter are related:
k = C (d)
2
(1)
k = intrinsic permeability (mD)
C = Hazen’s empirical coefficient (dimensionless) between 0.4 and 10 with an
average value of 1, related to the configuration of the flow-paths
d = average or effective pore diameter (µm)
This equation is valid for ideal spherical particles with all the same size. It is a
pragmatic approach to estimate the average pore diameter in sandstone reservoir.
However, often clay particles are present between the sand particles which make
the system less ideal.
In Table 4 the average pore size and the critical plugging range have been
calculated, assuming C in equation 1 = 1. With this information, recommendation
can be done for a filter rating in a filter installation. The critical plugging range is
based on (pore size/10) to (pore size/3). When C = 1, equation (1) can be simplified
to:
d = √k
(2)
Example:
If the permeability at the injection zone is 250 millidarcy, the critical pore size is 15.8
µm. When particles between 1/3 and 1/10 the size of the pore throat will plug the
pore channels, the critical plugging range is 1.6 and 5.3 µm. If all the particles in the
critical plugging range need to be removed, a filter with a 2 µm absolute rating is
recommended.
Table 4. Relation between permeability and pore size.
Permeability
(milliDarcy)
100
250
500
750
1000
1500
2500
Pore size
(microns)
10
15.8
22.4
27.4
31.6
38.7
50.0
Critical plugging
range (microns)
3.3-1.0
5.3-1.6
7.5-2.2
9.1-2.7
10.5-3.2
12.9-3.9
16.7-5.0
The average permeability can be estimated for the entire formation. Realistically, it
is more probable that only certain zones (stratigraphic zones or fault zones) are
productive, with a much lower thickness. As a result, the average permeability will
be much higher, resulting in a higher critical plugging range.
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Methods to determine sediment properties at the injection zone
Relevant reservoir properties like particle size distribution of the rock, porosity and
permeability in the reservoir at the injection zones of the geothermal wells in The
Netherlands are currently based on assumptions and calculations. They must be
known in more detail to determine the pore size distribution at the injection zone
more precisely and therewith the critical plugging range of injected particles.
Porosity and permeability of the reservoir cannot directly be measured from the
bore holes.
For a petrophysical evaluation, reservoir (rock) properties are calculated from bore
hole measurements (logs) and core plug analysis. The clay content is often
determined based on a gamma ray log. The porosity is calculated based on bore
hole measurements like the density log and the neutron log. As a calibration, the
porosity that is determined from the plugs from the cores of the reservoir formation
is used. To be able to compare with the bore hole measurements (reservoir
conditions) the plug porosities determined in the laboratory must be corrected for
reservoir conditions.
In addition to porosity, also permeability is measured during core plug analysis.
From the porosity and permeability measurements from the core plugs, a so called
poro-perm relationship is determined. This relationship is used to translate the
reservoir porosity (determined from porosity logs) to reservoir permeability.
(Raemakers et al., 2006).
No cores are available from the wells of the geothermal doublets in The
Netherlands. A reliable poresize distribution can only be done with samples from
the cores. On the other hand, the cuttings can be analysed. They are collected from
a specific interval, although caving may corrupt the pure interval signal.
To determine the grain size distribution of a representative sample a laser particle
analysis (light scattering analysis) and/or sieving analysis can be done to
determine the grain size distribution of the sediment.
Several laser diffraction particle size analyzers are on the market. The technique of
laser diffraction is used to measure the size of particles. It does this by measuring
the intensity of light scattered as a laser beam passes through a dispersed
particulate sample. This data is then analyzed to calculate the size of the particles
that created the scattering pattern. These analysers typically can measure particles
in the range between 0.02 µm and 2-3 mm in a number of size classes (source:
www.malvern.com/LabEng/technology/laser_diffraction/laser_diffraction_systems.ht
m?gclid=CJSFnf3o-rkCFdHMtAodgx0Abg ).
At the department Functional Ingredients at TNO Zeist, particle size distribution can
be determined in the range of 0.02 µm to 2000 µm by laser diffraction with the
Malvern Mastersizer 2000. It is capable of accurately characterising emulsions,
suspension and dry powders. A dry sample must be wetted first and diluted. The
sample is stirred and pumped through a cuvette. This can break aggregates.
Ultrasonic treatment is an option the break aggregates of the cutting before the
determination of the psd. The costs of such a psd determination are about €200 per
sample (duplo).
A sieving analysis is less advanced. Particles varying between 2 µm and 250 mm
are separated in regular size class intervals. More (dry) sample is needed
compared to a laser particle analysis. The costs of a sieving analysis are
comparable to the costs of a laser particle analysis. A laser particle analysis is
recommended above a sieving analysis, because the former can measure in a
lower size range, which is relevant for our purpose. The analysis can be done if the
cutting is a sample of a fragile sediment with loose grains.
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When the cutting is a hard rock, the sample must first be destructed into its grains
before the analysis can be done.
If the cutting/sample cannot be disintegrated to separate individual grains, an
optical analysis by microscope can be done. For that, a 2 by 2 cm slice or plaquette
of a few mm thick is prepared from mm-sized particles. By measuring the grain
sizes and distances between the grains under the microscope, estimations can be
Easier, but less precise is measurement of grain sizes and distances between the
grains from the cutting by using a magnifying glass.
Chemical characterisation of particles
Both particles in the produced/injected water and sediment/cuttings from the
injection zone of the injection well can be characterised on the chemical
composition. A water sample first need to be filtered and dried before it can be
analysed.
At the TNO department Applied Environmental Chemistry three (analytical)
techniques are used to characterize and chemically analyse a sample: Stereo light
microscopy (SLM), Scanning Electron Microscopy combined with energy dispersive
X-ray microanalysis (SEM / XRMA) and µ-Fourier Transformed Infrared
spectroscopy (µ-FTIR). These techniques combines the possibility of micro
morphological investigation of solid materials and simultaneous (local) elemental
analysis. These analyses are further described in Appendix 9.3.
The costs of such analyses depend on the homogeneity of the composition. When
the sample is homogeneous, less particles need to be analysed. It takes more time
to analyse a sample which consists of a variety of elements. The costs of an
SEM/XRMA and µ-FTIR analysis are about 1000 to 3500 per sample.
3.5
Estimating permeability based on grain size
Several methods can be used to estimate the permeability of a sediment based on
grain size of the sediment:
- Krumbein and Monk's equation
- Berg’s model
- Van Baaren’s model
These methods are further descripted in Appendix 9.1. Permeability can be
estimated when the following data are known:
Krumbein and Monk's equation
- Geometric mean grain diameter
- Standard deviation of grain diameter
Berg’s model
- Median grain diameter
- Spread in grain size
- Porosity
Van Baaren’s model
- Dominant or medium grain diameter
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Archie cementation exponent, common values for this cementation
exponent for consolidated sandstones are 1.8-2.0, see Appendix 9.1.
Sorting index, that ranges 0.7 for very well sorted to 1.0 for poorly sorted
sandstones.
Porosity
Thus, for estimating the permeability at the injection zone the mean or medium
grain diameter must be known from analysis of the cuttings and the grain size
distribution. If Berg’s or Van Baaren’s model are used, also the porosity must be
known. Achie cementation index and the sorting index are known from literature.
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Filtration theory for geothermal doublets
4.1
Introduction
32 / 70
In this chapter the critical factors that determine the choice of a filter system in the
surface installation of a geothermal doublet are discussed and the theory behind
the filtration of particles is described. In appendix 9.4 information is given on the
commercially available filtration systems for geothermal doublets to remove
particles and dispersed oil.
4.2
Critical factors for filter choice
Filters in the surface installation of the geothermal doublet are aimed to remove the
produced particles like sand, clay particles and scales from the water before it is
injected. Critical factors for the selection of a filter system are:
-
Flow rate
Temperature
Total suspended solids (TSS)
Particle size distribution (psd)
Compatibility of the filter system with total geothermal doublet
Batch and/or continuous flow
Expected life time
Properties of the injection reservoir
Filtration degree
Flow rate
The total flow rate of the water stream determines the required capacity of the filter
system. Typical flow rates in geothermal doublets in The Netherlands are between
3
3
100 m /h and 200 m /h and one or more parallelly operated filter houses with
multiple bag and/or cartridge filters are used to filter the entire stream. Obviously, a
higher flow rate requires a larger filter installation.
Temperature
Bag and cartridge filters have their maximum operating temperature depending on
the materials used. The filters in the geothermal doublet in the Netherlands are
either used before or after the heat exchangers. Before the heat exchangers, the
water has normally a temperature between 60ºC and 90ºC and after the heat
exchangers between 30ºC and 45ºC. The filters in the surface installation must be
compatible in this temperature range.
Total suspended solids
The total suspended solids (TSS) is the dry-weight of particles obtained by
separating particles from a water sample using a filter (laboratory test). It is normally
expressed in mg/L. This parameter together with the flow rate determines the total
solids load on the filters in the geothermal doublet. When the TSS is high, the filter
in the doublet will be block fast. To increase the replacement time of the filters, the
filter capacity can be expanded or a prefilter can be applied.
The life time of the filter can be estimated when the dirt holding capacity of the filter
is known (see paragraph 4.6).
Particle size distribution
The particle size distribution (psd) in fluids is a list of size intervals with a relative
amount (normally mass) of particles present in that size interval.
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The psd is a critical factor for the selection of a filter system. The TSS only
determines the total amount (concentration) of particles, but the psd indicates how
they are distributed over size. For example when 99% of the particles has a size of
10 µm or bigger, it is probably not needed to install an absolute filter with a rating of
5 µm. This information is obtained by doing a psd analysis.
Compatibility with system
The used filters in the surface installation must be compatible with the system. E.g.
materials of the filters, and filter vessels/hardware must be compatible with the pH,
temperature and high salt concentration of the water and with the flow rate.
The filters must not disturb the process in the surface installations in a negative
way, like excessive pressure build up or release of filter material.
Batch and/or continuous flow
In principle the geothermal doublet is operated continuously. However during
maintenance the production could temporary stop. Filters must be able to cope with
start and stop operations. For a continuous filtration like in a geothermal doublet,
the filters are sized for a maximal life time.
In a batch operation, filters are sized based on the volume of the batch. It is aimed
to treat the batch with only one set of filters in a preset time limit.
Expected life time
Different filter systems have various life times. The expected life time of the
geothermal doublet is about 25 year. Therefore, a filter system should not be
designed for a lifetime longer than 25 years. In practice, operators change their filter
system several times during the lifetime of the doublet.
Properties of injection reservoir
The permeability, porosity and pore size distribution of the reservoir at the injection
zone are important parameters for selecting the right type of filter and filter rating
(micron size). This is discussed in Chapter 3. In practice, filter suppliers do not yet
use reservoir properties in their advice for a filter system, because of a lack of data.
Filtration degree
The selection for the type of filters depends on the desired filtration degree. If the
filtered water needs to be 90% free of particles of a size of 50 µm, another type of
filter can be used than in the case that the filtered water must be 100% free of
particles of 5 µm and bigger. Thus, before a filter can be chosen it must be clear
what particles have to be removed and to what extent (goal of the filtration). This
follows from the determination of the TSS, psd and permeability of injection zone
and critical plugging range.
Furthermore, the expected maximum pressure drop over the filter system should be
known for the choice of the required pumping capacity. Viscosity and specific
gravity of the water play a minor role in the filter choice.
4.3
Surface or depth filtration
Cartridge filters (see appendix 9.4) are most often depth-type filters, but sometimes
they are surface-type filters or a combination of both types. Bag filters (see
appendix 9.4) are most often based on surface filtration but they also exist as depth
filters.
Depth-type filters capture particles through the total thickness of the medium,
providing a tortuous (meandering) path with many points for impingement (collision)
of particles.
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In surface filters (usually made of thin materials like papers, woven wire, cloths)
particles are blocked on the surface of the filter and start to form a filter cake (Figure
7). Building up a filter cake without particle invasion in the pores of the membrane
extends the life time of the filter.
Surface filters are advisable if sediment of similar-sized particles are filtered. If all
particles are for example five micron, a pleated 5-micron filter works best because it
has more surface area than other filters. Compared with pleated surface filters,
depth filters have a limited surface area, but they have the advantage of depth.
It can be generally stated that if the size of filter surface is increased, higher flows
are possible, the filter lasts longer, and the dirt holding capacity increases.
(www.lenntech.com).
Figure 7. Principle of surface filtration (left) and depth filtration (right).
4.4
Pressure drop
The pressure drop over a filter depends on the filter medium, the filter housing and
the flow. The pressure drop increases with the life time of the filter. Figure 8 shows
a typical curve of in increasing pressure over a cartridge filter.
The recommended change-out pressure depends on the application, but is normally
around 1.5 bars for cartridge filters. They can be used till 2.5 bar but a pressure
drop above 1.5 bar hardly results in a longer life time and higher pressure drops can
lead to penetration of pollution, lower flow rates, and mechanical burst.
In should be noted that a pump must be selected that can overcome a pressure of
1.5-2.5 bar to ensure that the maximal advisable life time of the filter can be
reached.
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Figure 8. Typical pressure drop of a cartridge filters during its life time (source:
www.vanborselen.nl)
The pressure drop should be below 0.1 bar over the cartridge filter house when new
cartridge filters are installed. The required number of cartridge filters in a process
can be calculated when the total flow rate through the geothermal doublet is known.
Filter suppliers use graphs indicating the relation between the pressure build up and
3
the flow rate through the filter (Figure 9). For example, when the flow is 80 m /h and
the 1 µm filters are used, two 40’’ filters are needed to start with a pressure
difference of 0.1 bar.
Figure 9. Typical example of the relation between the flow through a cartridge filter (length 40 inch
(102 cm) or 60 inch (152 cm)) and the pressure build up (source: Hitma Filters – 3M
Filtration, Full line Catalogue).
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Absolute rating and nominal rating for filters
Filters are rated on their ability to remove particles of a specific size from a fluid, but
the problem is that a variety of very different methods are applied to specify
performance in this way.
Pore size ratings refer to the size of a specific particle or organism retained by the
filter media to a specific degree of efficiency. A filter that is marked '10 micron' has
some capability to capture particle as small as 10 micrometers. However you do not
know exactly what this means unless you also have a description of the test
methods and standards used to determine the filter rating. The two most used
reported media ratings are nominal and absolute micron rating
(www.lenntech.com).
Absolute rating
The absolute rating or cut-off point of a filter refers to the diameter of the largest
spherical glass particle, normally expressed in micrometers (µm), which will pass
through
the
filter
under
laboratory
conditions.
It represents the pore opening size of the filter medium. Filter media with an exact
and consistent pore size or opening thus, theoretically at least, have an exact
absolute rating.
In other words: an absolute pore size rating specifies the pore size at which a
challenge organism or particle of a particular size will be retained with 100%
efficiency under strictly defined test conditions. Among the conditions that must be
specified are: test organism (or particle size), challenge pressure, concentration and
detection method used to identify the contaminant (http://doultonusa.com).
The absolute rating should not be confused with the largest particle passed by a
filter under operating conditions: the absolute rating simply determines the size of
the largest glass bead which will pass through the filter under very low pressure
differentials and nonpulsating conditions.
This does not usually apply in practice: pore size is modified by the form of the filter
element and it is not necessarily consistent with the actual open areas. Furthermore
the actual form of the contaminants are not spherical and the two linear dimension
of the particle can be very much smaller than its nominal one, permitting it to pass
through a very much smaller hole (i.e. cylindrical particles with a thickness less than
the slot opening of the filter, Figure 10).
Figure 10. Cylindrical particles with a length exceeding the cut-off point of a filter can sometimes
pass through the filter.
The passage of oversize particles in this manner depends very largely on the size
and shape of the opening and on the depth over which filtering is provided.
Most of filters generate a filter bed: contaminants collecting on the surface impart a
blocking action decreasing the permeability of the element bad improving filter
efficiency. When the blocking is so severe that the pressure drop is excessive, the
flow rate through the system decrease seriously.
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This explains why the performance of a filter can often exceed its given rating
based on the performance of a clean element and why test figures can differ widely
with different test conditions on identical elements.
It may be argued that the term absolute rating is not a realistic description. Strictly
speaking the term absolute indicates that no particle larger than that rating can pass
through the filter, limiting the type of media to those of consistent pore size where
they show 100% retention of particles.
The beta ratio is often used to the express the efficiency of a filter. It is defined as:
“the influent particle count > X micron” / “the effluent particle count > X micron”
A beta ratio of e.g. 50 (98% efficiency) can be considered as nominal for that
micron size, a beta ratio of e.g. 5000 (99.98% efficiency) is considered as an
absolute rating.
Nominal rating
A nominal pore size rating describes the ability of the filter media to retain a
nominated minimum percentage by weight of solid particles of a specific
contaminant (usually again glass beads) greater than a stated micron size, for
example 90% of 10 micron. The chosen percentage for the stated micron size can
vary largely, from 35% to more than 95% and therefore, nominal filter cannot
always easily be compared.
Process conditions such as operating pressure and concentration of contaminant
have a significant effect on the retention of the filters. Many filter manufacturers use
similar tests but, due to the lack of uniformity and reproducibility of the basic
method, the use of nominal ratings has fallen into disfavor (http://doultonusa.com).
4.6
Dirt holding capacity
The dirt holding capacity of a filter is the quantity of contaminant a filter element can
trap and hold before the maximum allowable back pressure or delta P level is
reached. For bag filters this is about 2 kg of suspended solids (size: 810 mm length
x 430 mm perimeter).
For cartridge filters this is:
0.45 kg for 10 inch cartridge 100 µm retention rating
0.15 kg for 10 inch cartridge 15 µm retention rating
2
0.54 kg/m for 3M cartridge
2
0.48 kg/m for string wound cartridge
2
0.19 kg/m for pleated cartridge (www.lenntech.com)
According to Twin Filter, the average dirt holding capacity is 500-800 grams for a 40
inch filter cartridge and above mentioned values seem quite high. The dirt holding
capacity partly depends on the flow rate. A lower flow velocity per filter element
improves the dirt holding capacity, because a cake layer can be formed on the filter.
Hitma states that surface filtration has a lower capacity than depth filtration. The
capacity is also determined by pore structure, percentage open area, and the
amount of dirt that is rejected before the filter clogs.
4.7
Relation surface area/flow velocity and lifetime of a filter
Decreasing the filter velocity leads to more effective filtration. At lower flow velocity,
the filter has a higher dirt-holding-capacity.
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A geothermal doubled is operated at a constant water flow. To decrease the flow
velocity in the filter installation, more filter surface area can be installed (in case of
surface filtration) or more depth (more depth filters) can be installed.
As a rule of thumb, it can be stated that doubling the filtration area increases the life
time of the filters by a factor up to 4. In practice, this will normally be a factor 3 to 4.
Also for depth filtration, as a rule of thumb, it can be stated that doubling the depth
results in an quadratic life time extension at the most.
Therefore, it can be beneficial to have a higher filtering capacity then strictly
necessary for the water flow. After an investment for extra filter houses and piping
system, the costs can be earned back by using less filter bags and cartridges.
A boundary conditions is that enough space must be available in the building of the
surface installation.
In addition, an oversized filter installation has the advantage that higher flows can
be treated when the capacity of the geothermal doublet is extended in future. The
theoretical background of the relation between the surface area/depth and the
expected lifetime is described in Appendix 9.2.
4.8
Filterability of a liquid
Before installation of filters in a geothermal doublet, the filterability of the produced
water must be determined. The water is passed through a standard membrane to
determine what volume of water can be passed before that membrane plugs from
accumulated solids. Or the rate is determined at which the standard membrane
loses permeability. A popular standard membrane is a cellulose acetate membrane
of about 150 µm thick, with a porosity of 0.8 and rated pore diameter of 0.2 µm or
0.4 µm (Johnston, 1990).
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Conclusions
General
- The main factors that determine the capacity, type and filter rating (micron
size) of the filter system in the surface installation are:
o flow rate,
o temperature of the water,
o concentration total suspended solids (TSS) in the water,
o particle size distribution (psd) of the particles in the water,
o compatibility of the filter system with rest of the geothermal doublet,
o properties of the injection reservoir (permeability, porosity, pore
size distribution).
-
Recommendations for the type of filters and the filter rating in the surface
installation of a geothermal doublet remain case specific, and depend
mainly on:
o the dirt loading of the particles,
o particle size distribution of the particles and
o their composition in relation to the reservoir properties.
Current practice
- Particles analysis (TSS, psd) is often done during the well test and
sometime chemical characterization of the particles. These measurements
in produced water are not representative for particle concentration and size
distribution during stable operation after e.g. half a year or a year.
- Little data are available on TSS and psd of the production water. These
data are essential for a well underpinned filter advice.
- Particle concentration (total suspended solids, TSS), particle size
distribution (psd) and particle composition in the produced water before and
after the filter installation must be measured to determine the removal
efficiency of the filter installation and the quality of the injected water
(regarding particles). The psd of the injected water must be compared with
the calculated critical plugging range.
- The main particles in the produced water in the geothermal doublet in The
Netherlands are sand and clay particles (for Sandstone type reservoirs).
Quartz and feldspar (tectosilicate minerals: K-Na-Ca and an alumina
silicate, KAlSi3O8, NaAlSi3O8, CaAl2Si2O8) are the main components in
these categories. In addition, Fe-Cr-Ni steels can be found, which can
origin from the casing/tubing system of the doublet. Furthermore, iron
hydroxide and calcite (CaCO3) particles can be present in the produced
water.
- Currently 10 µm filtration - nominal bag filtration, sometime followed by 10
µm absolute filtration by cartridge filters - is the standard in The
Netherlands.
Only in one geothermal project experience has been gained with automatic
filtration (25 µm), but this is in combination with pre and post filtration. The
main advantages of automatic self-cleaning filters are: reliability, no down
time for cleaning/continuous water supply, labor saving/low maintenance,
less consumables needed (bag and cartridge filters). As drawbacks can be
mentioned: pre filtration needed, higher investment than for bag or cartridge
filtration, flushing water amount is 0.5-3%, which have to be disposed of.
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For most of the geothermal doublets in The Netherlands, additional oil
removal by oil filters after the oil-gas-water separator is not an issue.
Currently, only in one case additional oil filters are used as a safety barrier.
Particle injection and reservoir properties
- The rate of impairment (damaging) of the injection zone by particles in the
injection water is influenced by the flow velocity at the wellbore. A lower
velocity decreases the risk of clogging by injected particles. This means
that in the winter period, when higher flow are applied than in summer, the
risk of damaging the injection well is higher than in the summer period.
- The critical particle/ pore size ratio for injected particles in the reservoir is
between 1/10 and 1/3. These particles have the highest tendency to block
the pores channels at the injection zone.
- A first estimation of the permeability of the reservoirs/doublets under
consideration in this report is that the critical plugging range is between 1
and 9 µm.
- Currently, the choice for the filter type in the surface installation is not
based on reservoir properties at the injection zone, because of a lack of
reliable data.
- Relevant reservoir properties like particle size distribution of the rock,
porosity and permeability in the reservoir at the injection zones of the
geothermal wells in The Netherlands are currently based on assumptions
and calculations. They must be known in more detail to determine the pore
size distribution at the injection zone more precisely and therewith the
critical plugging range of the particles that are injected.
- No cores are available from the wells of the geothermal doublets in The
Netherlands. A direct determination of the pore size distribution can only be
done with samples from the cores.
- A laser particle analysis from the cutting of the injection well must be done
to determine the grain size distribution of the sediment. Based on the grain
size analysis of the sediment/cutting of the injection zone, the permeability
and pore size distribution can be estimated more precisely than the current
estimations, by using existing relations. With this information the critical
plugging range can be estimated more precisely.
Prevent the risk of plugging of the injection well
- Filtration in the surface installation must focus on the removal of particles
in the critical plugging range. However, bigger particles will automatically be
removed at the same time. To minimize the risk of plugging in the injection
well, the final filter before injection must remove all the remaining particles
in the critical plugging range.
- The investment costs of a filter installation in a geothermal doublet can vary
between < 10 k€ for a clean production well to about 50-80 k€ for a well
that produces high amounts of oil and solids.
- The maintenance costs (labor + material) of the filter installation in the
surface installation are estimated on 100-150 k€ per year in the first year
3
3
after the start-up, based on flow rate between 100 m /h and 200 m /h. After
more than a year of operation and experience with filter usage, the costs
will go down and are estimated on about 50 k€ or even 20 k€ per year. This
drop in filter costs has to do with the lower solids load in the production
water after a year of operation, compared to the start-up period.
- Filtering with a lower flow velocity is more effective. At lower flow velocity,
the filter has a higher dirt-holding-capacity. As a rule of thumb, it can be
stated that doubling the filtration area or depth will increase the life time of
the filters by a factor 3 to 4.
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Therefore, it can be cost saving to have a higher filtering capacity then
strictly necessary for the water flow. Theoretically, operational costs for
filters will decrease by a factor 1.5 to 2 when the capacity of a filter
installation is doubled.
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Recommendations for best practice
Use of filters in the surface installation
- As a final filter (just before injection), it is recommended to install an
absolute filter that removes the particles in the critical plugging range. In
practice this will be and absolute (cartridge) filter with a filter rating between
1 and 10 µm. This filter reduces greatly the risk of plugging in the injection
zone. Such a filter also removes potential corrosion or loose biofilms the
might be formed in dead parts (corners) of the last part of the surface
installation. In addition, mechanical produced nominal filters can sometimes
unexpectedly let through particles they were initially blocking. Therefore an
absolute filter after a nominal (bag) filter works as a safety barrier.
-
Overcapacity of the filter installation is recommended. When e.g. two filter
houses/filter lines have the capacity of the maximal (winter) flow of the
geothermal doublet, it is still recommendable to have a third filter line with
the same capacity as the other two. Two of the three filter lines can be used
alternately. When filters in one filter line are replaced, the complete flow can
be directed through the other two filter lines, maintaining a constant flow
through the filter system. This filter configuration is an improvement
compared to the use of only two filter lines, because in the latter case,
during replacement of filters in one filter line, the flow through the other filter
line is doubled leading to shorter life times.
-
Instead of using a third filter line as spare filter line, it can be chosen to
operate the three filter lines continuously to lower the flow velocity in the
filters and increase the total filtration area. This will increase the filter life, as
discussed in paragraph 4.7. Only during replacement temporary two filter
lines can be used.
-
It can be recommended to increase the capacity of the filter installation.
Doubling the filtration area or the filter depth squares the life time
(maximally) of the filter. Therefore, it is recommended to install a higher
filtering capacity then strictly necessary for the water flow. An initial
investment in extra filter houses and material will be earned back by
reduction in the use of filter bags and cartridges. Using filter bags with
higher surface area is a cheap option to increase the total filtration capacity.
-
Replacement of filters must be done at the over pressure indicated by the
manufacturer. This is normally at about 90% of the life time. To utilise the
last 10% of the filter life time is not recommended, because this leads to a
disproportionate amount of energy (electricity) use. Also an earlier
replacement is not recommended, because this leads to an unnecessary
high amount of consumed filter bags/cartridges. A pump must be selected
that can overcome a pressure drop of 1.5-2.5 bar to ensure that the
maximal advisable life time of the filter can be reached.
-
In a multi stage filter system, the overpressure should be measured over
every filtration step and not only over the total system, to have detailed
information about the pressure build up in a single filtration step. This will
ensure that filters are only replaced when this is really necessary.
Operation of the surface installation
- It is recommended to operate the geothermal doublet at a constant flow
rate during the day. Higher flow rates during night time when electricity
CONFIDENTIAL
43 / 70
costs are lower seem attractive, but can lead to instable water quality and
production of mud and extra dirt particles, which can harm the surface
installation and the injection well or faster block the filters. Fluctuations in
the water flow rate might also result in slight fluctuation of the temperature
of the produced water, which affects the solubility of sparingly soluble salts,
running the risks of precipitation in the surface installation.
-
Increasing the flow rate for the winter period should be done gradually to
minimize the production of solids in the produced water.
-
It is recommended to operate the surface installation at a constant pressure
to reduce the chance of scaling of e.g. CaCO 3 and other sparingly soluble
salts on pipes, pumps, heat exchangers and other equipment. This topic is
further discussed in the other Geothermal Technologiecluster by Wasch
(2013).
-
During replacement of the filters, air intrusion in the system must be
prevented as much as possible. Oxygen intrusion can lead to oxidation and
precipitation of e.g. iron hydroxide and biological growth. At this stage it is
not clear if the replacement leads to significant air intrusion to cause
operational problems. Operators do not indicate that corrosion problems
occur in the filter houses.
Measurements of particles in produced water
- Particle concentration (total suspended solids, TSS), particle size
distribution (psd) and particle composition in the produced water (sample of
1-5 L) before the filter installation must be measured (laser particle
analysis) to be able to optimise the filter choice in the surface installation. It
is recommended to do this once a year. Often the particle characterisation
in the produced water is only done during the well test, but these data are
not representative for operation after e.g. half a year or a year.
CONFIDENTIAL
-
The psd must be measured over wide range to cover all potential particle
size in the produced water. A range between 0.02 µm and 2000 µm is
recommended. Particles between 1 and 10 µm are often critical for
plugging. This size range must be measured precisely.
-
It is also recommended to measure the particle concentration, particle size
distribution and particle composition after the filter installation once a year
at the same time as the measurements before the filter installation. In
combination with the measurements before the filter installation the removal
efficiency of the filter installation can be determined and the quality of the
injected water (regarding particles) is known. The psd of the injected water
must be compared with the calculated critical plugging range.
-
Alternatively, a sludge sample from bag filters can be taken during
replacement for the determination of particle concentration (TSS), particle
size distribution (psd) and particle composition. Drawback of sampling from
the bag filter is that only particles with a bigger size than the mesh size of
the bag filter can be characterized. To determine the TSS the total amount
of sludge must be collected and the total flow through the filter must be
known.
44 / 70
-
The TSS can then be determined as an average during the life time of the
bag filter. When the total amount of sludge in the filter is determined, also
the dirt-holding capacity of the filter is known. Collecting sludge from a
cartridge filters is more difficult and not recommended.
-
Characterisation and chemical analysis of the particles in the
produced/injected water and sediment/cuttings from the injection zone of
the injection well can be done by stereo light microscopy (SLM), Scanning
Electron Microscopy combined with energy dispersive X-ray microanalysis
(SEM / XRMA) and µ-Fourier Transformed Infrared spectroscopy (µ-FTIR).
-
In addition to the measurements of particles (TSS, psd, composition) it is
recommended to monitor the life time of the filters carefully. This gives an
indication of the produced water quality (regarding particles). If a
spontaneous and permanent significant change occurs in the life time of the
filters (e.g. three times in a row a fast clogging), it is recommended to do an
extra particle characterization (TSS, psd, composition) before and after the
filter installation and evaluate the filter system.
Analysis reservoir material
- It is recommended to analyse the cuttings from the reservoir at the injection
zone. A selection must be made of representative cutting from the injection
zone.
CONFIDENTIAL
-
A laser particle analysis and/or sieving analysis is recommended to
determine the grain size distribution of the sediment/cuttings. A laser
particle analyser is able to detect particles down to a diameter of 0.02 µm
and up to a diameter of about 2-3 mm divided in many intervals (similar to
psd analysis of particles in water). A sieving analysis is less advanced.
Particles varying between 2 µm and 250 mm are separated in regular size
class intervals. A laser particle analysis is recommended above a sieving
analysis, because the former can measure in a lower size range, which is
relevant for our purpose. The costs will be about 200 euro per sample. The
analysis can be done if the cutting is a sample of a fragile sediment with
loose grains. When the cutting is a hard rock, the sample must first be
destructed into its grains before the analysis can be done.
-
If the cutting cannot be destructed, an optical analysis by microscope can
be done. For that, a 2 by 2 cm slice or plaquette of a few mm thick is
prepared from mm-sized particles. By measuring the grain sizes and
distances between the grains under the microscope, estimations can be
-
Easier, but less precise is measurement of grain sizes and distances
between the grains from the cutting by using a magnifying glass.
-
When the above recommended measurements are done, more reliable
estimations can be made for the permeability, pore size distribution and
critical plugging range in the injection zone of the reservoir. With these data
a more underpinned recommendation can be given for the final filtration
step before injection.
-
For new geothermal projects, ideally it is recommended to take cores
samples during the drilling phase. Poresize distribution can be determined
from samples of the cores. From the plugs of the cores, porosity and
45 / 70
permeability can be measured (see paragraph 3.4). Costs are however
high. Drilling of whole cores (9 m) costs about € 200.000 to 250.000 per
well. For geothermal installations only the injection well have to be drilled
for cores. Alternatively, sidewall cores can be taken. This can be done with
a wireline. It can be carried out during the regular logging operation. Costs
are about € 50.000 in addition to regular logging costs. Instead of investing
in the analysis of reservoir properties, a more practical solution can be to
install an absolute filter just before the injection well, that removes all the
particles in the critical plugging range.
Mineral oil
- When mineral oil is present in excessive amounts in the produced water
(low percentage range), the first step for the oil treatment is to optimize the
oil gas separator to remove the mineral oil as far as possible. Additional oil
separation technology must be considered, like a settler, hydrocyclone,
chemical injection, before high amounts of oil filters are being installed and
used. This will also improve the particle removal by which less filters are
needed. This falls outside the scope of this report.
CONFIDENTIAL
-
A careful estimation is that concentrations of dispersed oil (droplets of oil in
water) below 1 mg/L do not harm the filter installation in the surface
installation. This is based on observations at a geothermal installation with
oil filter cloths inside the regular filter bags. This would mean that below a
concentration of 1 mg/L dispersed oil, oil filters are not recommended.
However, more research must be done on this critical dispersed oil
concentration.
-
Oil clog and oil block filters have an optimal performance till an oil
concentration up to 50 mg/L. When concentrations of dispersed oil after the
oil-gas-water separator above 1 mg/L are (incidentally) expected it is
recommended to install oil filters as a safety barrier.
-
During test runs and early operation of the geothermal doublet, oil and
grease can be detected in the produced water from the production well, that
originates from the construction of the well (casings) and not from the
reservoir itself. Therefore, it is recommended to repeat mineral oil
measurement after 2 to 3 months of stable production. At that time point,
mineral oil might not or hardly be detected any more.
-
It is recommended to measure the mineral oil concentration before and
after the gas separator (if present) periodically (twice a year) and to visually
inspect the bag filters on remainders of oil.
7
46 / 70
Literature
-
-
-
-
-
-
-
-
-
-
-
-
CONFIDENTIAL
Barkman JH and DH Davidson. Measuring Water Quality and predicting
Well Impairment. Journal of Petroleum Technology, July 1972, pp 865-873.
Broothaers, M. and B Laenen (2012). Execution and interpretation of the
pump tests on geothermal well CAL-GT-01 (S02) Report for the SEI
application. Interim report. VITO, September 2012.
BWG Geochemische Beratung (2012). Geochemical Investigations of water
and solid samples at Honselersdijk GT1 during the hydraulic test on 07 and
08 March 2012
BWG Geochemische Beratung (2012). Geochemical Investigations of water
and solid samples at Honselersdijk GT2 during the hydraulic test on 02 Mai
2012
De Haan, AB and H Bosch (2006). Fundamentals of industrial separations.
Degens GP, MPD Pluymaekers, T Benedictus, F edari Eyvazi, CR Geel
(2012). Productivity/injectivity investigation of geothermal wells –
Aardwarmte Den Haag. TNO 2012 R10268
Degens GP, MHAA Zijp, JP de Boer, ANM Obdam, CR Geel (2012b). BIA
geothermal – TNO umbrella report into the causes and solutions to poor
well performance in Dutch geothermal projects. Appendix 3: Operator
review: Pijnacker Duijvestijn
Degens GP, MHAA Zijp, JP de Boer, A Obdam, F. Jedari Eyvazi, CR Geel
(2012c). BIA geothermal – TNO umbrella report into the causes and
solutions to poor well performance in Dutch geothermal projects. Appendix
2: Operator review: Koekoekspolder
GPC IP (2011). Van den Bosch (VDB). Diagnosis of the geothermal
reservoir/well system of two VDB greenhouse heating doublets. 14 January
2011.
GPC IP (2012). Van den Bosch (VDB). Fluid-rock interactions as function of
reinjection temperature of 13ºC. Mineral reaction processes and resulting
changes of formation properties near the well face.
GPC IP-KWR. Design and implementation of a standard monitoring
protocol adressing the improvement of well injectivitities on selected
geothermal sites. Presented at TNO-Delft at 25 March 2013
Herzig, JP, Leclerc DM and Le goff, P (1970). Flow of suspensions through
porous media. Application to deep filtration. Am. Chem. Soc. Publ. Flow
through porous media, pp. 129-158.
Hybrid Energy Solutions/Ammerlaan (2010). Puttest PNA-GT-01. Okt – Nov
2010.
IF Technology (2012). Proces Ontwerp Specificatie voor Aardwarmte Den
Haag Project Productie fase
IF WEB (2010). Geothermal energy Noord-Holland. Geological study of the
Slochteren Formation in the southern part of the province Noord-Holland.
Johnston, PR (1990). Fundamentals of fluid filtration. Tall Oaks Publishing.
Littleton. Colorado.
Klarenaar, W (2012). Analyse zand- en slibmonsters ADH, SGS INTRON
BV, Juni 2012
Langeveld J and L van Leeuwen (without date). Sandstone quality for
geothermal use. Delft Sandstone, Ammerlaan well. TU Delft.
Ramaekers, J, K Geel, A Lokhorst, HJ Simmelink. Nader onderzoek naar
mogelijkheden van aardwarmtewinning voor de vleestomaatkwekerij van Fa
A&G van den Bosch BV te Bleiswijk, TNO-rapport 2006-U-R0016/B, 26
januari 2006.
Saripalli, KP, PB Gadde, SL Bryant, MM Sharma. Role of fracture face and
formation plugging in injection well fracturing and injectivity decline. SPE
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52731. Presented at the 1999 SPE/EPA exploration and production
environmental conference, Austin, Texas, 28 February – 3 March 1999.
Smit, WH. Millipore filter tests for water injection wells. Interpretation of
Millipore data for the determination of the half life of water injection wells.
Without year.
T&A Survey BV (2012). Well test Honselersdijk GT 2. Documentation of
well test analysis. Technical report.
T&A Survey BV (2012b). Welltest analyse en log interpretatie HON-GT-01
en HON-GT-01-S1.
T&A (2012c). Welltest analyse HON-GT-02, 22 juni 2012 (BJ Vrouwe and
RFX Rutten)
TNO (2012). BIA Geothermal – TNO umbrella report into the causes and
solutions to poor well performance in Dutch geothermal projects. Pijnacker
Ammerlaan
Ungemach, P (2003). Reinjection of cooled geothermal brines in sandstone
reservoirs. Proceedings of the European Geothermal Conference 2003,
Szeged, Hungary, I-2-04 16 pp.
Van Velzen, JFG and Leerlooijer, K. (1992). Impairment of a water injection
well by suspended solids. Testing and prediction. SPE paper 23822
presented at the SPE Int. symp. on formation damage control. Feb. 26-27.
Lafayette. La. USA. pp 148-165.
Vernoux, JF et al. (1997). Improvement of the Injectivity Index of
Argillaceous Sandstone: Final Report; Research Funded in Part by the
Commission of the European Communities in the Framework of the JOULE
[II] Programme, Sub- programme Non Nuclear Energy.
Wasch, LJ, Geothermal energy – Scaling potential with cooling and CO2,
TNO report TNO 2013 R11661
www.hpwellscreen.com, November 2013
www.lenntech.com, November 2013
www.tradekorea.com, November 2013
www.twinfilter.com, November 2013
www.vanborselen.nl, November 2013
TNO repod I TNO 2013 R11739 |
I
I
CONFIDENTIAL
Authentication
Name and address of the prineipal
TNO Programma MKB Kennisoverdracht met inzet SMO in samenwerking met
Platform Geothermie en leden
Date upon which, or period in which the research took plaee
February 2013
-
November 201 3
Name and signature reviewer:
René Jurgens
Monique Oldenburg
Research Manager
49 / 70
9
Appendices
9.1
Appendix 1: Estimating permeability based on grain size
Krumbein and Monk's equation
[1]
Using experimental procedures that were later adopted by Beard and Weyl,
[2]
Krumbein and Monk measured permeability in sandpacks of constant 40%
porosity for specified size and sorting ranges. Analysis of their data, coupled with
dimensional analysis of the definition of permeability, led to
(1)
where:



k is given in darcies
dg is the geometric mean grain diameter (in mm)
σD is the standard deviation of grain diameter in phi units, where phi= log2(d) and d is expressed in millimeters
Although the Krumbein and Monk equation is based on sandpacks of 40% porosity
and does not include porosity as a parameter, Beard and Weyl showed that Eq. 1
fits their own data fairly well even though porosity of the Beard and Weyl samples
ranges from 23% to 43%.
Berg’s model
An interesting model linking petrological variables—grain size, shape, and sorting—
[4]
to permeability is that of Berg. Berg considers "rectilinear pores," defined as those
pores that penetrate the solid without change in shape or direction, in various
packings of spheres. Simple expressions for k are derived from each packing,
which form a linear trend when log(k) is plotted against log(Φ). From these
geometrical considerations comes an expression relating k to Φ raised to a power
and to the square of the grain diameter,
(2a)
where:
k is given in darciesd (in mm) is the median grain diameterΦ is porosity in percent
p, a sorting term.
If permeability is expressed in millidarcies, d in micrometers, and Φ as fractional
porosity, this expression becomes
(2b)
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To account for a range of grain sizes, Berg considered two mixtures of spheres and
assumed that k will be controlled primarily by the smaller grains. This introduces a
sorting term p=P90-P10, called the percentile deviation, to account for the spread in
grain size. The p term is expressed in phi units, where phi=-log2d (in mm). For a
sample with a median grain diameter of 0.177 mm, a value of 1.0 for p implies that
10% of the grains are >0.25 mm and 10% are <0.125 mm.
Berg’s expression (Eq. 2b) is illustrated in Fig. 2 for p=1 and varying d.
Permeability increases rapidly with increasing porosity, depending on Φ to the fifth
power, and the curves migrate downward and to the right with decreasing grain
[5]
size. Nelson finds that Fig. 2 is remarkably concordant with several published
data sets. Berg’s model appears to be a usable means of estimating permeability in
unconsolidated sands and in relatively clean consolidated quartzose rocks. This is
true even though Berg did not expect his model to be applicable for porosity values
<30%.
Van Baaren’s model
[6]
Proceeding along more empirical lines, Van Baaren obtains a result nearly
identical to that of Berg. Van Baaren begins with Kozeny-Carman's equation where
surface area is based on the ratio of pore surface area to rock volume and makes a
[5]
series of substitutions (see summary by Nelson ) that result in
(3a)
where dd (in μm) is the dominant grain size from petrological observation, m is the
cementation exponent, and C is a sorting index that ranges from 0.7 for very well
sorted to 1.0 for poorly sorted sandstones. Consequently, Eq. 3a can be used to
estimate k from petrological observations on dominant grain diameter dd and
degree of sorting, along with a porosity estimate obtained from either core or logs.
Assuming that the dominant grain size dd is equivalent to Berg’s median grain
diameter d, then Eq. 3a is very similar in form to Eq. 2a. For example, a sorting
parameter p=1 in Berg’s Eq. 2b results in
(2c)
where k is given in millidarcies, whereas for a well-sorted sandstone, C=0.84 and
Eq. 3a becomes
(3b)
Van Baaren’s Eq. 3b is so close to Berg’s Eq. 2c that a separate log(k)-Φ plot is not
warranted here. Van Baaren’s expression is probably easier to use because the
parameters are directly related to practical petrological variables. Both models
display a porosity exponent > 5, and both are compatible with the data of Beard and
Weyl on unconsolidated sands in that k increases with the square of grain size.
Source:http://petrowiki.spe.org/Estimating_permeability_based_on_grain_size
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Cementation exponent
The cementation exponent models how much the pore network increases the
resistivity, as the rock itself is assumed to be non-conductive. If the pore network
were to be modelled as a set of parallel capillary tubes, a cross-section area
average of the rock's resistivity would yield porosity dependence equivalent to a
cementation exponent of 1. However, the tortuosity of the rock increases this to a
higher number than 1. This relates the cementation exponent to the permeability of
the rock, increasing permeability decreases the cementation exponent. The
exponent m has been observed near 1.3 for unconsolidated sands, and is believed
to increase with cementation. Common values for this cementation exponent for
consolidated sandstones are 1.8-2.0. In carbonate rocks, the cementation exponent
shows higher variance due to strong diagenetic affinity and complex pore
structures. Values between 1.7 and 4.1 have been observed. The cementation
exponent is usually assumed not to be dependent on temperature.
Source: http://en.wikipedia.org/wiki/Archie's_law
CONFIDENTIAL
9.2
52 / 70
Appendix 2: Relation surface area/flow velocity and lifetime of a filter
The relationship between the approach velocity of the liquid to a filter and pressure
drop over the depth of the filter is the following:
ΔP/z = αηu +βρu
2
2
ΔP = difference in pressure over filter (N/m )
z = thickness of the medium (m)
-2
α = viscous term coefficient (m )
2
η = absolute viscosity of the liquid (Ns/m )
u = approach velocity (m/s)
-1
β = inertia term coefficient (m )
3
ρ = density of the liquid (kg/m )
At low velocity and low pressure drop the left part of the equation (αηu) is
determining the ΔP/z. At high velocity, pressure drop is proportional to the square of
the velocity (Johnston, 1990).
Values of α and β can be determined from a filter test (pressure build up at
increasing approach velocity). α and β can be determined from a straight line plot,
on linear coordinates, of ΔP/zu versus u as:
ΔP/zu = αη +βρu
where the intercept is αη and the slope is βρu.
Constant rate filtration is encountered when a positive displacement pump feeds a
pressure filter. Due to the increasing cake resistance the pressure delivered by the
pump must increase during the filtration process to maintain a constant filtration
rate. For incompressible cake layer formation on a surface filter the following
equation applies:
 R V 
 cV 
P  
V  M 
2 
 2A t 
 At 
(1)
2
A = surface area (m )
3
c = mass of dry solids per unit volume suspension (kg/m )
α = constant (m/kg)
3
V = volume (m )
t = time (s)
-1
RM = resistence of filter medium (m )
Values for specific cake and filter medium resistance can be determined
graphically. It can be seen from equation 1 that the ΔP decreases up to
quadratically with an increase in filter area, A (De Haan and Bosch, 2006).
Johnston (1990) distinguishes five types of blocking by a filter medium (Figure 11).
When complete cake filtration occurs, the filter medium does not plug. The
increasing resistance to the constant water flow through the geothermal doublet is
only the result of increasing thickness of the cake of the dirt (e.g. sand) building up
on the surface of the filter. Curve A is valid when the cake does not compress, to
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become less permeable, with increased driving pressure in the case of constant
flow rate.
The other curves in Figure 11 represent expressions derived for situations where
relatively small amounts of collected solids do either block the surface of the filter
(surface filtration) or enter in depth in the filter (depth filtration) to plug the pores.
Figure 11 shows the standardized rate of plugging (pressure increase) in constant
flow rate filtration. The pressure drop across the filter (ΔP) rises with increased
volume of water filtered (V).
Figure 11. Standardized rate of plugging (pressure increase) in constant flow rate filtration. The
pressure drop across the filter (ΔP) rises with increased volume of water filtered (V).
A) ΔP = 1 + V cake filtration. Slope reaches 1.0
B1) ΔP = exp (V) intermediate filtration
2
B2) ΔP = exp (V ) intermediate filtration
-2
C) ΔP = (1-V) standard blocking
-1
D) ΔP = (1-V) complete blocking
Cake filtration is the most ideal blocking type (curve A). Cake formation will be more
predominant when the flow velocity is lowered. This can be done by scaling up the
filter installation (more filters, more filter area).
Lowering the flow velocity can lead to:
a more preferable blocking mechanism (cake filtration)
more than proportional slower pressure build up (nonlinear relationships)
Filter suppliers use a rule of thumb that doubling the filtration depth (depth filtration)
squares the life time.
Doubling the surface of a filter installation (surface filtration) based on the same
pore sizes, increase the life time of the filters by a factor 3. When a double cake
layer is formed, this can increase to a factor 4.
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9.3
54 / 70
Appendix 3: Beschrijving diverse microscopisch analysetechnieken
Stereomicroscopie
M.b.v. stereo (licht)microscopie (SLM) kan het oppervlak van materialen bij lage
(6x) tot hogere vergroting (max. 60x) in beeld worden gebracht, waarbij
kleurenfoto’s kunnen worden gemaakt. Deze techniek wordt vaak gebruikt als
voorbereiding op nader analytisch/morfologisch onderzoek van het oppervlak van
materialen met Scanning Elektronenmicroscopisch onderzoek.
Scanning elektronenmicroscopie (SEM) in combinatie met röntgen microanalyse
(XRMA)
SEM/XRMA biedt de mogelijkheid het oppervlak van vaste materialen te
bestuderen en in beeld te brengen, waarbij tegelijkertijd (lokale) elementanalyses
kunnen worden uitgevoerd. De onderste bepalingsgrens bedraagt voor de meeste
elementen (natrium en hoger atoomnummer) circa 0,1 gewichtsprocent in het
analysevolume. Met SEM/XRMA is het mogelijk standaardloze, semikwantitatieve
elementanalyses uit te voeren. Hierbij wordt de som van de elementoxides op 100
% gesteld en worden aan de hand van de netto röntgenintensiteiten de
percentages van deze elementoxiden bepaald. De resultaten zijn derhalve niet
absoluut, doch relatief ten opzichte van elkaar. Het element koolstof kan (nog) niet
semi-kwantitatief kan worden bepaald. Bij de analyses wordt dit element dan ook
meestal niet meegenomen, maar indien nodig, apart bepaald.
Details van mogelijke technieken van SEM/XRMA (beeldvorming en analyse)
Met behulp van SEM/XRMA kan het oppervlak van een vast materiaal op een
aantal wijzen in beeld worden gebracht, t.w.met behulp van zogenaamde
secundaire elektronen (SEI) (morfologie), teruggekaatste primaire elektronen (BEI)
(onderscheid "lichte" en "zware" elementen) en met behulp van zogenaamde
elementverdelingsbeelden (de positie van de elementen in beeld).
Secundair elektronenbeeld (SEI)
Met behulp van de secundaire elektronen kan het uiterste oppervlak ( < 0,2um diep)
van een vast materiaal in beeld gebracht worden en fotografisch vastgelegd. De
vergroting varieert hierbij tussen (10x) a> 10.000x.
Backscattered elektronenbeeld (BEI)
Met behulp van de teruggekaatste (backscattered) primaire elektronen kan
informatie verkregen worden over de (gemiddeld) chemische samenstelling van het
te onderzoeken materiaal, gebaseerd op het gemiddeld atoomnummer. Delen met
een relatief laag gemiddeld atoomnummer (b.v. koolstof) worden hierbij donkerder
afgebeeld dan delen met een relatief hoog gemiddeld atoomnummer (b.v.
ijzerdeeltjes). Omdat de informatie die met deze techniek verkregen wordt ook uit
dieper (0,1 - 0,5 μm) gelegen delen van het materiaal is de resolutie beduidend
minder.
Elementanalyses en elementverdelingsbeelden (X-ray-map)
Met behulp van SEM/XRMA kunnen elementanalyses worden uitgevoerd. Bij het
bombarderen van het oppervlak van het monster met de primaire elektronen komt
namelijk naast de secundaire elektronen ook röntgenstraling (X-ray's) vrij. Elk
element heeft hierbij zijn eigen specifieke energie of golflengte. Alle röntgenstraling
wordt door een zogenaamde energie- dispersieve röntgendetector gedetecteerd en
in aparte energiegebiedjes vastgelegd. Door een bepaald energiegebiedje te
selecteren en de informatie ervan apart weer te geven in een beeld kan op deze
wijze per element een beeld verkregen worden, waarbij in dat beeld de posities
waar dit element aanwezig is (> 0,1%) als heldere puntjes zichtbaar worden.
Tevens is de intensiteit van deze puntjes een maat voor de relatieve concentratie
van dat element in dat beeld.
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Door dit tegelijkertijd voor meerdere elementen afzonderlijk te doen kan een
totaalbeeld verkregen worden waar bepaalde elementen zich bevinden. Op deze
wijze zijn ook combinaties van elementen (= verbindingen) weer te geven.
Dergelijke beelden worden elementsverdelingsbeelden genoemd. Deze beelden
kunnen tot op zekere hoogte bij diverse vergrotingen worden gemaakt, zij het dat bij
zeer grillig gevormde materialen bepaalde delen (gaten, "ravijnen") afgeschermd
worden.
μ-FTIR analyse
Terwijl SEM/XRMA op lokaal niveau de elementsamenstelling kan vaststellen kan
FTIR analyse (Fourier Transform Infrarood Spectrometrie) indien gekoppeld met
een lichtmicroscoop het type verbinding/component van kleine deeltjes vaststellen.
Normaal gesproken moeten deze deeltjes minimaal 50 μm of groter zijn en apart
analyseerbaar of isoleerbaar zijn. Bovengenoemde technieken zijn aldus
complementair t.o.v. elkaar.
(Bron: afdeling Apllied Enviromental Chemistry, TNO)
CONFIDENTIAL
9.4
56 / 70
Appendix 4: Overview commercially available filter systems for geothermal
doublets
In this Appendix, the commonly used filters for geothermal doublet are discussed.
These are:
- Bag filters
- Cartridge filters and high flow cartridge filters
- Oil removing filters
- Automatic filters
- Well screens (downhole)
In addition, shortly the role of the settler and the hydro cyclone will be discussed.
9.4.1
Bag filters
A bag filter works by the principle of microfiltration. The liquid is cleaned in bags
(Figure 12) by passing small permeable pores. Filtration occurs on the inner face of
the bag by impingement, inertial impact and diffusion. Thus all contaminants are
collected in the bag, simplifying disposal of the bag, and contaminants, on changeout. The liquid flows from the top of the filter house (manufactured in either stainless
or epoxy coated carbon steel) and is distributed equally amongst the bags. The
liquid comes out at the bottom leaving the solids behind. Since the bag is locked at
the top of the vessel the solids are trapped inside the bag. During replacement of
the filters, the vessel must be drained to remove particles from the vessel. Usually
the filter bags can be replaced manually.
The sizes of the pores are between 1-1000 µm. The capacity depends on the
2
surface area of the bags, typically 0.50 m . A typical maximum flow rate for a single
3
3
bag filter is 50 m /h. The capacity of L2 filter bags is 10-15 m /h per bag. Big
3
systems can treat flows exceeding 500 m /h (multi bag filters).
Bag filters can be made of polyester, polypropylene, nylon, NMO (nylon
monofilament), PMO (polypropylene monofilament), PEM (polyester multifilament)
or even other materials. Maximum operational temperatures are in the range of 95
ºC to 135 ºC, depending on the material.
The filtration method of a bag filter is normally surface filtration (source:
www.lenntech.com; product information Twin Filter).
Bag filters normally have a nominal pore size rating, but filter bags with absolute
pore size rating (see paragraph 4.5) also exist.
Figure 12. Some examples of filter bags.
CONFIDENTIAL
9.4.2
57 / 70
Cartridge filtration
Cartridge filtration units generally operate most effectively and economically on
applications having contamination levels of between 0 and 100 ppm solids. For
heavier contamination, a bag filter can be used first, followed by a cartridge filter as
polishing step.
3
3
The flow rate through a cartridge filter is typically maximal 5 m /h and 2 m /h is
recommended (Table 5). Therefore, when bag and cartridge filters are used in
series, about ten times more cartridge filters are needed than bag filters.
Table 5. Properties of absolute rated filter cartridges (source: Twin Filter).
Property
3
Maximum flow rate per cartridge (m /h)
3
Recommended flow rate per cartridge (m /h)
2
Filtration area (m per 10 inch (24 cm) cartridge)
Max. differential pressure (bar at 25 ºC)
Advised change out differential pressure (bar)
Max. working temperature (ºC)
Value
5
2
0.25-0.75
5.5
2.5
80
Basic cartridge filter systems are: wound cartridge, melt-blown cartridge, stainless
steel cartridge filters, pleated cartridge filters. These filters can be used for the
removal of sand, scales, lime, rust, and other fine particles. The pore size can be
chosen between about 0.1 and 250 µm. Stainless steel cartridges are not used in
geothermal doublets. They are considered too expensive. Cartridge filters are
normally designed disposable. They have to be replaced when the filter is clogged.
Twin Filter gives the following cartridge filter description:
- Wound cartridge (nominal) 1-200 µm
- Spunbonded cartridge (nominal) 0.5-50 µm
- Pleated cartridge (absolute) 0.5-25 µm
- Magnum 0.5-25 µm
- Oil absorption cartridge
- Oil block and oilclog
Wound cartridge (nominal rated)
Wound cartridge filters (method: depth filtration) are available in separation sizes of
varying from 1-200 µm or even with bigger or smaller separation sizes (Figure 13).
Several materials are used: polypropylene, polyethene, cotton, glass fibre, nylon,
ryton (polyphenylene sulfide).
The maximum allowable pressure drop over the wound cartridge filter is 5.6 bar. It
is advised to replace the filters at a pressure drop of 2.5-3 bar
(www.vanborselen.nl).
A more tightly woven wound cartridge filter and/or a thinner wire leads to a finer
filtration.
CONFIDENTIAL
58 / 70
Figure 13. Wound cartridge filter (www.vanborselen.nl, l; www.twinfilter.com, r).
Meltblown, spun bonded, cartridge
Spun bonded cartridge filters (method: depth filtration) are thermally bonded micro
fibres. They are available in separation sizes of varying from about 0.5-50 µm
(Figure 14). Several materials are used: polypropylene, nylon and ryton
(polyphenylene sulfide). These kind of filters have a very accurate separation size.
Figure 14. Meltblown (spun bonded) cartridge filter ( www.vanborselen.nl).
Pleated cartridge (absolute rated)
Pleated cartridge filters (method: surface filtration) are available in separation sizes
of varying from about 0.5-25 µm (Figure 15). Several materials are used:
polypropylene, glass fibre, cellulose, polyester.
Figure 15. Pleated cartridge filters (www.tradekorea.com; (l) www.twinfilter.com (r)).
CONFIDENTIAL
59 / 70
The discussed types of cartridge filters are all feasible for the application in the
surface installation of the geothermal doublet. No standardization has been made
on the type of cartridge filters for the geothermal doublet.
In a commercially available combi unit that can consist of two vessels of bag filters,
two vessels of cartridge filters or a combination, different types of cartridge or bag
filters can be easily changed out when necessary.
9.4.3
High flow filter system
High flow filters can be used as an alternative for conventional cartridge filters. They
deliver a high flow in a compact housing design. A 3M High Flow filter delivers a
3
flow rates of up to 113 m /h (outside-to-in flow path) in a single 60 inch (152 cm)
cartridge. The result is a compact filter system (Figure 16). The high flow in one
filter reduces the filter usage, saves labor and disposal costs, and downtime for filter
change-out.
The elements use a pleat design that results in a high usable filtering surface area
per filter. Each grade of the 3M High Flow filter system is manufactured from
meltblown polypropylene microfibre media, providing high particle removal
efficiency (absolute rated) with broad chemical compatibility.
The length of the filters is either 40 inch (102 cm) or 60 inch (152 cm). The
maximum operating temperature is 71ºC. Removal ratings are available between 1
µm and 70 µm (source: product information Hitma).
Figure 16. 3M High Flow filter housing (source: product information Hitma).
Twin Filter offers the TF HFC high surface area cartridges. They utilize pleated
depth media with high efficiency and high flow capabilities. Two options are offered
as standard, the polypropylene and glass microfibre (Table 6).
CONFIDENTIAL
60 / 70
Table 6. Properties of TF high flow cartridges (source: Twin Filter).
Property
3
Maximum flow rate per cartridge (m /h)
3
Recommended flow rate per cartridge (m /h)
2
Filtration area (m per 10 inch (24 cm) cartridge)
Max. differential pressure (bar at 20 ºC)
Advised change out differential pressure (bar)
Value
1.75 - 2.25
4.0
1.5
140
The advantage of high flow cartridge filters is a low footprint for the filter installation.
Two or three high flow cartridges are needed where for conventional filter cartridges
tens of filters need to be used.
9.4.4
Oil removing cartridge filters
If oil is present in the produced hot water, a well designed oil-gas separator
removes the majority of the mineral oil before the water enters the surface
installations and the filters. In most geothermal doublets additional oil removing
filters are not needed. In case oil is produced, oil filters can be used as a safety
barrier.
Oil block
Oil block cartridge filters exist of polypropylene caps and the outer shell is filled with
oil block absorption media. They can be used for the removal of free, dispersed and
emulsified oils (www.lenntech.com). Oil droplets react irreversibly with the
polypropylene polymer in the filter forming a gel. The filter blocks when it gets
saturated and when the pressure limit is exceeded (low flow) at about 2.5 bar, the
filter must be replaced. The filter is non regenerable (source: Twin Filter). The
standard oil block cartridges absorb about 2 Liters of oil (0.5 L per kg of filter
medium, Table 7).
Table 7. Properties of oil block cartridge (source: Twin Filter).
Property
Removal efficiency free, dispersed and emulsified oil (%)
3
Max. recommended flow rate per cartridge (m /h)
Required pre-filtration (µm)
Absorption capacity/cartridge
pH range
Cartridge length (cm)
Value
99
0.5
25
60
2 kg hydrocarbon
1-9
102
The oil block needs a pre-filtration and should be used after the heat exchanger as
the maximum working temperature is 60ºC (Table 7).
Oilclog
Oilclog absorption cartridges from Twin Filter (www.twinfilter.com) are used for the
removal of free and dispersed hydrocarbons, emulsified and dissolved oil from
water (Table 8). The oilclog absorption cartridge is filled with granulated organic
material, impregnated with a powerful surfactant.
CONFIDENTIAL
61 / 70
This element absorbs the hydrocarbons very quickly and bonds with the oil. The
water flows from top to bottom through the cartridge and the absorption media
immediately reacts with the hydrocarbons. When this clog cartridge is saturated, the
oil is let through.
Table 8. Properties of oil clog cartridge (source: Twin Filter).
Property
Removal efficiency free, dispersed and emulsified oil (%)
3
Max. recommended flow rate per cartridge (m /h)
Required pre-filtration (µm)
pH range
Cartridge length (cm)
Value
99
0.2
25
60
1-9
102
These oil block and oil clog filters remove dissolved oil from the water phase from
concentrations up to 50 ppm to 10 ppm or lower. If the oil content is higher than 50
ppm a pretreatment must be applied. Otherwise filters must be replaced too often
and removal efficiency will be low.
An alternative for oil cartridge filters is the use of (polypropylene) oil bag filters.
They have a capacity of 2.8 L oil/bag and can be placed inside the regular (solid
removing) bag filters to protect the filter system from oil contamination and clogging.
9.4.5
Filter vessel and capacity
Filter houses can contain multiple bag filters (e.g. 4) or cartridge filters (e.g. 50) to
create a multiple capacity of a single filter. The total capacity can be increased by
using more filter houses parallelly. Figure 17 give an example of a vessel containing
40 cartridge filters.
Figure 17. Single vessels for cartridge or bag filters (source: www.twinfilter.com)
CONFIDENTIAL
9.4.6
62 / 70
Automatic filter
Automatic filters can be provided with separation sizes of 10-3000 µm. For a
geothermal doublets often a separation size of 10 µm, 25 µm or 50 µm is advised.
Water flows from inside to the outside of a cylindrical screen and the particles are
retained inside forming a cake layer and building up a pressure difference over the
screen. The screen material is SMO254 or Super Duplex RVS. When the limit of the
differential pressure over the screen is reached (at about 0.3 bar) the self-cleaning
process (suction mechanism) is automatically started by:
- signal from the Pressure Differential Switch
- signal from a timer
During automatic self-cleaning cycle (about 25 seconds) there is no interruption of
the outlet flow. The amount of flushing water is 0.5% to 1%, with peaks of 3%. At a
3
3
flow of 200 m /h and 1% of flushing water this is 2 m /h. The amount of flushing
water depends on the dirt load on the filter. This water can be stored in a buffer tank
to wait for disposal. The back flush volume depends on the size of the filter surface
and is on average 130 L per flush (range of 80-175 L). The life time of an automatic
filter is about 3 to 5 years.
After the automatic filter a cartridge filter (2-5 µm) can be used for polishing.
Advantages of automatic self-cleaning filters are:
Reliability
Ability to clean in extreme conditions
No down time for cleaning
Continuous water supply
Low space
Labor saving/low maintenance
Less consumables needed (bag and cartridge filters)
Potentially lower costs
As drawbacks can be mentioned:
The investment for an automatic filter is higher than for bag or cartridge
filtration. These costs need to be earned back by lower consumable use
and lower maintenance costs during the operation.
Flushing water amounts of 0.5-3% which have to be disposed of.
Normally in combination with other filters.
CONFIDENTIAL
63 / 70
Figure 18. TwinOmatic, a self-cleaning filter (Source: www.twinfilter.com)
An alternative automatic filtration system is the Filtomat from Amiad (Figure 19). It
3
treats water flows up to 320 m /hour and has a filtration degree of 2-20 µm. The
minimal operating pressure is 1.2 bar. The amount of cleaning water is claimed to
be less than 1% of the total flow. The temperature working range is 4-50ºC. This
means this automatic filter can only be used after the heat exchanger of a
geothermal doublet.
The filters remove particles as water flows through multi-layered microfiber
cassettes. Dirt particles that accumulate on and in-between the microfiber layers
create a pressure differential. At a preset pressure differential value or time interval,
the control unit activates the self-cleaning cycle.
Cleaning is carried out by pressurized water. Both sides of a cassette are sprayed
with high powered jet streams that penetrate the microfiber layers and dislodge the
debris.
After cleaning all rows of cassettes, the filter is clean. The drain valve closes and
the inlet valve re-opens, filling the filter vessel. After the vessel is full, a “filter to
waste” valve opens. This eliminates any residual contaminant that may have
entered the collector pipes during the flush process. Then, the “filter to waste” valve
closes, the outlet valve opens and the filter is back on-line.
CONFIDENTIAL
64 / 70
Figure 19. Filtomat AMF self-cleaning filter system by Amiad (source: product information Hitma).
For higher temperatures, up to 110 ºC, the SAF 4500 en SAF 6000 self-cleaning
filter from Amid Water Systems can be used (Figure 20, Table 9). Filtration degrees
are possible between 10 en 800 µm (four-layer or molded weave wire stainless
steel screen). Working pressure is between 2 and 10 bars. It is claimed that the
cleaning water is less than 1% of the total flow.
The filter is flushed at a differential pressure of 0.5 bar, or at fixed intervals and
manual operation.
Raw water enters the filter inlet through a coarse screen which protects the cleaning
mechanism from large debris. The water passes through a fine screen, trapping dirt
particles which accumulate inside the filter. Clean water flows through the filter
outlet. The gradual dirt buildup on the inner screen surface causes a filter cake to
develop, with a corresponding increase in the pressure differential across the
screen. A pressure differential switch senses the increased pressure differential and
when it reaches a pre-set value, the cleaning process begins.
Cleaning of the filter is carried out by the suction scanner which spirals across the
screen; the open exhaust valve creates a high velocity suction stream at the
nozzles tip which “vacuums” the filter cake from the screen. During the self-cleaning
process filtered water continues to flow downstream.
Table 9. Specifications SAF 4500 and 6000 automatic self-cleaning filters (source: product
information Hitma).
3
Max flow rate (m /h)*
3
Min. flow for flushing (m /h)
Flushing time cycle (s)
SAF 4500
250
15
20
SAF 6000
400
25
40
* based on hydraulic capacity with clean water, max. flow rate also depends on mesh size of
the filter.
CONFIDENTIAL
65 / 70
Figure 20. SAF 4500 automatic self-cleaning filter (source: Production information Hitma).
9.4.7
Hydrocyclone and settling tank
A hydrocyclone and a settling tank are pretreatment options for the removal of
solids before they enter the surface installation and the filter installation. They fall
outside the scope of the report and are there only shortly mentioned.
A hydrocyclone is a device to classify, separate or sort particles in a liquid
suspension based on the ratio of their centripetal force to fluid resistance. This ratio
is high for dense (where separation by density is required) and coarse (where
separation by size is required) particles, and low for light and fine particles.
A hydrocyclone will normally have a cylindrical section at the top where liquid is
being fed tangentially, and a conical base (Figure 21). The angle, and hence length
of the conical section, plays a role in determining operating characteristics.
In the project of Aardwarmte DenHaag, hydrocyclones were used during the well
test, when high amount of particles were produced.
Figure 21. Typical example of a hydrocyclone.
CONFIDENTIAL
66 / 70
Alternatively, a settling tank can be used for the primary removal of solids. When an
advanced oil-gas-water separator is used with several compartments and sufficient
retention time, this also serves as a settler.
In practice, course screens (e.g. 200 µm, 300 µm) are sometimes used before the
surface installation instead of a hydrocyclone or a settler.
9.4.8
Well screens
In most of the wells of the geothermal doublets in The Netherlands, wire wrapped
filter screens of HP Well Screen (Figure 22) are installed at reservoir depth, with a
separation size of 300 µm to prevent the production of course sand and particles in
the produced water. Slot opening of 50 µm to 2000 µm are available.
The screen jacket is fully pickled and passivated for maximum corrosion resistance
(source:www.hpwellscreen.com).
CONFIDENTIAL
67 / 70
Figure 22. Example of a wire wrapped filter screen of HP Well Screen (www.hpwellscreen.com).
CONFIDENTIAL
9.5
68 / 70
Appendix 5: Summarizing table of nine geothermal doublets (data mid 2013)
Geothermal
Formation
Doublet
type
at
injection zone
Average
Flow
estimated
(m3/hour)
rate
permeability
of
target
formation
T
Particles
Down-
Set-up
Set-up
Criteria
Estimated
in produced
hole
surface
filter
replaceme
annual
water :
screens
instal-
installation
nt filters
costs
in prod.
lation
pr/inj
(ºC)
Distribution
(mD)
(k€, filters
and inj.
+ labor)
well
Thermal
Replace-
TSS (mg/L)
ment time
capacity –
max. (MW)
Estimated
(days)
Composition
critical
Operating
Procedure
plugging
pressure
replace-
range (µm)
doublet
ment
(bar)
A
Delft
143
120-210
Sandstone
86 – 35
Member and
0.45 µm –
OG
2
154 µm
300 µm
BF
filter lines:
D50 = 28 µm
CF
partly
Alblasserdam
11
Sandstone
TSS
10.6
total
for
surface
IP
BF, 10µm
BF:
(N)
days
(May 2012)
installation
4-5
Not known
CF:
1 vessel: 2
quartz 60 %
hf CF, 10
feldspar 20-
µm (A)
2
months
yet
for
filters,
because
of varying
30 %
1.7-4
200
BF and CF
1 vessel: 6
after
degasser
1 bar over
HEX
mg/L
3-3.5
parallel
During
Till
14% Fe-Cr-
2013: HC
ment
Ni steels 4 %
filters
in
filter
iron
BF
to
used, total
hydroxide
remove
flow
5.5 %
mineral oil
reduced
OG
2
After
Not known
BF
filter lines
pressure
yet
HEX
containing
limit
IP
BF.
reached
(May
July
life times.
CaCO3 6.5-
replace1
line
50%
2012
well test)
B
Rijswijk
350
150
Sandstone,
(injection
(intended)
Delft
well)
and Pijnacker
76 – 40
Sandstone,
Alblasserdam
300 µm
2.7-6.2
parallel
25
µm, 5 µm
6.3
-
Sandstone
and 2 µm
-
tested
5
filters
(after
bypassed,
degassing)
Calcite
or
Quarts, clay
stream
water
Iron
through
hydroxide
one
filter
house, or
installation
stopped
CONFIDENTIAL
C
Slochteren
80 – 140
250
69 / 70
-
300 µm
Sandstone
74-35
2.2 – 8.2
5.5
OG
2
BF
HEX
0.3-0.4 bar
125
filter lines
(before
total
containing
HEX) 0.4-
surface
BF
4 BF 10
0.5
installation
IP
µm before
(after
and
HEX)
2-3
parallel
after
bar
for
Not known
HEX
yet
6-8 weeks
total
for
filters
flow
through
one
D
Rijswijk
and
400-500
160
Delft
D50 = 188
300 µm
µm
Sandstone
filter
line
77-30/40
2.9 – 7.4
OG
3
0.8
bar
BF
filter lines
over
filter
HEX
containing
house
IP
4 BF 10
7
parallel
µm
15-20
for
filter
installation
6-7 days
4-6
2 of the 3
filter lines
Calcium
are
carbonate,
alternately
iron
used
oxide,
halite (NaCl),
pyrite (FeS2)
E
Rijswijk
and
150 – 730
Delft
(production
Sandstone
well)
90
-
300 µm
72 – 35/45
8.6 (at 200
-
m3/h)
1.7 – 9.0
?
OG
Course
aut. F 0.3
course
filter
bar
filter
µm,
(flushing)
HEX
HEX,
CF 2 µm
Aut. F
2
1.5-2 bar
CF (oil)
aut. F 25
CF (N) 2
µm,
µm
3
IP
PP
300
parallel
CF 2 µm:
parallel
3 months
CF
(oil)
-
35
?
vessels,
50 CF 2
µm N
F
Berkel
and
731 ± 485
Rijswijk
(Berkel,
Sandstone
surrounding
200 - 240
0.20 – 8 um
measured
60 - 28
300 µm
Wire
200 µm or
150
mesh
40
entire
screen
wire mesh
surface
screen
installation
wells)
7
354 ± 506
(200 m3/h)
(Rijswijk,
HEX
TSS
67.5 mg/L
surrounding
10
mesh
negligible
1.9 – 9.0
CONFIDENTIAL
Costs wire
screens
wells)
(based
for
before
HEX
IP
µm
on
-
70 / 70
average
permeability
surrounding
wells)
G
Rijswijk
354 ± 506
Sandstone
(surroundin
150
0.20 – 8 um
300 µm
measured
65 – 27/36
g wells)
6
(150 m3/h)
1.9 – 6.3
(based
on
Wire
200 µm or
150
mesh
50
entire
screen
wire mesh
surface
screen
installation
HEX
TSS
µm
before
HEX
11 mg/L
for
Costs wire
IP
mesh
average
screens
permeability
negligible
surrounding
wells)
H
Carboniferous
162
240
Limestone
(production
(expected)
Group
well,
estimation)
80 – 40
-
No
BF
2
parallel
1 bar over
Not known
screens
CF
filter lines
BF. CF not
yet
HEX
containing
known yet
IP
8 BF and
-
3 hf
11
µm CF
1.8 – 4.2
Could
10
BF 1 week
expected
CF
be
≤ 20
not
known yet
higher
because of
-
fracture
Entire flow
openings
through 1
filter
line
(reduced
flow)
I
Slochteren
Not known
Sandstone
yet
135 (P90)
90 – 45
Not known
yet
D50 = 0.55-
300 µm
(OG)
BF
Not known
Not known
0.61 µm
pr. well;
BF
CF
yet
yet
Range 0.020
no
CF
µm – 63 µm
screen
HEX
inj. well
IP
6
Not known
yet
Not known
-
yet
Entire flow
through 1
filter line
-
OG: oil gas separator or degasser only; HEX: heat exchangers; BF: bag filters; CF: cartridge
filters; hf: high flow; aut. F: automatic filter; IP: injection pump; A: absolute filter rating; N:
nominal filter rating; HC: hydrocarbons; pr: produced water; inj: injected water; TSS: total
suspended solids; PP: polypropylene
CONFIDENTIAL

Filtration in the surface installation of a geothermal doublet: from

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