CO2_Kreft_Schweinric..

international journal of greenhouse gas control 1 (2007) 69–74
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/ijggc
‘‘The Schweinrich structure’’, a potential site for industrial
scale CO2 storage and a test case for safety assessment in
Germany
Eric Kreft a,*, Christian Bernstone b, Robert Meyer c, Franz May c, Rob Arts a, Arie Obdam a,
Rickard Svensson b, Sara Eriksson b, Pierre Durst d, Irina Gaus d, Bert van der Meer a,
Cees Geel a
a
TNO, PO Box 80015, 3508 TA Utrecht, The Netherlands
Vattenfall Utveckling AB, SE-162, 87 Stockholm, Sweden
c
BGR, Stilleweg 2, 30655 Hannover, Germany
d
BRGM, F-45060 Orleans, France
b
article info
abstract
Article history:
The identification of risks associated with the geological storage of CO2 requires methods
Received 31 July 2006
that can analyse and assess potential safety hazards. This paper evaluates how perfor-
Received in revised form
mance assessment can be used as a method for assessing the impact of CO2 storage on
20 December 2006
health, safety and the environment (HSE) with particular respect to potential future aquifer
Accepted 22 December 2006
storage in the anticlinal structure Schweinrich in Germany. The performance assessment
Published on line 9 March 2007
was conducted under the CO2STORE European Fifth Framework project as one of the four
cases on the aquifer storage of CO2. It is known as the Schwarze Pumpe case study.
Keywords:
Being a case study, it is restrictive from a feasibility study point of view—i.e., the
CO2 storage
extended identification of the key safety factors where an actual CO2 storage project would
Aquifer storage
be considered for the Schweinrich structure. The study is based on data currently available,
Geological storage
gathered in prior surveys, and on the use of simplified models, with CO2 leakage levels from
Safety assessment
natural analogues being the evaluation criteria. While the results should be interpreted as
FEP analysis
provisional, they point out clearly which additional data should be gathered in relation to
the long-term storage performance in the event that the site warrants further investigation.
# 2007 Elsevier Ltd. All rights reserved.
1.
Introduction
Assessing and managing the risks associated with the
geological storage of CO2 is a relatively new area of research;
there is no detailed knowledge base as a frame of reference.
This therefore shifts the focus to learning from studies
conducted for other substances whose risks are similar yet
contain differences. A thorough safety assessment method
called performance assessment has been adopted on this
basis. Natural gas storage teaches us that several HSE risks
associated with subsurface storage can be managed through
the strict regulation of site selection and well construction.
Thus, it is also important to establish guidelines, in addition to
performance assessment evaluation, for inclusion in a future
safety standard for CO2 storage.
The performance has been applied to the Schwarze Pumpe
case study. The Schwarze Pumpe plant is located in
Brandenburg (Niederlausitz) 150 km southeast of Berlin and
* Corresponding author. Current address: EBN, 3511 DX, Utrecht, The Netherlands.
E-mail address: eric.kreft@ebn.nl (E. Kreft).
1750-5836/$ – see front matter # 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/S1750-5836(07)00009-6
70
international journal of greenhouse gas control 1 (2007) 69–74
operated by Vattenfall Europe Generation. The potential
Schweinrich storage site is located in the north-eastern region
of Germany, about 100 km north-west of Berlin, at a depth of
approximately 1600 m. It was selected as the most suitable
candidate in the north-eastern German basin for the underground storage of 400 million tonnes of CO2, which corresponds to 40 years’ production from a 1600 MW lignite-fired
power plant.
2.
The geology of the structure
The Schweinrich structure covers an area of about 100 km2
and its estimated storage potential is between 500 and
840 tonnes of CO2 (Meyer et al., 2006). Its anticlinal structure
(Fig. 1) contains two main reservoirs: the shallower one in the
Lower Jurassic (Lias, Hettang) and the deeper one in the Upper
Triassic (Keuper, Contorta). The total reservoir thickness
ranges between 270 and 380 m and consists of several layers
of fine-grained, highly porous sandstone, which is overlain by
several hundred meters of thick Jurassic clay formations that
cap the storage system.
The current geological model of Schweinrich is based on
the information available from 2D seismic lines and from
exploration wells near the structure, mostly recorded and
drilled in the early 70s. No wells penetrate the anticline.
Refinement of the geological model combined with new
data as it is gathered will continue throughout the
performance assessment process. The Schweinrich structure has been used as a representative generic model that is
also valid for other potential storage sites in north-east
Germany.
3.
The methodology involves the:
Definition of a basis for the assessment;
analysis of features, events and processes (FEPs)
formation of safety scenarios;
development of dedicated models for probabilistic simulation of safety scenarios;
evaluation of HSE effects on safety.
The basis of the methodology is a comprehensive evaluation of potential safety factors (FEPs) that may affect the future
performance of the storage site. A large number of FEPs are
evaluated and the most relevant and critical regarding safety
are selected for further evaluation. The FEPs are the building
blocks for the construction of safety scenarios, which are
simulated by numerical models. The long-term storage
performance is evaluated using probabilistic simulation in
order to cover the uncertainty related to the future impact of
the safety factors. This study compares the results from the
simulation models against the CO2 leakage levels from natural
analogues (e.g., reported in Streit and Watson, 2004).
The FEP analysis of the Schweinrich structure evaluates
potential HSE factors within the next 1000 years following CO2
injection. However, since the safety factors that are identified
may generate hazards, the simulations are run for additional
9000 years. The outcome of the safety scenarios was expressed
as the maximum concentration and maximum flux of CO2 in
the pore system in the Pleistocene sediments in the shallow
subsurface and represented in the simulation models by the
topmost subsurface layer. Since no modelling of the flow and
fate of CO2 in the shallow groundwater compartment was
conducted, no outcome was given with respect to groundwater
deterioration and mobilisation of heavy metals. This has been
planned for the next phase of the performance assessment.
Methodology
The purpose of the performance assessment study has been to
evaluate its suitability as a method for determining the Health,
Safety and Environmental (HSE) effects of CO2 storage. These
effects have also been evaluated from a feasibility point of
view, thereby identifying and evaluating the key safety factors
at an early stage (for further examination in a follow-up
project).
4.
Identified safety scenarios
Four safety scenarios were identified through FEP combinations:
1. Reference: no failure of the containment zone occurs. This
scenario, considered to be the most likely, reflects the CO2
Fig. 1 – Cross-section of the Schweinrich anticline between two salt diapers. The hatched area indicates the reservoir and
storage position.
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international journal of greenhouse gas control 1 (2007) 69–74
Table 1 – Number of grid blocks of different Schweinrich simulation models
Simplified radial model (2D)
Simplified Cartesian model (3D)
Deterministic 3D model
# Grid blocks X
# Grid blocks Y
# Grid blocks Z
25
20
84
21
40
16
17
22
injection process and the flow and fate of CO2 in the
reservoir after abandonment of the site.
2. Leaking-seal: the leaking seal scenario reflects the CO2
injection process and the flow and fate of CO2 through the
cap rock due to geochemical deterioration. The reason for
the possible release of CO2 through the cap rock might be
due to small amounts of carbonates and thin marl layers in
the shale layers that form the cap rock.
3. Leaking-fault: the leaking fault scenario reflects the flow and
fate of CO2 through a fault system running from the cap
rock to the shallow subsurface. The interpretation of the
existing seismic lines over the Schweinrich structure are
not conclusive due to the poor data quality, but the
existence of fault systems in the Mesozoic and Caenozoic
overburden cannot be ruled out (Fig. 1). At this moment, the
constituency of such a fault system and its permeability are
simply unknown and require additional data acquisition.
4. Leaking-well: the leaking well scenario reflects the CO2
injection process and the flow and fate of CO2 along the
well trajectory due to several events and processes. The
drilling and completion schedule of future wells are
unknown. Therefore a ‘‘generic’’ abandoned old well
safety scenario based on a previous study was applied to
the FEP evaluation (Wildenborg et al., 2005). This scenario
was chosen mainly to evaluate the differences in outcome
with the above scenarios. It must be noted, that no
abandoned wells penetrate the Schweinrich structure
and that all possible precautions can still be taken for
future injection wells making the occurrence of leaking
wells highly unlikely.
5.
Model development
The safety scenarios present the possible future flow and fate
of CO2 for 10,000 years after injection. The scenarios are
represented in simplified 2D and 3D models with stochastically varied input parameters using the multi-component flow
Time per run
22 s
20 min
2 days
simulator SIMED-II (Stevenson and Pinczewski, 1995). Because
Simed II does not allow mechanical and chemical processes to
be modelled, the mechanical and chemical safety factors that
apply to the identified safety scenarios are represented by
adjustments in hydrodynamic properties (Svensson et al.,
2005). The advantage of using simplified models is their
limited run time, which allows a large number of stochastic
input combinations to be modelled (Table 1). Simplifications
relate mainly to the limited number of grid cells and the
homogeneous layer properties. The simplified models have
been calibrated to a detailed, deterministic, finely scaled
model of the Schweinrich structure over an injection period of
40 years (Fig. 2) based on the following:
1. Similar input properties: the stochastic models should have
similar input properties, such as porosity and (relative)
permeability. For the coarser stochastic models these
properties have been averaged on the basis of the detailed
deterministic model.
2. Similar pressure development: the development of the
pressure of the reservoir in time needs to be similar to
the development in the deterministic model over the first 40
years.
3. Similar CO2 spread/distribution: the spread of the CO2 front in
the stochastic models should be similar to the spread in the
deterministic model where the CO2 front has a lateral
spread of more than 2 km after 40 years.
Buoyancy is the main cause of CO2 rising. One thousand
simulations were carried out for each safety scenario, with the
variation of the stochastic input parameters, such as permeability, constrained on the basis of related studies (Hildenbrand et al., 2004; Schlömer and Krooss, 1997). An example of
the magnitude is given in Table 2 (Svensson et al., 2005). In the
event of uncertainty about input parameters that were not
varied stochastically, worst-case values were generally
selected. Moreover, neither the CO2 dissolution in the aqueous
phase nor capillary entry pressures were taken into account. A
Fig. 2 – Development of the CO2 gas saturation in time (3D deterministic model).
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international journal of greenhouse gas control 1 (2007) 69–74
Table 2 – Stochastic input parameters of the leaking fault scenario
Parameter
Reservoir horizontal permeability
Distance between fault and well
Fault vert. permeability (clay)
Fault vert. permeability (silt)
Fault vert. permeability (sand)
Units
Type of distribution
Low
High
mD
m
mD
mD
mD
Uniform
Uniform
Uniform
100 fault vert. perm. (clay)
10,000 fault vert. perm (clay)
50
50
10 3
10 1
10 1
1000
2500
10 1
10 1
10 3
few sensitivities were run, including CO2 dissolution, in order
to evaluate the effects on the outcome of the model. Changes
in flux and concentration with respect to the case without CO2
dissolution varied between 2% and 25% in flux and between 0%
and 7% in concentration for high and low release rates,
respectively. For these reasons the outcome expressed as the
maximum flux of CO2 in the shallow subsurface Pleistocene
sediments are biased towards the worst-case scenarios.
6.
Simulation results
The reference scenario and the leaking-seal scenario show no
increase of CO2 in the Pleistocene sediments over 10,000 years.
The CO2 escaping from the seal is sufficiently held up and
spread over time that it does not reach the shallow subsurface
(Fig. 3).
The leaking-fault scenario, i.e. where it is assumed that a
fault extends from the cap rock to the shallow subsurface,
shows a relatively slow migration process of CO2 along the
fault plane. An example of one of the simplified model runs is
presented in Fig. 4. Maximum CO2 fluxes vary between 0.00025
and 0.62 tonnes/(year m2) in the Pleistocene sediments (Fig. 5).
These values are comparable to leakage rates from natural CO2
accumulations in Europe and Australia (Streit and Watson,
2004). The maximum CO2 gas concentration in the shallow
subsurface Pleistocene sediments is less than 4% at a depth of
80 m, which is close to the lower limit of moderate effects on
trees and crops (Saripalli et al., 2002). The effects of the fluxes
and concentrations on the shallow subsurface ecosystem will
be investigated in a later phase.
The ranges in outcome show that further research on the
existence of the faults through the cap rock is required. Such a
study would be a priority if actual CO2 storage project were to
be considered at Schweinrich. The location of the faults can be
investigated by running a 3D seismic survey, and the fault
properties determined by conducting a special study on the
local fault permeability. These fault property values would be
needed for detailed coupled THMC models in order to restrict
the range of fluxes obtained from the stochastic models. Note
that the simulation results should be interpreted as worstcase scenarios, especially since the presence of faults cutting
the cap rock has not yet been established.
The leaking-well scenario (Fig. 6) is the most dramatic,
with average release percentages of 60% of the total amount
of injected CO2. The release of CO2 is directly proportional to
the permeability of the well zone, which increases in time as
a result of various FEPs that apply specifically to this
scenario. It should be noted is that the study is based on the
data of an existing abandoned old well and not the quality to
Fig. 3 – Cross-sections of the simplified 2D flow model presenting subsurface CO2 saturation in the leaking-seal scenario.
The CO2 injection is positioned on the left-hand side of the sections.
international journal of greenhouse gas control 1 (2007) 69–74
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Fig. 4 – Quadrant of the simplified 3D flow model presenting subsurface CO2 saturation in the leaking-fault scenario. The CO2
injection is positioned in the lower left corner. Note that this scenario assumes that a permeable fault from the cap rock to
shallow subsurface is present, but this cannot be confirmed at this stage. More data (i.e., seismic data) is needed to explore
the extent of the fault system.
Fig. 5 – Simulated maximum fluxes and maximum concentrations in local groundwater for the leaking-fault scenario
(assuming there is a leaking fault). Results were acquired without modelling CO2 dissolution in the aqueous phase.
Fig. 6 – Cross-sections of the simplified 2D leaking-well scenario, presenting the subsurface CO2 saturation.
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international journal of greenhouse gas control 1 (2007) 69–74
be expected from a purpose-designed abandoned CO2
injection well.
Maximum fluxes in the Pleistocene vary between
15 tonnes/(year m2) and 350 tonnes/(year m2). This flux is
about 1–10 times the fluxes measured at the Cava dei Selci
near Rome, where fatal accidents were caused by natural CO2
emissions (Carapezza et al., 2003). However, this is not a
realistic scenario for Schweinrich since new wells would be
better designed and high leakage rates in the well zone
detected early enabling mitigating actions to be taken to avoid
further leakage. Most probably, the injection wells would be
placed on the lower flanks of the Schweinrich structure. As
mentioned previously, this scenario was run mainly to
evaluate the differences in outcome with the scenarios above.
7.
Conclusions
This first HSE performance assessment of the Schweinrich
structure was conducted on the basis of the available existing
and limited input data prior to commercial site exploration.
The reference scenario and some worst case scenarios have
been analysed using simplified models. The outcome is biased
towards worst-case scenarios because of the uncertainty
about the input parameters and the use of simplified models.
The results are provisional, given the ongoing data-gathering
process and refinement of the geological model.
The methodology evaluation shows that the performance
assessment methodology is a powerful tool for use in safety
assessments of CO2 storage projects, one that is able to
distinguish relevant safety scenarios at an early stage. The
performance assessment clearly reveals which additional data
should be gathered in respect of long-term storage performance if further investigation of the site should be needed.
Acknowledgements
This study is part of the European CO2STORE project. The
authors thank the European Commission and the industrial
consortium partners for funding this research.
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