150617 Intro to Muph and Wet Gas (Final) [Compatibility Mode]

Introduction to Multiphase
& Wet Gas Flows
Terri Leonard
Flow Measurement Engineer
Contents
Introduction to Multiphase & Wet Gas Flow
Flow Patterns
Characterisation & Terminology
Traditional Measurement Methods
Multiphase & Wet-Gas Flow Measurement
Technologies
Flow Meter Selection & Verification
Why is Measurement Important?
Measurement plays an important role in
the UK economy
Trade requires a regulatory framework
based upon measurement confidence
National Measurement Office (NMO)
National Measurement System (NMS)
3 National Measurement Institutes deliver
the UK’s NMS
Responsible for stimulating good
measurement practice
Represents position of UK measurement
internationally
INTRODUCTION TO MULTIPHASE &
WET GAS FLOW
Oil / Gas not produced as a single phase fluid
Water and gas present
GAS
OIL
WATER
Technically speaking it’s actually multi-component flow
CHARACTERISATION & TERMINOLOGY
Void Fraction and
Liquid Hold-Up
Simultaneous flow of two or more immiscible fluids
Typically OIL / WATER / GAS
α Gas
GAS
α OIL
OIL
Q GAS
Q OIL
WATER
α WATER
Gas “Void Fraction”
Liquid “Hold-up”
Q WATER
= α GAS
= α WATER
+ α OIL
Volumetric Flowrate Ratios
CHARACTERISED BY : Volumetric Flowrate Ratios
α Gas
αOIL
GAS
OIL
WATER
QGAS
QOIL
QWATER
αWATER
Gas Volume Fraction (GVF) =
Water to Liquid Ratio (WLR) =
QG / (QG + QO + QW)
QW / (QO + QW)
ALSO CHARACTERISED BY : Flow Regime / Pattern
GVF & Gas Void Fraction
Important to distinguish between gas volume fraction and
gas void fraction
Gas Volume Fraction based on flowrates (GVF)
Gas Void Fraction based on local areas
They are usually unequal
For example:
70% gas void fraction could
be 95% gas volume fraction
as the gas is travelling at
higher velocity.
Phase Slip
Gas and liquid travel at different velocities
Mean gas velocity is greater than mean liquid velocity
Difference known as “slip”:
or “slip ratio”:
vR = vg – vl
K = vg /vl
Note: GVF is related to void fraction eg and slip ratio K
through


εgK

GVF = 
1− ε + ε K 
g
g


Example: If εg = 70% and K = 8.1, then GVF = 95%
Homogeneous Flow
Liquid and gas travel at same mean velocity (v)
vl = v g
For homogeneous flow, K = 1, so equation from
previous slide gives
ε g = GVF
A homogeneous flow can be assigned a single value of
properties like density, viscosity, etc based on weighted
average of phase mass fractions
Inversion Region
oil continuous
water continuous
inversion region
45% < water cut < 75%
THE INVERSION
POINT MOVES
Superficial Phase Velocity
The velocity a particular phase would have if the
same volume flowrate flowed alone in the pipe
e.g. Pipe diameter
Gas flowrate
Liquid flowrate
= 6 inch
= 950 m3/hr
= 50 m3/hr
Superficial Gas Velocity (SGV) = 14.9 m/s
Superficial Liquid Velocity (SLV) = 0.8 m/s
Description of Wet-Gas Flow
Wet gas is a mixture consisting mostly of gas
with a small amount of liquid.
Liquid can be water and /or hydrocarbon
Water cut - 0% to 100%
How is wet gas defined?
Gas volume fraction > 90%
Lockhart-Martinelli parameter < 0.3
gas
liquid
Froude Number
Froude number, Fr, of each phase
Liquid
Frl =
Gas
v s ,l
gD
High Fr:
Low Fr:
ρl
ρl − ρ g
Frg =
vs , g
gD
Kinetic energy dominates
Gravity forces dominate
ρg
ρl − ρ g
Lockhart-Martinelli Parameter
Lockhart-Martinelli parameter, X
General definition
X
=
FrL
FrG
=
ml
mg
ρg
ρl
=
Ql
Qg
ρl
ρg
(subscripts G, L refer to gas or liquid phase)
Used to describe wet-gas flows where X < 0.3
Wet-gas flow are normally with GVF > 90%
FLOW PATTERNS / REGIMES
Flow Patterns / Regimes
The phases can be distributed over a pipe cross
section in many different ways
Flow pattern depends on the amount of each
phase, liquid and vapour properties, pressure
and velocities
Different for horizontal and vertical pipe
orientations
Horizontal Flow Patterns
Separated flow regimes
QGAS
QLIQ
STRATIFIED
QGAS
QLIQ
STRATIFIED WAVY
Occur at relatively low velocities for both phases
Surface becomes wavy as gas velocity increases or pipe inclines
Horizontal Flow Patterns
Intermittent flow regimes
QGAS
QLIQ
PLUG
QGAS
QLIQ
SLUG
Alternating regions of high and low liquid hold-up
As liquid flowrate increases liquid phase dominates flow
Horizontal Flow Patterns
Occurs at high liquid velocities
Gas bubbles are suspended in continuous liquid phase
QGAS
QLIQ
BUBBLE
Occurs at high gas velocities
Gas flows in central core / liquid as film on pipe walls
QGAS
QLIQ
ANNULAR
Horizontal Flow Patterns
As gas velocity and / or gas density increases the liquid starts to
becomes entrained as droplets in the gas flow
QGAS
QLIQ
ANNULAR MIST
Increasing gas velocity and/or gas density
The liquid becomes completely entrained as droplets in the gas flow
QGAS
QLIQ
MIST
Horizontal Flow Patterns
Typical Horizontal Flow Pattern Map (2 phases)
Not general!
Applies only to a
specific fluid and
pressure
Vertical Flow Patterns
Bubble Flow
Liquid phase continuous
Dispersed gas bubbles
Slug Flow
Small bubbles coalesce
Taylor bubbles (slugs)
Churn Flow
Irregular gas slugs
Liquid rises and falls
Annular Flow
Gas flows in core
Liquid flows in annulus
Bubble
Mist Flow
Liquid entrained as
droplets
Slug
Churn
Annular
Annular
Mist
INCREASING GAS VELOCITY
Mist
Vertical Flow Patterns
Typical Vertical Flow Pattern Map (2 phases)
Not general!
Applies only to a
specific fluid and
pressure
Multiphase Flow Patterns
Three-phase Separation
OIL can separate from WATER at low velocities
–
–
More likely in horizontal flow
Occurs in stratified and slug flow regimes
GAS
OIL
WATER
–
QGAS
QOIL
QWATER
Oil and water remain better mixed in vertical (up) flow
Effect of Upstream Conditions
Flow pattern maps are based on test sections in
well developed flow
• Long, straight pipe lengths
Upstream conditions (bends, valve, etc.) affect
flow pattern
Can use this to advantage by conditioning flow
• For example, use mixer to get closer to homogeneous flow
• Blind tee used in multiphase flow measurement
TRADITIONAL MEASUREMENT
METHODS
Traditional Measurement
MULTIPHASE
FLOW
GAS
WATER
OIL
Measure separated phases – using traditional meters:
GAS:
Orifice, Vortex …
LIQUID: Turbine, PD, Coriolis …
WLR:
Coriolis, grab samples … (2-phase separators)
Carry-over and Carry-under
GAS
WATER
OIL
Traditional Measurement
GAS
MULTIPHASE
FLOW
WATER
OIL
Poor level control, foaming, emulsions etc. ⇒ phase contamination
Liquid carry-over, gas carry-under, water-in-oil ⇒ may be
unmonitored
Capital, operating and infrastructure costs can be high
Only periodic testing may be possible (oilfield “well test”)
WET GAS FLOW MEASUREMENT
TECHNOLOGY
- High GVF Multiphase Flows
Wet-Gas Flow Measurement
Wet-Gas Metering
Differential Pressure
Meters
Commercial Wet-gas
Meters
Must correct for presence of liquid
as causes meter to over-read
Provides water, oil and gas
flowrates
Cheapest option
Can use multiphase
metering technology
Need info on wetness to correct
the gas flowrate
New ISO TR 11583
MULTIPHASE FLOW MEASUREMENT
TECHNOLOGY
Multiphase Metering Technologies
MULTIPHASE METERS – GENERAL METHODOLOGY
Measure BULK flowrate of MIXTURE :
QMIX
Differential Pressure device
Positive Displacement meter
Cross Correlation technique etc.
Measure PHASE FRACTIONS :
αO ,
αW , αG
Gamma-Ray Absorption
Electrical Properties
Microwave etc.
Calculate INDIVIDUAL phase flowrates from:
QWAT = αW . QMIX
QOIL = αO . QMIX
QGAS = αG . QMIX
Multiphase Metering Technologies
BULK FLOWRATE : ∆P METER (e.g. VENTURI)
Mass flow is function of DENSITY (ρ) and ∆P
& = CD E ε A t 2 ρ ∆P
m
Simple, robust design
Need separate density measurement
CD = f (WC, GVF, fluid properties, …)
Must be characterised by testing
Performance improved by mixing
Vertical up-flow, Blinded-T on inlet
Generally still require Slip Model
Low turndown / Finite pressure loss
∆P
Multiphase Metering Technologies
DP METER OPERATION IN VERY UNSTEADY FLOWS
Fast sampling required to reduce “averaging errors”
Q (l/s)
30
20
10
∆P (mbar)
900
10sec
400
100
10 sec
Multiphase Metering Technologies
BULK FLOWRATE : CROSS CORRELATION
Compare response of 2 (+) axially displaced sensors
Capacitance probes
Densitometers
Pressure Gauges
SENSOR 1
SENSOR 2
Not applicable in single-phase, homogenous flow etc.
Multiphase Metering Technologies
DETECTOR
PHASE FRACTION : GAMMA RAY ABSORPTION
Number of gamma-rays detected :
I = IO exp( - µ D )
Linear Absorption Coefficient µ depends on fluid in pipe
GAS is weak absorber (µ low), WAT is strong absorber (µ
high)
Absorption probability also depends on gamma-ray energy
(Eγ)
Multiphase Metering Technologies
PHASE FRACTION : GAMMA RAY ABSORPTION
HIGH Energy Gamma: Absorption ∝ Fluid DENSITY only
Transmitted
Counts
GAS
OIL
WATER
Multiphase Metering Technologies
PHASE FRACTION : GAMMA RAY ABSORPTION
HIGH Energy Gamma: MIXTURE in pipeline
Interpolate between LIQ and GAS calibration rates: GVF
Observe where flow is liquid dominant
Or gas dominant
Transmitted Counts
GAS
GAS BUBBLE
MIX
Gives GAS/LIQ ratio
Poor discrimination of OIL from WATER
SLUG
LIQ
Multiphase Metering Technologies
PHASE FRACTION : GAMMA RAY ABSORPTION
For WLR include LOW Eγ : Absorption ∝ Fluid DENSITY + TYPE
Transmitted
Counts
GAS
OIL
WATER
Multiphase Metering Technologies
PHASE FRACTION : GAMMA RAY ABSORPTION
For WLR and GVF need both LOW Eγ and HIGH Eγ
Plot I(EHIGH) vs I(ELOW)
Gas
I (EHIGH)
Corners = Pure Phases
Internal points = Mixtures
Interpolate for GVF + WC
20%
Mix
Salinity changes = errors
GVF
Oil
Wat
50% WC
I (ELOW)
Multiphase Metering Technologies
PHASE FRACTION : ELECTRICAL PROPERTIES
Capacitance Sensor
−
−
−
−
Electrodes embedded in pipe wall
Measure permittivity of mixture
Use to derive WC of liquid phase
OIL- continuous flow only
Conductivity Sensor
− Measure conductivity of mixture
− WATER-continuous flow only
Separate density measurement
− Correct for effect of void fraction
− Used to derive all three fractions
Multiphase Metering Technologies
PHASE FRACTION : MICROWAVE METHOD
Microwave Cavity (~GHz)
−
−
−
−
Flow passes through resonance cavity
Microwaves reflect back and forth
Adjust frequency fR for resonance
fR depends on εMIX (permittivity)
Microwave by energy absorption
Microwave generator and receiver
Microwave absorption related to bulk electrical properties
Combine with density
− To derive all three fractions
FLOW METER SELECTION &
VERIFICATION
Flow Meter Selection
WHAT IS THE ROLE OF THE MULTIPHASE METER?
• Well Testing?
• Control and Monitoring?
• Production Allocation?
WHERE WILL THE METERING BE APPLIED ?
•
•
•
•
Onshore?
Offshore topside (manned / unmanned)?
Subsea?
Mobile ?
WHAT ARE THE CONDITIONS TO BE METERED ?
• Will the conditions change over time?
HOW MANY METERS WILL BE APPROPRIATE ?
• Replacing test separator?
Validation of Flow Meters
Before installation
Joint Industry Projects (JIP) to evaluate technology
Factory Acceptance Testing (FAT)
• at a fully traceable independent facility such as NEL’s
Multiphase & Wet Gas Flow Facilities
After installation
Against a test separator
Other options.....
• Against another meter?
• Sampling?
• Check sensors
Refer to API 20.3 for further info
Summary
Flow patterns/regimes for horizontal and vertical flows
Key definitions & characterising a multiphase flow
Traditional method vs. Multiphase metering
Wet-gas & Multiphase flow measurement technologies
Considerations for selecting a meter and verifying the
performance
Multiphase flow measurement is much more
challenging than single phase metering
Thank you for listening
Any questions?
Email: terri.leonard@tuv-sud.co.uk
Website: www.tuvnel.com
NEL Contact Tel: + 44 (0) 1355 220222
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