introduction to multiphase & wet gas flows - nel to muph and wet gas.pdf · introduction to...

Introduction to Multiphase

& Wet Gas Flows

Emmelyn Graham

Flow Measurement Engineer

• Introduction to Multiphase & Wet Gas Flows

• Flow Patterns

• Characterisation & Terminology

• Traditional Measurement Methods

• Wet-Gas Flow Measurement

• Multiphase Flow Measurement Technologies

• Flow Meter Selection & Verification

Contents

Oil / Gas not produced as a single phase fluid • Water and gas present

Technically speaking it’s actually multi-component flow

OIL

GAS

WATER

Multiphase Flow Measurement

CHARACTERISED BY : Volumetric Flowrate Ratios

GAS

OIL

WATER

ALSO CHARACTERISED BY : Flow Regime / Pattern

GASQ

OILQ

WATERQ

Gas

OIL

WATER

• Simultaneous flow of oil, water and gas

• Gas Volume Fraction, GVF : 0 -100%

• Water Cut, WC (or Water liquid ratio, WLR) : 0 – 100%

• Variety of flow patterns

• May contain sand, wax, etc

Description of Multiphase Flow

gas

water

oil

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

These terms will be

covered shortly

gas

liquid

Flow 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 flows

Flow Patterns / Regimes

Horizontal Flow Patterns

Stratified flow regimes

STRATIFIED

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

PLUG

Alternating regions of high and low liquid hold-up

As liquid flowrate increases liquid phase dominates flow

SLUG

Horizontal Flow Patterns

Occurs at high liquid velocities

Gas bubbles are suspended in continuous liquid phase

BUBBLE

ANNULAR

Occurs at high gas velocities

Gas flows in central core / liquid as film on pipe walls

Horizontal Flow Patterns

Increasing gas velocity and / or gas density

The liquid becomes completely entrained as droplets in the gas flow

MIST

As gas velocity and / or gas density increases the liquid starts to

becomes entrained as droplets in the gas flow

ANNULAR MIST

Horizontal Flow Patterns

TYPICAL HORIZONTAL FLOW PATTERN MAP (2 phases)

Not general!

Applies only to a

specific fluid and

pressure

Horizontal Three-Phase

Flow Patterns Separation of oil and water • Occurs in stratified and slug flow • Possibility of oil and water travelling at different velocities

Mixing of oil and water • Emulsions • Increased liquid viscosity • Slower film drainage

GAS

OIL

WATER

GASQ

OILQ

WATERQ

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

Mist Flow Liquid entrained as droplets

INCREASING GAS VELOCITY

Bubble Slug Churn Annular Annular

Mist

Mist

Vertical Flow Patterns

TYPICAL VERTICAL FLOW PATTERN MAP (2 phases)

Not general!

Applies only to a

specific fluid and

pressure

Vertical Flow Patterns

• No major effects

–Oil and water are usually well mixed

–No separation due to gravity

• Transitions are not affected by water cut

–Effects due to liquid density changes eliminated

• 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

Effect of Upstream Conditions

Characterisation & Terminology

Void Fraction and

Liquid Hold-Up

GAS

OIL

WATER

GASQ

OILQ

WATERQ

Gas

OIL

WATERGas “Void Fraction” = GAS

Liquid “Hold-up” = OILWATER

Carry-over and Carry-under

GAS

WATER OIL

Volume flowrate of one phase relative to total

volume flowrate

e.g. GVF = Gas volume flowrate / Total volume

flowrate

GVF = Gas Volume Fraction

Phase Volume Fraction

21 llg

g

QQQ

QGVF

Cross sectional area locally occupied by one

phase relative to the total cross sectional area of

the pipe at that point.

Also referred to as:

• Void fraction (gas)

• Hold up (liquid)

Phase Area Fraction

gas

liquid

• 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.

GVF

Phase Slip

• Gas and liquid travel at different velocities

• Mean gas velocity is greater than mean liquid velocity

• Difference known as “slip”: vR = vg – vl

or “slip ratio”: K = vg /vl

• Note: GVF is related to void fraction eg and slip ratio K through

K

KGVF

gg

g

ee

e

1

Example: If εg = 70% and K = 8.1, then GVF = 95%

• If a multiphase flow meter measures local mean density

given by

• Use phase densities to obtain the void fraction εg

• Use K to obtain GVF from εg

• How to determine K (Slip Model)?

• Apply semi-empirical model for flow pattern, OR

• Perform laboratory or test facility experiments with known GVF, and develop correlation for K using appropriate physical variables

• Strictly only applies to range of conditions of experiments and ideally should not be extrapolated

Phase Slip (contd)

lggg ee 1

εg

1-εg

Areas

Homogeneous Flow

• Liquid and gas travel at same mean velocity (v)

gl vv

• For homogeneous flow, K = 1, so equation from previous slide gives

GVFg e

• A homogeneous flow can be assigned a single value of properties like density, viscosity, etc based on weighted average of phase mass fractions

oil - continuous

water - continuous

inversion region

45% < water cut < 75%

THE INVERSION POINT MOVES

Inversion Region

The velocity a particular phase would have if the

same volume flowrate flowed alone in the pipe

e.g. Pipe diameter = 6 inch

Gas flowrate = 950 m3/hr

Liquid flowrate = 50 m3/hr

Superficial Gas Velocity (SGV) = 14.9 m/s

Superficial Liquid Velocity (SLV) = 0.8 m/s

Superficial Phase Velocity

Froude number, Fr, of each phase

• High Fr: Kinetic energy dominates

• Low Fr: Gravity forces dominate

Froude Number

gl

ggs

ggD

v

,Fr

gl

lls

lgD

vFr

,

Liquid Gas

• Lockhart-Martinelli parameter, X

• General definition

(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%

Key Definitions

g

l

g

l

l

g

g

l

G

L

Q

Q

m

m

Fr

FrX

CFD Modelling of

Multiphase Flows

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)

Traditional Measurement

MULTIPHASE FLOW

GAS

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

- High GVF Multiphase Flows

Differential Pressure Meters

• Must correct for presence

of liquid as causes meter

to over-read

• Need info on wetness to

correct the gas flowrate

• Cheapest option

• New ISO Technical Report

Wet-Gas Flow Measurement

Commercial Wet-gas

Meters

• Provides water, oil and

gas flowrates

• Can use multiphase

metering technology

Wet-Gas Metering

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 : O , W , G Gamma-Ray Absorption

Electrical Properties

Microwave etc.

– Calculate INDIVIDUAL phase flowrates from: QWAT = W . QMIX

QOIL = O . QMIX

QGAS = G . QMIX

Multiphase Metering

Technologies

• BULK FLOWRATE : P METER (e.g. VENTURI)

– Mass flow is function of DENSITY () and P

– 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

ΔPρ2AεECm tD

P

• DP METER OPERATION IN VERY UNSTEADY FLOWS

• Fast sampling required to reduce “averaging errors”

1 sec

Q (l/s)

20

10

30

1 sec

P (mbar)

400

100

900

Multiphase Metering

Technologies

• BULK FLOWRATE : CROSS CORRELATION

– Compare response of 2 (+) axially displaced sensors

Capacitance probes

Densitometers

Pressure Gauges

SENSOR 2 SENSOR 1

Multiphase Metering

Technologies

• PHASE FRACTION : GAMMA RAY ABSORPTION

DE

TE

CT

OR

– Number of gamma-rays detected :

I = IO exp( - D )

– Linear Absorption Coefficient depends on fluid in pipe GAS is weak absorber ( low), WAT is strong absorber ( high)

Absorption probability also depends on gamma-ray energy (E)

Multiphase Metering

Technologies

• PHASE FRACTION : GAMMA RAY ABSORPTION − HIGH Energy Gamma: Absorption Fluid DENSITY only

GAS

OIL

WATER

Transmitted

Counts

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

MIX

GAS

LIQ SLUG

GAS BUBBLE

Gives GAS / LIQ ratio but poor discrimination of OIL from WATER

Multiphase Metering

Technologies

• PHASE FRACTION : GAMMA RAY ABSORPTION

− For WLR include LOW E : Absorption Fluid DENSITY + TYPE

GAS

OIL

WATER

Transmitted

Counts

Multiphase Metering

Technologies

• PHASE FRACTION : GAMMA RAY ABSORPTION

− For WLR and GVF need both LOW E and HIGH E

− Plot I(EHIGH) vs I(ELOW)

− Corners = Pure Phases

− Internal points = Mixtures

− Interpolate for GVF + WC

− Salinity changes = errors

I (ELOW)

I (EHIGH)

Wat

Oil

Gas

Mix

50% WC

20% GVF

Multiphase Metering

Technologies

Flow Meter Selection and Verification

• 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?

Flow Meter Selection

• Before installation

• Joint Industry Projects (JIP) to evaluate technology

• Factory Acceptance Testing (FAT)

• at independent facility like NEL

• at manufacturer’s facility

• After installation

• Against a test separator

• Other options.....

• Against another meter?

• Sampling?

• Check sensors

Validation of Flow Meters

• Flow regimes for horizontal and vertical flows

• How to describe a multiphase flow

• Challenges with measuring flow rates

• Wet-gas flow measurement

• Different multiphase metering technologies

• Considerations for selecting a meter and verifying the

performance

Multiphase flow measurement is much more

challenging than single phase metering

Summary

Any questions?

Thank you for listening

Multiphase & Wet-Gas Flow Measurement

Training Course, Aberdeen

25 February 2014

Introduction to Measurement Uncertainty

Measurement Training Course, Aberdeen

26 February 2014

Contact me at email: egraham@tuvnel.com

Forthcoming NEL Presentations

NEL Contacts Audit & Allocation Alick MacGillivray amacgillivray@tuvnel.com

CFD Neil Bowman nbowman@tuvnel.com

Densitometers Norman Glen nglen@tuvnel.com

Erosion John Peters jpeters@tuvnel.com

Flow Consortium Phil Mark pmark@tuvnel.com

Heavy Oil Chris Mills cmills@tuvnel.com

Measurement Consultancy

Calum Hardie

Craig Marshall

Chris Mills

chardie@tuvnel.com

cmarshall@tuvnel.com

cmills@tuvnel.com

Measurement Uncertainty Alick MacGillivray amacgillivray@tuvnel.com

Meter Diagnostics Craig Marshall cmarshall@tuvnel.com

MeterVue Phil Mark pmark@tuvnel.com

Multiphase Terri Leonard tleonard@tuvnel.com

PPDS Lynn Hunter lhunter@tuvnel.com

Single Phase Metering Bob Belshaw bbelshaw@tuvnel.com

Training Helen Tulloch htulloch@tuvnel.com

Umbilicals Janice MacLeod jmacleod@tuvnel.com

Valve Testing John Dods jdods@tuvnel.com

Wet Gas Emmelyn Graham egraham@tuvnel.com

NEL Contact Tel: + 44 (0) 1355 220222

For general queries contact the sales team on sales@tuvnel.com