Multiphase pipe flow – a key technology for oil and gas production

Pipe Flow: Some considerations related to single and multiphase flow

Calculation of flow in pipes

in

out

• Conservation of• Energy• Mass • Momentum

• Thermodynamics

Mass conservation

• Single-phase : Mass in - mass out = accumulated mass

• Multiphase: Mass transfer comes in addition, e.g. for condensate:Mass in - mass out + local condensation = accumulated mass

• Steady state single-phase flow: G = (density) * (pipe area) * (mean velocity)

= ρUA = constant along a pipeline

in

out

Momentum balance – single-phase:

PL

PR

)(sin)( gmAPP RL

L

Friction

Friction

• Pressure gradient large enough for flow: Velocity depends on friction

• Friction = Friction force per area * wall area

LDUFriction w ...),(

Veggskjærspenning

Multiphase Pipe Flow Depends on:

Fluid properties Pipe geometry Environment

Density Diameter T, externalViscosity Wall roughness Insulation Phase fractions Pipeline profile/ T at inletConductivity topography P at inletHeat capacity P at outletSurface tensionEtc...Varies with P and T !

P=pressure, T=temperature

Oil samples -large differences in

fluid properties

Crude oils• Njord• Visund• Grane• Statfjord C

Condensates• Sleipner• Midgard

Midgard

Multiphase flowThree-phase flow (here):Simultaneous flow of oil-gas-water in the same pipeline

Flow regimes: Describes (intuitively) how the phases are distributed in the pipe cross section and along the pipeline

Superficial velocity:The velocity a phase will have if it were the only fluid present

Flow regimes steeply inclined pipes

Bubbly flow:Little gas, large Uoil(All inclinations)

”Churn”-flow:More gas, large Uoil(steep inclinations)

Annular flow:High Ugas, low Uoil(wide range of incl.)

Stratified/wavy- near horizontal pipeline

Large waves: More effective liquid transport

Stratified flow. Ugas normally >> Uoil

Hydrodynamic slugging

• Large waves that eventually block the pipe cross section pressure build up

• Intermittent flow – liquid slugs divided by gas pockets

• Effective liquid transport • Void in slug: Volume fraction of

entrained gas bubbles in the slug

Liquid slug

Taylor-bubble

Slug front in three-phase flow

Need for experimental data

• MP-flows are complex due to the simultaneous presence of different phases and, usually, different compounds in the same stream.

• The combination of empirical observations and numerical modelling has proved to enhance the understanding of multiphase flow

• Models to represent flows in pipes were traditionally based on empirical correlations for holdup and pressure gradient. This implied problems with extrapolation outside the range of the data

• Today, simulators are based on the multi-fluid models, where averaged and separate continuity and momentum equations are established for the individual phases

• For these models, closure relations are required for e.g. interface and pipe-wall friction, dispersion mechanisms,

turbulence, slug propagation velocities and many more• These can only be established with access to detailed,

multidimensional, data from relevant and well-controlled flows

Conclusion: we need models based on physics to extrapolate beyond lab data

Lab Field

Dimensionless numbers – dynamic similarity

• Reynolds number, ratio of the inertial forces to the viscous forces,

Re= =rvL/m

• Froude number, ratio of a body's inertia to gravitational forces or ratio of a characteristic velocity to a gravitational wave velocity

• Weber number, relative importance of the fluid's inertia compared to its surface tensions:

Laminar vsturbulent flow

Wave propagation, outlet effects, obstructions

Formation of droplets and bubbles.

P = 100 bar1 m/s

Corresponds to 10 m/s

Conditions in pipeline

1 m/s ρ = 1 kg/m3

Hydrodynamic forces proportional to rU2

Wind = 3 m/sLight breeze

Gas – liquid interaction: governed by Dρ*DU2

P = 100 bar

ρ = 600 kg/s

Ug = 3 m/s

Corresponds to more than 30 m/s, i.e. Full Storm

Typical gas-condensate pipe: Gas velocity of 6 – 7 m/s, corresponding to twice Hurricane force winds

Conditions in pipeline

Conditions in pipeline – Drops and bubbles

Liquid layer can be significantly aerated (40% - 70%)

Hydrocarbon systems can have very low surface tension, in particular gas-condensate systems. Encourages generation of smaller drops and bubbles.Typical values: Air – water: 0.07 N/m vs. Gas – condensate: < 0.005 N/m

P = 100 bar

3 – 6 m/s

3 – 6 m/s

2

2

tensionSurfacenalGravitatio

tensionSurfaceInertial

dgEo

dUWe

Drop/bubble sizesCapillary waves

60 mm/h

90 000 mm/hmeasured in lab

Test facilities for study of multiphase flow behaviour

Open and closed loopsOpen loops with air as the gas phase – atmospheric pressure

• Simple to build, relatively low cost• Few safety barriers• Liquid phase e.g. water, vegetable oil • Common at universities

Closed, pressurised flow loops • More complex design, higher costs• More realistic gas-liquid density ratio• Crude oils possible (unstable, EX)• Safety barriers against pressure burst

and explosion

MEK 4450 Multiphase Flow - IFE Oct. 22, 2013

Design considerations Main goal for a test loop: • Establish well controlled and relevant multiphase flows Common requirements: • Length/diameter ratio , L>300 D – flow develops along the pipe• Large diameter – diameter scaling difficult • Easily changeable pipe inclination• High gas density to give relevant gas-liquid density ratio• Large span in flow rates

Cost-benefit:• Pressure vs gas density; pressure drives costs• Flow velocities vs pipe diameter; Flow rates drives costs – pumps and

separator • High L/D and pipe inclination drives cost of building

Some test facilities in Norway• IFE Well Flow Loop

• + All inclinations • + Indoor • + High gas density • + Transparent pipes • + Cost effective

• SINTEF – Large Sc.• + Large L/D • + Large diameter • + High pressure, N2

• Statoil - Herøya• + Real oil-gas system • + Formation water • + High pressure• + Long, large L/D

• - Short, low L/D • +/- Medium diam.

• - Fixed inclination• - Expensive to run• - Outdoor

• - Cumbersome to change inclination

• - Small diameter• - Steel pipe • – Expensive to run• - Outdoor

MEK 4450 Multiphase Flow - IFE Oct. 22, 2013

The Well Flow Loop – Principal LayoutComponent list:1: Oil-water separator2: Gas-liquid separator3: Gas compressor 4: Water pump5: Oil pump6: Heat exchanger, gas 7: Heat exchanger, water8: Heat exchanger, oil9: Main el. board10: Flow rate meter, gas

11. Flow rate meter, water12: Flow rate meter, oil 13: Inlet mixing section14: Slug catcher, pre-separator15: Return pipe, gas 16: Return pipe, liquid17: Test section

18: Winch

Worldwide test loops

Worldwide test loops

Instrumentation

• Gamma densitometers• PIV (Particle image velocimetry)• X-Ray tomography• LDA/PDA (Laser Doppler anemometry/Phase Doppler anemometry)• ECT (electrical capacitance tomography)• FBRM (Focused beam reflectance measurement)• PVM (Particle vision and measurement)• Shear stress probes

Pressure gradients• Differential pressure transducers;

many measurement principles, accuracy, response times etc.

• Connected to an upstream and downstream pressure tap (small holes in the wall)

• The connecting pipe is called impulse pipe.

• Pressure tap can be top/bottom/side mounted

• Distance between pressure taps can vary widely (1 m – 100 m)

• Measures wall friction and the hydrostatic pressure difference between the taps

• dp/dz [Pa/m]= dp/dL, where dp is the differential pressure measured with the transducer and dL is the distance between the tappings

Holdup=Cross-sectional liquid fraction (H=1-a)• Gamma densitometer • Attenuation of photon flux due

to absorption and scattering

• Single media:

where N is the intensity, m is the attenuation coefficient (material property) and x is the distance travelled in the media

• Two-phase gas-liquid• This can be developed to

and explicit equation for the Holdup