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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