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 out in Conservation of Energy Mass Momentum Thermodynamics

Mass conservation out in 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

Momentum balance – single-phase: PR PL Friction Pressure gradient large enough for flow: Velocity depends on friction Stasjonær tilstand Friction = Friction force per area * wall area Veggskjærspenning

Multiphase Pipe Flow Depends on: Fluid properties Pipe geometry Environment Density Diameter T, external Viscosity Wall roughness Insulation Phase fractions Pipeline profile/ T at inlet Conductivity topography P at inlet Heat capacity P at outlet Surface tension Etc... 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 flow Three-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 Stratified flow. Ugas normally >> Uoil Large waves: More effective liquid transport

Hydrodynamic slugging Taylor-bubble Liquid slug 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 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 correlation Lab Field

Dimensionless numbers – dynamic similarity :4               Dimensionless numbers – dynamic similarity Laminar vs turbulent flow Wave propagation, outlet effects, obstructions Formation of droplets and bubbles. 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:

Conditions in pipeline Hydrodynamic forces proportional to rU2 1 m/s ρ = 1 kg/m3 P = 100 bar 1 m/s Corresponds to 10 m/s

Conditions in pipeline Gas – liquid interaction: governed by Dρ*DU2 Wind = 3 m/s Light breeze 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 – Drops and bubbles 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 60 mm/h 3 – 6 m/s Drop/bubble sizes Capillary waves P = 100 bar 3 – 6 m/s 90 000 mm/h measured in lab Liquid layer can be significantly aerated (40% - 70%)

Test facilities for study of multiphase flow behaviour

Open and closed loops Open 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

The Well Flow Loop – Principal Layout Component list: 1: Oil-water separator 2: Gas-liquid separator 3: Gas compressor 4: Water pump 5: Oil pump 6: Heat exchanger, gas 7: Heat exchanger, water 8: Heat exchanger, oil 9: Main el. board 10: Flow rate meter, gas 11. Flow rate meter, water 12: Flow rate meter, oil 13: Inlet mixing section 14: Slug catcher, pre- separator 15: Return pipe, gas 16: Return pipe, liquid 17: Test section 18: Winch MEK 4450 Multiphase Flow - IFE Oct. 22, 2013

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