Introduction To Multiphase Flow
Multiphase ?Gas + DropletsLiquid HydrocarbonsGas bubblesWater dropletsFree WaterOil dropletsGas bubblesHydratesWaxSand
Multiphase flowComplexitiesInteraction between the phasesFluid propertiesInteraction with the boundary conditionsi.e. reservoir and process plantNetworkswells, flow lines, process equipment and control system Pipe profiles
Multiphase flow predictionsare difficult:Several flow regimesSlip effects
Flow regimes in OLGA(Horizontal flow)Stratified flow(Annular flow)Dispersed bubble flowSlug flowSEPARATEDDISTRIBUTED
Flow regimes in OLGA(Vertical flow)Annular flowDispersed bubble flowSlug flowSEPARATEDDISTRIBUTED
Pressure and FlowAssume outlet pressure given and fixedFriction dominatedGravity dominated
Potential problems in multiphase flowHYDRODYNAMIC SLUGGING
Hydrodynamic sluggingTwo-phase flow pattern maps indicate hydrodynamic slugging, but slug length correlations are quite uncertain tracking of the development of the individual slugs along the pipeline is necessary to estimate the volume of the liquid surges out of the pipelines
Potential problems in multiphase flowTERRAIN SLUGGINGA: Low spots fills with liquid and flow is blockedB: Pressure builds up behind the blockageC&D: When pressure becomes high enough, gas blows liquid out of the low spot as a slug
Potential problems in multiphase flowTERRAIN SLUGGINGTerrain slugs can be very severe, causing large pressure variations and liquid surges out of pipeline. Terrain slugging is a transient situation which requires a dynamic model to predict and describe
Potential problems in multiphase flowPIGGINGPigging the line will create a large liquid slug ahead of the pig
Potential problems in multiphase flowRATE CHANGESPipe line liquid inventory decreases with increasing flow rate Rate changes may trigger slugging
Potential problems in multiphase flowSHUT-IN - RESTARTLiquid redistributes due to gravity during shutinOn startup, slugging can occur as flow is ramped up
Other concerns with multiphase flow
Shutdown and startup Temperature profilesLiquid surges during startupPipeline blow down Temperature profile, gas and liquid flow rates Liquid accumulation and water separation in low pointsPigging operationsPipeline ruptures and other accidents
The Transient Multiphase Flow Simulator
The dynamic three phase flow simulator
The OLGA three phase flow modelMass conservationGasHydrocarbon bulkHydrocarbon dropletsWater bulkWater dropletsMomentum conservationGas + dropletsLiquid bulkEnergy conservationMixtureConstitutive equations
VariablesPrimary variables5 mass fractions (specific mass)2 velocities1 pressure1 temperatureSecondary variablesVolume fractionsVelocitiesFlow ratesFluid properties
Modeling the pipeline profile in OLGAThe assumption
Fluid propertiesGeneralThe fluid properties are pre-calculated tables as a function of P and T and for one fluid compositionIt follows that the total composition is constant throughout a fluid table1) The exact value of a property for a given P and T is found by interpolating in the relevant property table1) Compositional tracking available with OLGA 2000 v3.00
OLGA 2000 ModulesSlugtracking with piggingWaterthree-phase flowFEM -Thermconductive 2-D (radial) heat transferfinite elementsintegrated with OLGA bundlegrid generator
OLGA 2000 Modules Comptrackcompositional trackingMEG-trackUBitTS Under Balanced interactive transient Training SimulatorAdvanced WellMultiphase Pumpspositive displacementrotodynamicCorrosionWax
OLGA 2000 file structureOLGA 2000reflex of the Input File +results from OUTPUTTrend Plot Fileresults from TRENDProfile Plot Fileresults from PROFILERestart File(binary)ASCII file
Principles of OLGA 2000 execution OLGA 2000
Present typical phenomena taking place in multiphase flow.-not a multiphase flow course
Includes some validation of OLGA against lab data (SINTEF).
Interaction between subsystems - extremely important- stable boundary conditions- possible big impact of varying boundary conditions- boundary conditions varies in real life- OLGA-ECLIPSE, OLGA-TOPSIDES- heat transferWhat can be neglected?
Annular only at very high velocities
HYDRODYNAMIC SLUGGINGFlow regime transportationTerrain effectSlug interactionsSlug length distribution
Gas condensate lines only get into slug flow regime at very low flowrates since flowregime is generally in stratified or slug flow.
Low GOR lines are frequently in slug flow region at design rate
Gas condensate lines, high GOR lines:Steady state models will not indicate terrain slugging. Models will usually show stratified flow in pipeline followed by bubble flow in riser under conditions where terrain slugging occurs.
Low GOR lines:Can experience terrain slugging at low rates if there are low spots along the pipeline. The transition to terrain slugging is a concern; extent of hydrodynamic slugging.
If terrain slugging is present, how can it be stopped or minimized:
Raise velocitiesGas lift riserChoke flow out of riser
For gas condensate lines predictions of holdup and pressure drop are very sensitive to pipeline profile at low rates.
Holdup increases rapidly as gas velocity increases.
Considerably entrainment of liquid at high rates.
Generally for gas condensate lines, only slugs leaving lines are caused by rate changes or pigging
Steady state methods can predict the holdup at the original and final conditions, but can not tell what happens during the transition.
Transient programs can determine whether slugging occurs, and estimate its severityWill slugging be a problem on startup.
Transient programs can assess potential for startup slugging.Temperature profile:Cooldown time to hydrate formation temperature.Time to exceed hydrate formation tempreature during restart (chemical injection).
Wax deposition is more related to normal production - Long term production at temperatures below the cloud point causes wax deposition on the pipe wall.
Can problems be solved with insulation, bundled pipelines or pipeline depressuration.The tables can be generated by PVTOL