doc150617 Intro to Muph and Wet Gas (Final) [Compatibility Mode]
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150617 Intro to Muph and Wet Gas (Final) [Compatibility Mode]
Introduction to Multiphase & Wet Gas Flows Terri Leonard Flow Measurement Engineer Contents Introduction to Multiphase & Wet Gas Flow Flow Patterns Characterisation & Terminology Traditional Measurement Methods Multiphase & Wet-Gas Flow Measurement Technologies Flow Meter Selection & Verification Why is Measurement Important? Measurement plays an important role in the UK economy Trade requires a regulatory framework based upon measurement confidence National Measurement Office (NMO) National Measurement System (NMS) 3 National Measurement Institutes deliver the UK’s NMS Responsible for stimulating good measurement practice Represents position of UK measurement internationally INTRODUCTION TO MULTIPHASE & WET GAS FLOW Oil / Gas not produced as a single phase fluid Water and gas present GAS OIL WATER Technically speaking it’s actually multi-component flow CHARACTERISATION & TERMINOLOGY Void Fraction and Liquid Hold-Up Simultaneous flow of two or more immiscible fluids Typically OIL / WATER / GAS α Gas GAS α OIL OIL Q GAS Q OIL WATER α WATER Gas “Void Fraction” Liquid “Hold-up” Q WATER = α GAS = α WATER + α OIL Volumetric Flowrate Ratios CHARACTERISED BY : Volumetric Flowrate Ratios α Gas αOIL GAS OIL WATER QGAS QOIL QWATER αWATER Gas Volume Fraction (GVF) = Water to Liquid Ratio (WLR) = QG / (QG + QO + QW) QW / (QO + QW) ALSO CHARACTERISED BY : Flow Regime / Pattern GVF & Gas Void Fraction 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. Phase Slip Gas and liquid travel at different velocities Mean gas velocity is greater than mean liquid velocity Difference known as “slip”: or “slip ratio”: vR = vg – vl K = vg /vl Note: GVF is related to void fraction eg and slip ratio K through εgK GVF = 1− ε + ε K g g Example: If εg = 70% and K = 8.1, then GVF = 95% Homogeneous Flow Liquid and gas travel at same mean velocity (v) vl = v g For homogeneous flow, K = 1, so equation from previous slide gives ε g = GVF A homogeneous flow can be assigned a single value of properties like density, viscosity, etc based on weighted average of phase mass fractions Inversion Region oil continuous water continuous inversion region 45% < water cut < 75% THE INVERSION POINT MOVES Superficial Phase Velocity The velocity a particular phase would have if the same volume flowrate flowed alone in the pipe e.g. Pipe diameter Gas flowrate Liquid flowrate = 6 inch = 950 m3/hr = 50 m3/hr Superficial Gas Velocity (SGV) = 14.9 m/s Superficial Liquid Velocity (SLV) = 0.8 m/s 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 gas liquid Froude Number Froude number, Fr, of each phase Liquid Frl = Gas v s ,l gD High Fr: Low Fr: ρl ρl − ρ g Frg = vs , g gD Kinetic energy dominates Gravity forces dominate ρg ρl − ρ g Lockhart-Martinelli Parameter Lockhart-Martinelli parameter, X General definition X = FrL FrG = ml mg ρg ρl = Ql Qg ρl ρg (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% FLOW PATTERNS / REGIMES Flow Patterns / 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 pipe orientations Horizontal Flow Patterns Separated flow regimes QGAS QLIQ STRATIFIED QGAS QLIQ 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 QGAS QLIQ PLUG QGAS QLIQ SLUG Alternating regions of high and low liquid hold-up As liquid flowrate increases liquid phase dominates flow Horizontal Flow Patterns Occurs at high liquid velocities Gas bubbles are suspended in continuous liquid phase QGAS QLIQ BUBBLE Occurs at high gas velocities Gas flows in central core / liquid as film on pipe walls QGAS QLIQ ANNULAR Horizontal Flow Patterns As gas velocity and / or gas density increases the liquid starts to becomes entrained as droplets in the gas flow QGAS QLIQ ANNULAR MIST Increasing gas velocity and/or gas density The liquid becomes completely entrained as droplets in the gas flow QGAS QLIQ MIST Horizontal Flow Patterns Typical Horizontal Flow Pattern Map (2 phases) Not general! Applies only to a specific fluid and pressure 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 Bubble Mist Flow Liquid entrained as droplets Slug Churn Annular Annular Mist INCREASING GAS VELOCITY Mist Vertical Flow Patterns Typical Vertical Flow Pattern Map (2 phases) Not general! Applies only to a specific fluid and pressure Multiphase Flow Patterns Three-phase Separation OIL can separate from WATER at low velocities – – More likely in horizontal flow Occurs in stratified and slug flow regimes GAS OIL WATER – QGAS QOIL QWATER Oil and water remain better mixed in vertical (up) flow Effect of Upstream Conditions 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 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) Carry-over and Carry-under GAS WATER OIL Traditional Measurement GAS MULTIPHASE FLOW 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 TECHNOLOGY - High GVF Multiphase Flows Wet-Gas Flow Measurement Wet-Gas Metering Differential Pressure Meters Commercial Wet-gas Meters Must correct for presence of liquid as causes meter to over-read Provides water, oil and gas flowrates Cheapest option Can use multiphase metering technology Need info on wetness to correct the gas flowrate New ISO TR 11583 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 : αO , αW , αG Gamma-Ray Absorption Electrical Properties Microwave etc. Calculate INDIVIDUAL phase flowrates from: QWAT = αW . QMIX QOIL = αO . QMIX QGAS = αG . QMIX Multiphase Metering Technologies BULK FLOWRATE : ∆P METER (e.g. VENTURI) Mass flow is function of DENSITY (ρ) and ∆P & = CD E ε A t 2 ρ ∆P m 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 ∆P Multiphase Metering Technologies DP METER OPERATION IN VERY UNSTEADY FLOWS Fast sampling required to reduce “averaging errors” Q (l/s) 30 20 10 ∆P (mbar) 900 10sec 400 100 10 sec Multiphase Metering Technologies BULK FLOWRATE : CROSS CORRELATION Compare response of 2 (+) axially displaced sensors Capacitance probes Densitometers Pressure Gauges SENSOR 1 SENSOR 2 Not applicable in single-phase, homogenous flow etc. Multiphase Metering Technologies DETECTOR PHASE FRACTION : GAMMA RAY ABSORPTION Number of gamma-rays detected : I = IO exp( - µ D ) Linear Absorption Coefficient µ depends on fluid in pipe GAS is weak absorber (µ low), WAT is strong absorber (µ high) Absorption probability also depends on gamma-ray energy (Eγ) Multiphase Metering Technologies PHASE FRACTION : GAMMA RAY ABSORPTION HIGH Energy Gamma: Absorption ∝ Fluid DENSITY only Transmitted Counts GAS OIL WATER 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 GAS GAS BUBBLE MIX Gives GAS/LIQ ratio Poor discrimination of OIL from WATER SLUG LIQ Multiphase Metering Technologies PHASE FRACTION : GAMMA RAY ABSORPTION For WLR include LOW Eγ : Absorption ∝ Fluid DENSITY + TYPE Transmitted Counts GAS OIL WATER Multiphase Metering Technologies PHASE FRACTION : GAMMA RAY ABSORPTION For WLR and GVF need both LOW Eγ and HIGH Eγ Plot I(EHIGH) vs I(ELOW) Gas I (EHIGH) Corners = Pure Phases Internal points = Mixtures Interpolate for GVF + WC 20% Mix Salinity changes = errors GVF Oil Wat 50% WC I (ELOW) Multiphase Metering Technologies PHASE FRACTION : ELECTRICAL PROPERTIES Capacitance Sensor − − − − Electrodes embedded in pipe wall Measure permittivity of mixture Use to derive WC of liquid phase OIL- continuous flow only Conductivity Sensor − Measure conductivity of mixture − WATER-continuous flow only Separate density measurement − Correct for effect of void fraction − Used to derive all three fractions Multiphase Metering Technologies PHASE FRACTION : MICROWAVE METHOD Microwave Cavity (~GHz) − − − − Flow passes through resonance cavity Microwaves reflect back and forth Adjust frequency fR for resonance fR depends on εMIX (permittivity) Microwave by energy absorption Microwave generator and receiver Microwave absorption related to bulk electrical properties Combine with density − To derive all three fractions FLOW METER SELECTION & VERIFICATION Flow Meter Selection 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? Validation of Flow Meters Before installation Joint Industry Projects (JIP) to evaluate technology Factory Acceptance Testing (FAT) • at a fully traceable independent facility such as NEL’s Multiphase & Wet Gas Flow Facilities After installation Against a test separator Other options..... • Against another meter? • Sampling? • Check sensors Refer to API 20.3 for further info Summary Flow patterns/regimes for horizontal and vertical flows Key definitions & characterising a multiphase flow Traditional method vs. Multiphase metering Wet-gas & Multiphase flow measurement technologies Considerations for selecting a meter and verifying the performance Multiphase flow measurement is much more challenging than single phase metering Thank you for listening Any questions? Email: [email protected] Website: www.tuvnel.com NEL Contact Tel: + 44 (0) 1355 220222 NEL Contacts Audit & Allocation CFD Densitometers Erosion Flow Consortium Heavy Oil Measurement Consultancy Measurement Uncertainty Meter Diagnostics MeterVue Multiphase PPDS Single Phase Metering Training Umbilicals Valve Testing Wet Gas Alick MacGillivray Neil Bowman Norman Glen John Peters Phil Mark Chris Mills Craig Marshall Chris Mills [email protected] [email protected] [email protected] [email protected] [email protected] [email protected] [email protected] [email protected] Alick MacGillivray Craig Marshall Phil Mark Terri Leonard Norman Glen Sarah Pedley John Dods John Dods John Dods Emmelyn Graham [email protected] [email protected] [email protected] [email protected] [email protected] [email protected] [email protected] [email protected] [email protected] [email protected] For general queries contact the sales team on [email protected]
