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WET GAS
BASIC WORKFLOW FLUID
EQUATIONS
Reservoir Fluid Fundamentals
Richard Henry [email protected]
Abstract A collection of equations used during basic workflows on wet gas fluids.
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ContentsRecombination .............................................................................................................................................. 2
Molecular Mass ............................................................................................................................................. 3
Pseudo‐Critical Properties ............................................................................................................................ 3
SI Conversions ........................................................................................................................................... 4
Output ................................................................................................................................................... 4
Input ...................................................................................................................................................... 4
Formation Volume Factor ............................................................................................................................. 4
Macros .......................................................................................................................................................... 5
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Recombination
𝛾𝑛 𝑀 𝑛 𝑀
𝑛 𝑛 𝑀
Equation 1: Reservoir gas gravity by recombination
𝑛𝑝𝑉
𝑍𝑅𝑇
𝑛𝑎 𝜌𝑀
Where: Oilfield SI
.γgR reservoir gas gravity [1/air] [1/air]
.Mwg surface gas molecular mass [lb/mol] [g/mol]
.Mwo surface oil molecular mass [lb/mol] [g/mol]
.Mwair air apparent molecular mass [lb/mol] [g/mol]
.nv moles of surface gas [lb‐mol] [g‐mol]
.nL moles of surface oil [lb‐mol] [g‐mol]
.V volume of surface gas [cuft] [m3]
.p pressure at which surface gas volume is measured [psia] [kPa]
.T temperature at which the surface gas volume is measured
[oR] [K]
.Z Z‐factor of surface gas [ ] [ ]
.R universal gas constant [psia.cuft]/[mol.oR] [J/g‐mol/K]
.a01 volume of surface oil 5.61458 [cuft] 1000 [m3]
.ρo density of surface oil [lb/cuft] [kg/m3]
𝛾𝛾 𝑅 𝑎 𝛾
𝑅 𝑎𝛾
𝑀
Equation 2: Reservoir gas gravity by recombination (field version)
Where: Oilfield SI
.γgR reservoir gas gravity [1/air] [1/air]
.Rs produced gas‐oil ratio [scf/stb] [sm3/sm3]
.γo specific gravity of surface oil [1/wtr] [1/wtr]
.γg specific gravity of surface gas [1/air] [1/air]
.Mwo surface oil molecular mass [lb/mol] [g/mol]
.a00 Conversion constant 4591 816.3
.a01 Conversion constant 132,983 23,644.1
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𝛾∑ 𝑅 𝛾
∑ 𝑅
𝑅 𝑅
Where: Oilfield SI
.Rspi produced separator gas/stock tank oil ratio for separator i. [scf/stb] [sm3/sm3]
.γgspi produced separator gas gravity for separator I [1/air] [1/air]
MolecularMass𝑀 𝛾 𝑀
Equation 3: Gas apparent molecular mass
𝑀𝑎
𝑎 𝛾Equation 4: Cragoe correlation for oil apparent molecular mass
Where: Oilfield SI
.γg gas specific gravity [1/air] [1/air]
.γAPI oil specific gravity [oAPI] [oAPI]
.Mwg surface gas molecular mass [lb/mol] [g/mol]
.Mwo surface oil molecular mass [lb/mol] [g/mol]
.Mwair air apparent molecular mass [lb/mol] [g/mol]
.a00 constant 6084
.a01 constant ‐5.9
Pseudo‐CriticalProperties𝑝 𝑎 𝑎 𝛾 𝑎 𝛾 Equation 5: Standing's pseudo‐critical pressure correlation
𝑇 𝑎 𝑎 𝛾 𝑎 𝛾 Equation 6: Standing's pseudo‐critical temperature correlation
Where: Oilfield
.ppc pseudo‐critical pressure [psia]
.Tpc pseudo‐critical temperature [oR]
.𝛾g specific gravity [1/air]
a00‐a12 are constants
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Constant Value Constant Value
a00 706 a10 187
a01 ‐51.7 a11 330
a02 ‐11.1 a12 ‐71.5
SI Conversions Output ppc [kPa] = ppc [psia] ÷ 0.145037738
Tpc [K] = Tpc [oR] ∙ 5 ÷ 9
Input [None]
FormationVolumeFactor
𝐵𝑍𝑇𝑝
𝑝𝑛 𝑍 𝑇
Equation 7: Wet gas formation volume factor
Where: Oilfield SI
.p pressure of interest [psia] [kPa]
.T temperature of interest [oR] [K]
.nV Mole fraction of reservoir gas that remains gas under surface conditions
[mol/mol] [mol/mol]
.Z compressibility factor at conditions of interest [ ] [ ]
.psc standard pressure [psia] [kPa]
.Tsc standard temperature [oR] [K]
.Zsc compressibility factor at standard conditions [ ] [ ]
𝑛𝑛
𝑛 𝑛
Where: Oilfield/SI
.nV Mole fraction of reservoir gas that remains gas under surface conditions [mol/mol]
.ng Moles of surface gas in reservoir gas [mol]
.no Moles of stock‐tank oil in reservoir gas [mol]
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MacrosName Purpose Qualifier Units Source Date Module
crag_mw0 Molecular Mass Oil [lb/mol] Cragoe 1929 Plus
stan_pc Critical Pressure Gas [psia] Brown & Standing 1948 Z
stan_Tc Critical Temperature Gas [oF] Brown & Standing 1948 Z
abou_Z Z‐Factor Gas [ ] Dranchuk & Abou‐Kassem
1975 Z
Lee1_Ugb Viscosity Gas, Live [cP] Lee, Gonzalez & Eakin 1966 Viscous
Lee1_Ugd Viscosity Gas, Dead [cP] Lee Gonzalez & Eakin 1966 Viscous
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