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Gas Material Balance Equation

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Gas Material Balance Equation

The gas material balance data is based on actual production data and is typically presented as a P/z plot. The  material balance equation is a simple expression of the conservation of mass in a reservoir.

Eq. 1

P / z = ( Pi / zi ) - [ ( Psc T ) / ( Tsc V ) ] Gp


( Pi/zi ) = Ratio of Pressure to Compressibility Factor at Initial Conditions (psi)
Psc = Pressure at Standard Conditions (psi)
Tsc = Temperature at Standard Conditions (°R)
T = Current Temperature (°R)
V = Original Gas Volume (ft3)
Gp = Cumulative Gas Production (SCF)
P/z = Ratio of Pressure to Compressibility Factor at Current Conditions for P/z vs Gp plot (psi)

mD = Millidarcy
MSCF/d = Thousand standard cubic feet per day
Standard conditions are normally set at 60 degF and 14.7

Source: Reference: Ahmed, T., McKinney, P.D. 2005. Advanced Reservoir Engineering.