Gas Material Balance Equation

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Gas Material Balance Equation

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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 = ( P_i / z_i ) - [ ( P_sc T ) / ( T_sc V ) ] G_p

Where:

( P_i/z_i ) = Ratio of Pressure to Compressibility Factor at Initial Conditions (psi)
P_sc = Pressure at Standard Conditions (psi)
T_sc = Temperature at Standard Conditions (°R)
T = Current Temperature (°R)
V = Original Gas Volume (ft³)
G_p = 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.

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