Gas Material Balance

Introduction

The base material balance equation used for analyses in conventional reservoirs is shown below:

In some cases, it may be expressed as:

And if we ignore compressiblity effects of water and formation, we have the basic equation shown below, which is effectively slope-intercept form (I.e., y=m+bx)

Which can be further simplified to:

Where:

P_r = Current pressure (psi, kPa)

z = Gas deviation factor at P, dimesionless

z_i = Initial gas deviation factor, dimesionless

OGIP = Original gas-in-place (Bcf, MMscf, Scf, 10³ m, etc), often displayed just as G

Gp = Gas produced (Bcf, MMscf, Scf, 10³ m, etc), also called cumulative gas production

C_w = Water compressibility , (1/psi, 1/kPa)

S_w = Water saturation, dimensionless

C_f = formation compressibility, (1/psi, 1/kPa)

P/z Plots

Using the above, a plot of P/z vs gas production (Gp) will produce an extrapolation to OGIP as shown below. When the sample data below is plotted, it results in an extrapolated OGIP of 13 MMscf.

See Also:

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