Derivation of Material Balance Equations

Gas Reservoir Material Balance Equation

5.6.1.1 Dry Gas Reservoir Without Water Influx

Applying the law of conservation of mass on Fig. 5.1, it states that the mass of the

gas initially in place in the reservoir is equal to the amount of gas produced plus the

amount of gas remaining in the reservoir. Recall that gas expands to fill the shape of

its container. Hence, in terms of volume balance, it simply states that the volume of

gas originally in place at the reservoir conditions is equal to the volume of gas

remaining in the reservoir at the new pressure-temperature conditions after some

amount of gas has been produced. Since the pressure of the reservoir has dropped

with a corresponding decrease in the volume of gas due to the amount that have been

produced, therefore the remaining amount of gas in the reservoir would have

expanded to occupy the same volume as that initially in place. Mathematically, we

have that;

A plot of P=z versus Gp gives the x-intercept as the initial gas in place and the

y-intercept as Pi =zi (Fig. 5.2)

Adjustment to Gas Saturation in Water Invaded Zone

The initial gas in place in reservoir volume expressed in terms of pore volume

(PV) is:

GBgi ¼ PVð