Page 30 - Principles of Applied Reservoir Simulation 2E

Part I: Reservoir Engineering Primer 1 5

production data is reliable; and gravity segregation of phases can be neglected.
A discussion of the relative importance of drive mechanisms obtained from Eq,
(2.3) is presented in Chapter 8.

Gas Reservoir Material Balance
The general material balance equation for a gas reservoir can be derived
from Eq. (2.3) by first recognizing the relationship

(2,6)
GB gi = NmB H
defines original gas in place G. Substituting Eq. (2.6) into Eq. (2.3) gives the
general material balance equation

N(B, - B,.
'

B giS wlg ( B» - B\ ( NB ti GB gi } (2.7)
' — __ _j. ^ ur
I- S.__( 5., J 1- 5..... 1- S...J f
*A + fas, + G^ - G,jy] - ]V pJR so5 g

Equation (2.7) is further simplified by recognizing that the material balance for
a gas reservoir does not include oil in place so that N - 0 and N p = 0. The
resulting material balance equation is

B B GB
~ -

Water compressibility and formation compressibility are relatively small
compared to gas compressibility. Consequently, Eq. (2.8) is often written in the
simplified form
GB ( B - B } = B G B w + w W B (2 9)
^ ^ « - < *' - < - ^ - -
- (