Page 274 - Introduction to Petroleum Engineering

GAS RESERVOIR MATERIAL BALANCE 261
TABLE 13.4 Drive Indices for the general Material Balance Equation
Drive Index
ND
Solution gas I o
sg
D HC
ND go
Gas cap I gc
D HC
W W B
Water I e p w
w
D HC
WB w GB g
i
Injected fluids I i
i D
HC
ND w D gw ND r
Connate water and rock expansion I
e
D
HC

Therefore, the remaining possible drive mechanisms are solution gas drive, water
drive from aquifer influx, and connate water and rock expansion drive. More
information is needed to determine which drive is dominating flow. In addition, it
is possible that the relative importance of the drive mechanisms will change during
the life of the reservoir.

13.5 gAS RESERVOIR MATERIAL BALANCE

Material balance in a gas reservoir can be derived from the general material balance
equation as follows. Original gas in place G is expressed in terms of the variables in
Table 13.2 as

GB gi NmB ti (13.19)
Combining Equation 13.19 with the general material balance equation gives

B B BS B B
N B B GB gc gi N tiwio tw twi
t ti gi
B 1 S B
gi wio twi i
BS B NB GB
gi wig B
G t w t wi i t gi cp (13.20)
1 S B 1 S 1 S f
wig t wi wio wig
NB GB GB GB NR B W W W W B
p o ps g pc gc i g p so g e i p w