Page 149 - Fundamentals of Reservoir Engineering

MATERIAL BALANCE APPLIED TO OIL RESERVOIRS 88

m - too small correct value of m
F
(rb)

m - too large

(E + mE ) (rb / stb)
o
g
Fig. 3.7 (a) Graphical method of interpretation of the material balance equation to
determine the size of the gascap (Havlena and Odeh)

N p and cumulative gas oil ratio R p are listed in table 3.1, as functions of the average
reservoir pressure, over the first few years of production. (Also listed are the relevant
PVT data, again taken from table 2.4, under the assumption that, for this particular
application, p I = p b = 3330 psia).

Pressure N p R p B o R s B g
psia MMstb scf/stb rb/stb scf/stb rb/scf

3330 (p i = p b) 1.2511 510 .00087
3150 3.295 1050 1.2353 477 .00092
3000 5.903 1060 1.2222 450 .00096
2850 8.852 1160 1.2122 425 .00101
2700 11.503 1235 1.2022 401 .00107
2550 14.513 1265 1.1922 375 .00113
2400 17.730 1300 1.1822 352 .00120
TABLE 3.1

The size of the gascap is uncertain with the best estimate, based on geological
information, giving the value of m = 0.4. Is this figure confirmed by the production and
pressure history? If not, what is the correct value of m?

EXERCISE 3.4 SOLUTION

Using the technique of Havlena and Odeh the material balance for a gascap drive
reservoir can be expressed as

F = N (E o + mE g ) (3.24)

where F, E o and E g are defined in equs. (3.8 − 10). The values of these parameters,
based on the production, pressure and PVT data of table 3.1, are listed in table 3.2.