Page 143 - Fundamentals of Reservoir Engineering

MATERIAL BALANCE APPLIED TO OIL RESERVOIRS 82

gas in the reservoir the total reservoir system compressibility in the simple material
balance

dV = cV ∆p

will be greatly increased by the presence of the gas and the dV, which is the
production, will be large for a given pressure drop.

N p
%
N 900
50

40

30

20

10

0
0 1000 2000 3000 4000
R (scf / stb)
p
Fig. 3.3 Oil recovery, at 900 psia abandonment pressure (% STOIIP), as a function of
the cumulative GOR, R p (Exercise 3.2)

2) The free gas saturation in the reservoir may be deduced in two ways, the most obvious
being to consider the overall gas balance

liberated total gas gasstill
gasinthe amount producedat dissolved
= − −
reservoir of gas thesurface intheoil

which in terms of the basic PVT parameters can be evaluated at any reservoir pressure
as

liberated gas (rb) = (NR si – N pR p − (N − N p) R s) B g
and expressing this as a saturation, which is conventionally required as a fraction of
the pore volume, then

S g = [ N (R si − R s) − N p (R p − R s) ] B g (1 − S wc) / NB oi (3.21)
where NB oi / (1 − S wc) = HCPV / (1 − S wc) = the pore volume.