Page 144 - Fundamentals of Reservoir Engineering

MATERIAL BALANCE APPLIED TO OIL RESERVOIRS 83

A simpler and more direct method is to consider that

liberated gas initial total current oil
in the volume of oil volume in
= −
reservoir in the reservoir the reservoir

i.e. liberated gas = NB oi − (N − N p) B o (rb)

and therefore

S g = (NB oi − (N − N p) B o) (1 − S wc) / NB oi

or
N
p B
−
S g = 1− 1− o (1 S ) (3.22)
wc
NB oi

which at abandonment pressure becomes
N p
×
S g = 1− 1− 0.88 0.8
N

again showing that if the gas is kept in the reservoir so that S g has a high value then
N p/N will be large, and vice versa.

Naturally equs. (3.21) and (3.22) are equatable through the material balance
equ. (3.20).
Although the lesson of the last exercise is quite clear, the practical means of keeping
the gas in the ground in a solution gas drive reservoir is not obvious. Once the free gas
saturation in the reservoir exceeds the critical saturation for flow, then as noted already
in Chapter 2, sec. 2, the gas will start to be produced in disproportionate quantities
compared to the oil and, in the majority of cases, there is little that can be done to avert
this situation during the primary production phase. Under very favourable conditions
the oil and gas will separate with the latter moving structurally updip in the reservoir.
This process of gravity segregation relies upon a high degree of structural relief and a
favourable permeability to flow in the updip direction. Under more normal
circumstances, the gas is prevented from moving towards the top of the structure by
inhomogeneities in the reservoir and capillary trapping forces. Reducing a well's offtake
rate or closing it in temporarily to allow gas-oil separation to occur may, under these
circumstances, do little to reduce the producing gas oil ratio.

A typical producing history of a solution gas drive reservoir under primary producing
conditions is shown in fig. 3.4.

As can be seen, the instantaneous or producing gas oil ratio R will greatly exceed R si
for pressures below bubble point and the same is true for the value of R p. The pressure
will initially decline rather sharply above bubble point because of the low
compressibility of the reservoir system but this decline will be partially arrested once