Page 133 - Fundamentals of Reservoir Engineering

MATERIAL BALANCE APPLIED TO OIL RESERVOIRS 72

p i p

Gascap gas B
mNB (rb)
oi

Oil + originally
dissolved gas ∆p

C A

NB (rb)
oi

(a) (b)

Fig. 3.1 Volume changes in the reservoir associated with a finite pressure drop ∆ ∆∆ ∆p;
(a) volumes at initial pressure, (b) at the reduced pressure

taking all the surface production back down to the reservoir at this lower pressure) then
it should fit into the volume A + B + C which is the total volume change of the original
HCPV. Conversely, volume A + B + C results from expansions which are allowed to
artificially occur in the reservoir. In reality, of course, these volume changes correspond
to reservoir fluid which would be expelled from the reservoir as production. Thus the
volume balance can be evaluated in reservoir barrels as

Underground = Expansion of oil + originally dissolved gas (rb)
withdrawal (rb)

+ Expansion of gascap gas (rb)

+ Reduction in HCPV due to connate water expansion and
decrease in the pore volume (rb)

Before evaluating the various components in the above equation it is first necessary to
define the following parameters.

N is the initial oil in place in stock tank barrels
= V φ (1−S wc) / B oi stb

m is the ratio

initial hydrocarbon volume of the gascap
initial hydrocarbon volume of the oil

(and, being defined under initial conditions, is a constant)

N p is the cumulative oil production in stock tank barrels, and

R p is the cumulative gas oil ratio