Page 134 - Fundamentals of Reservoir Engineering

MATERIAL BALANCE APPLIED TO OIL RESERVOIRS 73

Cumulative gas production (scf)
=
Cumulative oil production (stb)

Then the expansion terms in the material balance equation can be evaluated as
follows.

a) Expansion of oil plus originally dissolved gas
There are two components in this term:

- Liquid expansion

The N stb will occupy a reservoir volume of NB oi rb, at the initial pressure, while
at the lower pressure p, the reservoir volume occupied by the N stb will be NB o,
where B o is the oil formation volume factor at the lower pressure. The difference
gives the liquid expansion as

N(B − B ) (rb) (3.1)
oi
o
- Liberated gas expansion

Since the initial oil is in equilibrium with a gascap, the oil must be at saturation or
bubble point pressure. Reducing the pressure below p i will result in the liberation
of solution gas. The total amount of solution gas in the oil is NR si scf. The amount
still dissolved in the N stb of oil at the reduced pressure is NR s scf. Therefore, the
gas volume liberated during the pressure drop ∆p, expressed in reservoir barrels
at the lower pressure, is

N(R − R ) B g (rb) (3.2)
si
s
b) Expansion of the gascap gas
The total volume of gascap gas is mNB oi rb, which in scf may be expressed as

mNB
G = oi (scf)
B gi

This amount of gas, at the reduced pressure p, will occupy a reservoir volume

B
mNB g (rb)
oi
B gi

Therefore, the expansion of the gascap is

B
mNB oi g − 1 (rb) (3.3)

B gi

c) Change in the HCPV due to the connate water expansion and pore volume
reduction