Page 86 - Fundamentals of Reservoir Engineering

SOME BASIC CONCEPTS IN RESERVOIR ENGINEERING 25

To convert this number to a pressure gradient in psi/ft requires the following
manipulation.

dp Ib 1 ft 2 6.696
= 6.696 2 × × 2 = = 0.0465 psi/ ft.
dD gas ft ft inch 144

1.7 GAS MATERIAL BALANCE: RECOVERY FACTOR

The material balance equation, for any hydrocarbon system, is simply a volume
balance which equates the total production to the difference between the initial volume
of hydrocarbons in the reservoir and the current volume. In gas reservoir engineering
the equation is very simple and will now be considered for the separate cases in which
there is no water influx into the reservoir and also when there is a significant degree of
influx.

a) Volumetric depletion reservoirs

The term volumetric depletion, or simply depletion, applied to the performance of a
reservoir means that as the pressure declines, due to production, there is an
insignificant amount of water influx into the reservoir from the adjoining aquifer. This, in
turn, implies that the aquifer must be small (refer sec. 1.4). Thus the reservoir volume
occupied by hydrocarbons (HCPV) will not decrease during depletion. An expression
for the hydrocarbon pore volume can be obtained from equ. (1.26) as

HCPV = Vφ (1−S wc) = G/E i

where G is the initial gas in place expressed at standard conditions. The material
balance, also expressed at standard conditions, for a given volume of production G p,
and consequent drop in the average reservoir pressure ∆p = p i−p is then,

Production = GIIP − Unproduced Gas
(sc) (sc) (sc)
(1.33)
G p = G − (HCPV)E

G
G = G − E
p
E i
which can be expressed as

G p E (1.34)
G = 1 − E i

or, using equ. (1.25), as

p p G
p
= i 1 − (1.35)
Z Z i G