Page 147 - Fundamentals of Reservoir Engineering

MATERIAL BALANCE APPLIED TO OIL RESERVOIRS 86

producing wells. This demonstrates a method of keeping as much gas in the reservoir
as possible where it can serve its most useful purpose, as suggested in exercise 3.2.

The economic success of both water and solution gas injection depends upon the
additional recovery obtained as a result of the projects. The present day value of the
additional oil recovery must be greater than the cost of the injection wells, surface
treatment facilities (mainly for water) and compressor costs (mainly for gas). In many
cases, for small reservoirs, injection of water or gas is not economically viable and the
solution gas drive process must be allowed to run its full course resulting in low oil
recovery factors.

3.6 GASCAP DRIVE

A typical gascap drive reservoir is shown in fig. 3.6. Under initial conditions the oil at
the gas oil contact must be at saturation or bubble point pressure. The oil further
downdip becomes progressively less saturated at the higher pressure and
temperature. Generally this effect is relatively small and reservoirs can be described
using uniform PVT properties, as will be assumed in this text. There are exceptions,
6
however, one of the most remarkable being the Brent field in the North Sea in which at
the gas oil contact the oil has a saturation pressure of 5750 psi and a solution gas oil
ratio of 2000 scf/stb, while at the oil water contact, some 500 feet deeper, the
saturation pressure and solution gas oil ratio are 4000 psi and 1200 scf/stb,
respectively. Such extremes are rarely encountered and in the case of the Brent field
the anomaly is attributed to gravity segregation of the lighter hydrocarbon components.

For a reservoir in which gascap drive is the predominant mechanism it is still assumed
that the natural water influx is negligible (W e = 0) and, in the presence of so much high
compressibility gas, that the effect of water and pore compressibilities is also
negligible. Under these circumstances, the material balance equation, (3.7), can be
written as

(
NB + (R − R )B g )
p
s
p
o
(B − B ) (R − R )B B (3.23)
+
= NB o oi si s g + m g − 1

oi
B oi B gi
in which the right hand side contains the term describing the expansion of the oil plus
originally dissolved gas, since the solution gas drive mechanism is still active in the oil
column, together with the term for the expansion of the gascap gas. Equation (3.23) is
rather cumbersome and does not provide any kind of clear picture of the principles
involved in the gascap drive mechanism. A better understanding of the situation can be
gained by using the technique of Havlena and Odeh, described in sec. 3.3, for which
the material balance, equ. (3.12), can be reduced to the form

F = N (E o + mE g) (3.24)