Page 71 - Fundamentals of Reservoir Engineering

SOME BASIC CONCEPTS IN RESERVOIR ENGINEERING 10

And, while it is easy to say, "simply by multiplying by the recovery factor", it is much
less easy to determine what the recovery factor should be for any given reservoir and,
indeed, it is the determination of this figure which is the most important single task of
the reservoir engineer.

For a start, one can clearly distinguish between two types of recovery factor. There is
one which is governed by current economic circumstances and, ever increasingly, by
environmental and ecological considerations, while the second can be classed as a
purely technical recovery factor depending on the physics of the reservoir-fluid system.
Regrettably, the former, although possibly the more interesting, is not a subject for this
book.

The two main categories of hydrocarbon recovery are called primary and
supplementary. Primary recovery is the volume of hydrocarbons which can be
produced by virtue of utilising the natural energy available in the reservoir and its
adjacent aquifer. In contrast, supplementary recovery is the oil obtained by adding
energy to the reservoir-fluid system. The most common type of supplementary
recovery is water flooding in which water is injected into the reservoir and displaces oil
towards the producing wells, thus increasing the natural energy of the system. The
mechanics of supplementary recovery will be described later, in Chapter 4, sec. 9 and
in Chapter 10; for the moment only primary recovery will be considered.

The entire mechanics of primary recovery relies on the expansion of fluids in the
reservoir and can best be appreciated by considering the definition of isothermal
compressibility.

1 ∂ V
c = — (1.11)
V ∂ p T

The isothermal compressibility is commonly applied in the majority of reservoir
engineering calculations because it is considered a reasonable approximation that as
fluids are produced, and so remove heat from the reservoir by convection, the cap and
base rock, which are assumed to act as heat sources of infinite extent, immediately
replace this heat by conduction so that the reservoir temperature remains constant.
Therefore, compressibility, when referred to in this text, should always be interpreted
as the isothermal compressibility.

The negative sign convention is required in equ. (1.11) because compressibility is
defined as a positive number, whereas the differential, ∂V/∂p, is negative, since fluids
expand when their confining pressure is decreased. When using the compressibility
definition in isolation, to describe reservoir depletion, it is more illustrative to express it
in the form

∆
dV = cV p (1.12)
where dV is an expansion and ∆p a pressure drop, both of which are positive. This is
the very basic equation underlying all forms of primary recovery mechanism. In the
reservoir, if ∆p is taken as the pressure drop from initial to some lower pressure,