Page 138 - Fundamentals of Reservoir Engineering

MATERIAL BALANCE APPLIED TO OIL RESERVOIRS 77

F = NE o (3.13)

in which the observed production, evaluated as an underground withdrawal, should plot
as a linear function of the expansion of the oil plus its originally dissolved gas, the latter
being calculated from a knowledge of the PVT parameters at the current reservoir
pressure. This interpretation technique is useful, in that, if a simple linear relationship
such as equ. (3.13) is expected for a reservoir and yet the actual plot turns out to be
non-linear, then this deviation can itself be diagnostic in determining the actual drive
mechanisms in the reservoir. For instance, equ. (3.13) may turn out to be non-linear
because there is an unsuspected water influx into the reservoir helping to maintain the
pressure. In this case equ. (3.12) can still be expressed in a linear form as
F W e (3.14)
E = n + E
o o

in which F/E o should now plot as a linear function of W e /E o.

Once a straight line has been achieved, based on matching observed production and
pressure data, then the engineer has, in effect, built a suitable mathematical model to
describe the performance of the reservoir. As previously described, in Chapter 1,
sec. 7, this phase is commonly referred to as a history match. Once this has been
satisfactorily achieved, the next step is to use the same mathematical model to predict
how the reservoir will perform in the future, possibly for a variety of production
schemes. This prediction phase is facilitated by the mathematical ease in using the
simple linear expressions for the material balance equation, as presented by Havlena
and Odeh. The technique will be illustrated in greater detail in the following sections.

3.4 RESERVOIR DRIVE MECHANISMS

If none of the terms in the material balance equation can be neglected, then the
reservoir can be described as having a combination drive in which all possible sources
of energy contribute a significant part in producing the reservoir fluids and determining
the primary recovery factor. In many cases, however, reservoirs can be singled out as
having predominantly one main type of drive mechanism in comparison to which all
other mechanisms have a negligible effect. In the following sections, such reservoirs
will be described in order to isolate and study the contribution of the individual
components in the material balance in influencing the recovery factor and determining
the production policy of the field. The mechanisms which will be studied are:
- solution gas drive

- gascap drive

- natural water drive
- compaction drive

And these individual reservoir drive mechanisms will be investigated in terms of:
- reducing the material balance to a compact form, in many cases using the
technique of Havlena and Odeh, in order to quantify reservoir performance