Page 158 - Fundamentals of Reservoir Engineering

MATERIAL BALANCE APPLIED TO OIL RESERVOIRS 97

deformation in the reservoir is shown in fig. 3.12, which is taken from the paper of
Merle et al. 12

COMPACTION

COMPACTION DUE
TO PRODUCTION
∆h
h
START
PRODUCTION D

unloading B
C ′′ ′ ′

BURIAL
C

DEPOSITION
A grain pressure

Fig. 3.12 Compaction curve illustrating the effect of the geological history of the
reservoir on the value of the in-situ compressibility (after Merle)

When the reservoir sand is initially being deposited it is at point A on the compaction
curve, fig. 3.12. Over geological times, as more and more material is deposited, the
original sand becomes buried corresponding to point B, with grain pressure p B.
Following this normal deposition, events can occur which will reduce the grain pressure
below p B, such as:

- uplifting of the reservoir

- erosion of the surface layers above the reservoir
- overpressuring of the fluid in the reservoir.

As a result of one or more of these effects, in the extreme cases of either completely
elastic or completely inelastic deformation of the rock during deposition, the reservoir in
fig. 3.12 will be either at C or C′, respectively, corresponding to the reduced grain
pressure p C In the former case, for elastic deformation, if the reservoir is produced with
an initial grain pressure p C then the compaction will start immediately since the uniaxial
compressibility at point C is finite. In the completely inelastic case, however, there. will
be a time lag between starting to produce the reservoir and the occurrence of any
significant degree of compaction. This is because the uniaxial compressibility in this
latter case is the tangent to the compaction curve at point C′, which is extremely small.
As shown in fig. 3.12, there will be very little compaction in the reservoir until sufficient
fluids have been removed to increase the grain pressure to p B which is the maximum
grain pressure experienced by the reservoir in the past.