Page 156 - Fundamentals of Reservoir Engineering

MATERIAL BALANCE APPLIED TO OIL RESERVOIRS 95

Fig. 3.10 Schematic of the production history of an undersaturated oil reservoir under
strong natural water drive

3.8 COMPACTION DRIVE AND RELATED PORE COMPRESSIBILITY PHENOMENA

The withdrawal of liquid or gas from a reservoir results in a reduction in the fluid
pressure and consequently an increase in the effective or grain pressure, the latter
being defined in Chapter 1, sec. 3, as the difference between the overburden and fluid
pressures. This increased pressure between the grains will cause the reservoir to
compact and this in turn can lead to subsidence at the surface.

Various studies 7,8,9,10 have shown that compaction depends only upon the difference
between the vertically applied stress (overburden) and the internal stress (fluid
pressure) and therefore, compaction can conveniently be measured in the laboratory
by increasing the vertical stress on a rock sample while keeping the fluid pressure in
the pores constant.

If V b is the bulk volume of a rock sample of thickness h, then the uniaxial compaction

∆V b/V b = ∆h/h

can best be determined in the laboratory using the triaxial compaction cell described by
11
Teeuw , which is shown in fig. 3.11 (a).
The core sample, which is completely saturated with water, is contained in a cell which
has permeable cap and base plates and a cylindrical, flexible sleeve surrounding it.
Vertical stress is applied by means of a piston while the fluid pressure in the pores is
maintained at one atmosphere. The pressure in the fluid surrounding the flexible sleeve
can be increased independently so as to maintain the condition of

vertical B
stress

permeable
disc ∆ h
h

lateral sample A
stress

elastic
sleeve

grain pressure

(a) (b)

Fig. 3.11 (a) Triaxial compaction cell (Teeuw); (b) typical compaction curve