Page 64 - Fundamentals of Reservoir Engineering

SOME BASIC CONCEPTS IN RESERVOIR ENGINEERING 3

The manner in which the oil water contact, or fluid contacts in general, can be located
requires a knowledge of fluid pressure regimes in the reservoir which will be described
in the following section.

1.3 FLUID PRESSURE REGIMES

The total pressure at any depth, resulting from the combined weight of the formation
rock and fluids, whether water, oil or gas, is known as the overburden pressure. In the
majority of sedimentary basins the overburden pressure increases linearly with depth
and typically has a pressure gradient of 1 psi/ft, fig. 1.2.

14.7 Pressure (psia)

Depth
(ft)
FP GP

overburden
pressure
overpressure (OP)

underpressure
normal hydrostatic
pressure

Fig. 1.2 Overburden and hydrostatic pressure regimes (FP = fluid pressure;
GP = grain pressure)

At a given depth, the overburden pressure can be equated to the sum of the fluid
pressure (FP) and the grain or matrix pressure (GP) acting between the individual rock
particles, i.e.

OP FP GP (1.3)
+
=
and, in particular, since the overburden pressure remains constant at any particular
depth, then

)
( dFP =− ( d GP ) (1.4)

That is, a reduction in fluid pressure will lead to a corresponding increase in the grain
pressure, and vice versa.