Page 66 - Fundamentals of Reservoir Engineering

SOME BASIC CONCEPTS IN RESERVOIR ENGINEERING 5

− osmosis between waters having different salinity, the sealing shale acting as the
semi-permeable membrane in this ionic exchange; if the water within the seal is
more saline than the surrounding water the osmosis will cause an abnormally
high pressure and vice versa.

Some of these causes of abnormal pressuring are interactive, for instance, if a
reservoir block is uplifted the resulting overpressure is partially alleviated by a decrease
in reservoir temperature.

3
The geological textbook of Chapman provides a comprehensive description of the
mechanics of overpressuring. Reservoir engineers, however, tend to be more
pragmatic about the subject of abnormal pressures than geologists, the main questions
being; are the water bearing sands abnormally pressured and if so, what effect does
this have on the extent of any hydrocarbon accumulations?

So far only hydrostatic pressures have been considered. Hydrocarbon pressure
regimes are different in that the densities of oil and gas are less than that of water and
consequently, the pressure gradients are smaller, typical figures being

dp
= 0.45 psi/ ft
dD water

dp
= 0.35 psi/ ft
dD oil

dp
= 0.08 psi/ ft
dD gas

Thus for the reservoir containing both oil and a free gascap, shown in fig. 1.3; using the
above gradients would give the pressure distribution shown on the left hand side of the
diagram.

At the oil-water contact, at 5500 ft, the pressure in the oil and water must be equal
otherwise a static interface would not exist. The pressure in the water can be
determined using equ. (1.5), rounded off to the nearest psi, as
p w = 0.45 D + 15 (psia) (1.7)