Page 65 - Fundamentals of Reservoir Engineering

SOME BASIC CONCEPTS IN RESERVOIR ENGINEERING 4

Fluid pressure regimes in hydrocarbon columns are dictated by the prevailing water
pressure in the vicinity of the reservoir. In a perfectly normal case the water pressure at
any depth can be calculated as

dp
p w × D + 14.7 (psia) (1.5)
dD water

in which dp/dD, the water pressure gradient, is dependent on the chemical composition
(salinity), and for pure water has the value of 0.4335 psi/ft.

Addition of the surface pressure of one atmosphere (14.7 psia) results in the
expression of the pressure in absolute rather than gauge units (psig), which are
measured relative to atmospheric pressure. In many instances in reservoir engineering
the main concern is with pressure differences, which are the same whether absolute or
gauge pressures are employed, and are denoted simply as psi.

Equation (1.5) assumes that there is both continuity of water pressure to the surface
and that the salinity does not vary with depth. The former assumption is valid, in the
majority of cases, even though the water bearing sands are usually interspersed with
impermeable shales, since any break in the areal continuity of such apparent seals will
lead to the establishment of hydrostatic pressure continuity to the surface. The latter
assumption, however, is rather naive since the salinity can vary markedly with depth.
Nevertheless, for the moment, a constant hydrostatic pressure gradient will be
assumed, for illustrative purposes. As will be shown presently, what really matters to
the engineer is the definition of the hydrostatic pressure regime in the vicinity of the
hydrocarbon bearing sands.

In contrast to this normal situation, abnormal hydrostatic pressure are encountered
which can be defined by the equation

dp
p = × D 14.7 C(psia) (1.6)
+
+
w
dD water
where C is a constant which is positive if the water is overpressured and negative if
underpressured.

For the water in any sand to be abnormally pressured, the sand must be effectively
sealed off from the surrounding strata so that hydrostatic pressure continuity to the
2
surface cannot be established. Bradley has listed various conditions which can cause
abnormal fluid pressures in enclosed water bearing sands, which include:

− temperature change; an increase in temperature of one degree-Fahrenheit can
cause an increase in pressure of 125 psi in a sealed fresh water system.

− geological changes such as the uplifting of the reservoir, or the equivalent,
surface erosion, both of which result in the water pressure in the reservoir sand
being too high for its depth of burial; the opposite effect occurs in a downthrown
reservoir in which abnormally low fluid pressure can occur.