Page 80 - Petrophysics

54 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

HYDROSTATIC PRESSURE GRADIENT

An important physical property of reservoir fluids is the density and its
relationship to the hydrostatic gradient (the increase of the fluid pressure
with increasing depth due to the increasing weight of the overlying fluid).
Density measurements are made relative to the maximum density of
water, which is 1 .O g/cm3 at 15°C (bo0@ and one atmosphere of pressure.
When the specific weight (or mass) of any substance is divided by the
specific weight (or mass) of an equal volume of water at 15OC and one
atmosphere of pressure, the resulting dimensionless value is described
as the specific gravity (SG) relative to water. The pressure gradient (Gp)
of any fluid is determined from the specific gravity as follows:

Gp = 1,000 kg/m3 x 9.81 m/sZ x yw
= 9,SlOr, Pa/m (0.433~~ psi/ft) (2.7)

where: yw = specific weight of water in kg/m3 (lb/ft3).
The hydrostatic gradient of subsurface waters is greater than 9.81 kPa
per meter of depth (0.433 psi/ft) because the brines contain dissolved
solids that increase the density of the fluids. The gradient also is
affected by the temperature and in some areas by dissolved gas, both of
which decrease the hydrostatic pressure gradient. An average hydrostatic
gradient of 10.53 kPa/m (0.465 psi/ft) generally is used in the literature
for subsurface brines. This value corresponds to about 80,000 ppm of
dissolved solids at 25OC (SG = 1.074).

LITHOSTATIC PRESSURE GRADIENT

The lithostatic pressure gradient is caused by the density of the rocks
and is transmitted through the grain-to-grain contacts of successive layers
of rocks. The lithostatic weight is, however, supported by the pressure of
the subsurface fluids in the pore spaces. Thus, the overburden pressure
is equal to the grain-to-grain lithostatic pressure plus the fluid pressure of
the porous formation, yielding an average overburden pressure gradient
of 22.7 kPa per meter of depth (1.0 psi/ft), which corresponds to an
overall bulk specific gravity of the rocks plus the interstitial fluids equal
to 2.31 (Figure 2.10):

where: Pob = overburden pressure.
p1 = lithostatic pressure.
pf = fluid pressure.