Page 240 - Applied Petroleum Geomechanics

Abnormal pore pressure mechanisms 235

Ground surface
Piezometric surface

h
p = ρ gh
n f
f n
Confined aquifer

Figure 7.1 Schematic cross-section showing the hydrostatic pressure caused by water
column in a subsurface formation (aquifer).

the depth below the surface and to the density of the fluid in the pores.
Thus, hydrostatic pressure can be calculated using the following equation:
p n ¼ r gh (7.1)
f
where p n is the hydrostatic pressure; g is the acceleration due to gravity; r f is
the fluid density; and h is the vertical height of the fluid column (or depth),
as shown in Fig. 7.1 (Zhang, 2013).
Practically the following equation can be used to calculate the hydro-
static pressure in the metric unit:
p n ¼ 0:00981r h (7.2)
f
3
where p n is in MPa; r f is in g/cm ; and h is in meters.
In the English unit, the hydrostatic pressure can be expressed as follows:
p n ¼ 0:4335r h (7.3)
f
3
where p n is in psi; r f is in g/cm ; and h is in ft.

7.1.2 Salinity effect on hydrostatic pressure
Hydrostatic pore pressure depends on water density (Fig. 7.2), while the
density of water is a function of water salinity, temperature, and content of
dissolved gases (Chillingar et al., 2002). There is a general variation in
hydrostatic pressure gradient (r f g) at different locations owing to different
water densities. For instance, the average hydrostatic pressure gradient is
3
usually taken as 0.465 psi/ft (1.074 kg/cm ) in the Gulf of Mexico, and this
corresponds to water with a salinity of 80,000 parts per million (ppm) of
sodium chloride at 77 F (25 C) (Dickinson, 1953). Formation water varies