Page 295 - Applied Petroleum Geomechanics

284 Applied Petroleum Geomechanics

For a sandstone or other permeable formation, pore pressure can be ob-
tained based on hydraulic theory by assuming that the permeable formation
is hydraulically connected.

8.2 Pore pressure prediction from hydraulics
8.2.1 Pore pressure in a hydraulically connected formation

In a sandstone, limestone, or other permeable formation, the pore pressure
can be obtained by assuming the formation being hydraulically connected
and fully saturated, and theory of hydraulics can be applied. For an inclined
aquifer or a hydrocarbon-bearing formation, if a deeper overpressured
section is connected to the shallower sections by a permeable pathway, the
pressures in such a hydraulically connected formation can be calculated
based on the difference of the heights of fluid columns, i.e.,
p 2 ¼ p 1 þ r gðZ 2 Z 1 Þ (8.1)
f
where p 1 is the formation fluid pressure at depth of Z 1 ; p 2 is the formation
fluid pressure at depth of Z 2 ; r f is the in situ fluid density; g is the acceler-
ation of gravity.
Therefore, for a permeable formation if formation pressure at a certain
depth is available, then the pressures at other depths can be obtained from
Eq. (8.1). The calculation and principle are relatively simple. However, to
perform this calculation, the connectivity and extension area of the for-
mation need to be understood firstly. In other words, each individual fluid
compartment and seal need to be distinguished, which can be determined
from regional geology, well logging data and drilling data (Powley, 1990).
Fig. 8.2 shows an example of calculating formation pressure in an oil-
bearing sandstone using the hydraulic communication model (Eq. 8.1).
When the formation pressure and fluid density in Well 1 are known, the
pressures in other wells can be calculated using Eq. (8.1). If the formation is
hydraulically connected and saturated with the same fluid, the formation
pressures in the four wells should follow a single fluid gradient. Fig. 8.2
demonstrates that Eq. (8.1) gives an excellent prediction. Therefore, when
geological structure, fluid pressure, and density in a well are known, the fluid
pressures in other wells located in this hydraulically connected formation can
be fairly predicted. It should be noted that the permeability magnitude and
its variation may affect hydraulic connectivity of a formation. For extremely
low permeable formations (e.g., shales), the applicability of Eq. (8.1) may be
limited.