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238 Applied Petroleum Geomechanics
Figure 7.3 Underpressure, overpressure, hydrostatic pressure (normal pressure), and
overburden stress in the subsurface.
because underpressures uncommonly occur in the sedimentary formations
(excluding those caused by depletion from oil and gas production).
7.1.4 Pore pressure and pore pressure gradient
The hydrostatic pressure and formation pore pressure in a typical oil and gas
well are plotted in Fig. 7.4. The pore pressure profile with depth in this well
is similar to many geologically young sedimentary basins where over-
pressure is encountered at depth. At relatively shallow depths (less than
2000 m), pore pressure is hydrostatic, indicating that a continuous, inter-
connected column of pore fluid extends from the surface to that depth. At a
depth of more than 2000 m the overpressure starts, and pore pressure in-
creases with depth rapidly, implying that the deeper formations are hy-
draulically isolated from the shallower ones. By 3800 m, pore pressure
reaches a value close to the overburden stress, a condition referred to as hard
overpressure. The effective stress in pore pressure prediction community is
conventionally defined to be the subtraction of pore pressure from over-
burden stress, as shown in Fig. 7.4. The increase of overpressure causes
reduction in the effective stress.