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Subsurface Pressure
Pressure, which like temperature increases with depth, plays a relatively minor role
in the petroleum-generation process (Phillippi, 1965), but has other important
effects.
The total overburden pressure exerted at any point in the subsurface is the sum of
two forces: the weight due to the over-lying rock (lithostatic pressure) and pressure
due to fluids contained within the pore spaces (fluid or pore pressure); therefore,
Overburden Pressure = Lithostatic Pressure + Fluid Pressure.. Lithostatic pressure
is transmitted via grain-to-grain contacts and averages about .6 psi/ft (.136
kg/cm2. m) (13.6 kPa/m). Fluid pressure is usually transmitted via pore-to-pore
communication extending to the surface and is then called hydrostatic pressure. For
a typical subsurface brine, hydrostatic pressure gradient is about .465 psi/ft (.1052
kg/cm 2m) (10.52 kPa/m).
Pressures increase with burial depth and in a normally pressured well, the fluid
pressure is always slightly less, and the lithostatic pressure slightly more, than half
of the total overburden pressure, at any depth ( Figure 1 ).
Figure 1
However, abnormally pressured rocks are sometimes encountered in drilling, often
unexpectedly. This may cause serious problems. If the rocks are overpressured (i.e.
where a permeability barrier seals pore fluids off from communication with the
surface), the pressure exerted by the drilling mud may not be great enough to hold
back the fluids in the rock. This could cause a well to "blow out". Underpressured
rocks are less common. However, they too can cause problems, when high-
pressure drilling muds enter the lower-pressured formation causing loss of
circulation and plugging up of pore spaces. This can lower the mud column in the
well to such a degree that even a normally pressured formation may blow out.
Although abnormally high pressures may be encountered in various sedimentary
provinces, they are particularly prevalent in rocks deposited in delta environments
where sedimentation may be too rapid for deep shales to thoroughly compact and
dehydrate. In this case, some of the weight of the overlying sediment, which would
otherwise be taken up by grain-to-grain contacts in normally compacted rocks is
taken up by the fluid in the pore spaces.