Page 76 - Petroleum Geology

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PRESSURE

Fig. 3-9. Normal hydrostatic gradient, and overburden (or geostatic) pressure gradient,
generalized.
static pressures. In some areas they are significantly lower than normal hydro-
static; more commonly, they are higher. Pressures below normal hydrostatic,
called subnormal pressures, are usually due to a water table or aquifer outlet
that is considerably below ground level of the well in question. Similarly,
some modest excess pressure may be due to artesian conditions in which the
intake area of the aquifer is elevated above the ground level at the well. How-
ever, we are concerned more with those areas in which measured pressures
far exceed the normal hydrostatic, and cannot be explained in artesian terms
(for example, when such pressures are encountered in places such as the Niger
delta and the U.S. Gulf Coast). These high pressures, known as abnormal
pressures, have a limiting value close to the pressure exerted by the overbur-
den, solids and fluids:
Pmax Tbw (3.12)
where Tbw is the mean bulk wet density of the overburden above depth z
(Fig. 3-9). The limiting gradient and the pressures on it are called overburden,
geostatic, or lithostatic gradients or pressures (in that order of priority). Pres-
sures lying between the overburden value and the normal hydrostatic value
are called abnormal pressures or geopressures - or superpressures when they
are very high. Abnormal pressures are only abnormal in the sense that they
do not conform to the normal hydrostatic.
The overburden pressure gradient is also generalized as a straight line, but
of course it depends on lithologies and the state of compaction. Its value is
commonly taken as 22.6 kPa/m (0.23 kfg cm-2 m-I; 1 psilft), but this is
usually on the high side. Because porosities tend to decrease with depth, the
overburden pressure gradient tends to increase with depth.