Page 81 - Petrophysics

PROPERTIES OF SUBSURFACE FLUIDS 55

Figure 2.10. Subsurface pressure gradients.

When the hydrostatic pressure gradient for any region is approximately
10.53 kPa per meter, it is known as the normal pressure gradient.
Abnormal pressure gradients may be either abnormally low or abnormally
high. Abnormally high hydrostatic pressure gradients of 21.5 kPa per
meter (0.95 psi/ft) have been encountered, for example, in the geopres-
sured/geothermal zones (1) along the Gulf Coast of the United States
extending from New Orleans into Mexico, (2) the Niger delta, and
(3) the North Sea [6, 181. Abnormally low pressures have been encoun-
tered, for example, in some gas fields of Pennsylvania and the Morrow
Formation in Northwest Oklahoma.

GEOTHERMAL GRADIENT

Heat rising from the mantle produces a heat flux in midcontinent
regions ranging from 0.8 to 1.2 pcal/cm2-s (3.0 to 4.4 pBTU/ft2-s)
measured at the surface, which results in a geothermal gradient, Gt [5].
The geothermal gradient varies at different areas on the globe depending
on the annual mean surface temperature and the thermal conductivity of
the subsurface formations, but an overall average temperature gradient
Gt of 18.2"C/km (l.O°F/lOO ft) of depth has been recorded around the
world. Using this average value and the region's mean annual surface
temperature Ts, an estimate of subsurface formation temperatures Tf
can be obtained as follows:

When the bottomhole temperature Tf of a well is accurately measured,
the local geothermal gradient Gt may be obtained from Equation 2.9