Page 26 - Origin and Prediction of Abnormal Formation Pressures

INTRODUCTION TO ABNORMALLY PRESSURED FORMATIONS 9

Hence, water in the geopressured zone can be expected to have a temperature of 152~
(305~ Several factors affect the heat flux in subsurface formations: (1) the prevailing
temperature of the zone; (2) the specific heats of the matrix and fluids; (3) the porosity
and permeability of the sedimentary layers; (4) the density and thermal expansion of the
rock and fluids; and (5) the chemical composition of the rocks and fluids (Donaldson,
1980).
The geopressured zones along the Gulf Coast region (USA) generally occur at depths
below 2500 m and require special drilling technology whenever these zones are to
be penetrated. These zones usually contain a considerable amount of methane that is
frequently separated and recovered when the geopressured formations are penetrated
(Harkins and Baugher, 1969). Dickinson (1951) made a thorough study of the geologic
aspects of the high fluid pressures in the Tertiary Basin of the U.S. Gulf Coast region.
The high-pressured zones occur most frequently in isolated Miocene and Pliocene sand
beds surrounded by thick shale sections located below the main deltaic sand series.
The high fluid pressures appear to be independent of the depth or geologic age of the
formations. Where sedimentation has been rapid, the thick accumulation of shales and
mudstones having low permeability (< 10 -7 D) have retarded the expulsion of water
and hydrocarbons. The trapped pore fluids bear a portion of the overburden load that
would normally be supported by the grain-to-grain contacts.
In the geopressured/geothermal zones (at depths greater than 3000 m) with pressure
and temperature of about 70 MPa (10,000 psi) and 152~ respectively, the solubility of
methane in water is about 0.058 mole fraction (40 ft3/bbl). The actual gas production
from several zones, however, exhibits an approximate saturation of 0.029 mole fraction

Plain Shelf

Top of geopressured zone

Sand and shale
sequence---

Louann salt
Sandstone
Granite
Fig. 1-4. Top of geopressured zone in the Gulf Coast of the U.S. in relation to salt domes (Louann Salt).