Page 54 - Origin and Prediction of Abnormal Formation Pressures

ORIGIN OF ABNORMAL FORMATION PRESSURES 37

where Crh is horizontal stress in general. On assuming a Poisson ratio, v, of 0.18 to 0.27
for consolidated sedimentary rocks, the horizontal compressive stress would range from
0.22 to 0.37 psi/ft of depth. According to Harrison et al. (1954), for soft shales and
unconsolidated sands found in the Gulf Coast of Texas and Louisiana, which can be
considered to be in a plastic state of stress, the horizontal stresses are in excess of 0.37
psi/ft of depth. Faulting can occur in cemented rocks at stresses that will only cause
plastic deformation in uncemented rocks.
The effective pressure, Pe, may be either increased or decreased by the presence of
vertical dynamic flow and resulting fluid drag pressure on the grains, depending upon
the flow direction. An example of this would be quicksand, a case where intergranular
loading has been reduced to nearly zero by upward water seepage resulting in zero
bearing strength in the skeletal structure.

Load transfer
The overburden (lithostatic) pressure, Pt, is equal to:

Pt = Pbg Z (2-40)
which for all practical purposes, is the pressure exerted at any depth by the weight of
overlying sediments and fluids. The density term in this equation is the bulk density of
fluid-saturated rocks. If Pb is known, the pressure-depth relationships can be established
in a particular area.
Hubbert and Rubey (1959, p. 129) stated that within depths of 1 or 2 km, the pressure
of the water as a function of depth, D, can be closely approximated by the equation:

Ph = pwgD = ywD (2-41)
where Ph is the hydrostatic pressure of a column of water extending from the surface
of the ground to a depth of D, Pw is the density of the water, g is the acceleration of
gravity, and Yw is the specific weight of water.
Along the Gulf Coast, the fluid-pressure gradient is about 0.465 psi for each foot
of depth. This represents a hydrostatic pressure gradient for brine having a specific
weight, yw, of 67 lb/ft 3. The corresponding shale matrix pressure is 0.535 psi/ft, if
one assumes a total lithostatic pressure gradient of 1 psi/ft. Frederick (1967) presented
several examples of the relationship between the bottom hole fluid pressure and depth
for areas with abnormal pressures. Hubbert and Rubey (1959, p. 155) noted a lithostatic
pressure as high as 1.06 psi/ft occurring in the Khaur Field in Pakistan. Levorsen (1958,
p. 386) reported that the average gradient of oilfield brines is approximately 0.450 psi/ft.
Deviations are in part due to the varying salt concentrations in the brines. Table 2-3
gives the specific gravity and pressure gradients of various fluids that might occur in a
sand-shale sequence. Fig. 2-8 demonstrates the pressure versus depth relationship for
various brines.
AHFPs can form when a portion of the effective stress, ~e, normally assumed by
skeletal structure is transferred to the intergranular water. For example, when increasing
the weight of the overlying sediments by continued burial, Fz, at a rate faster than
the intergranular water can escape from the sediment, the percentage of the lithostatic