Page 52 - Origin and Prediction of Abnormal Formation Pressures

ORIGIN OF ABNORMAL FORMATION PRESSURES 35

!
O" v

~W

i t
O" H + O'w ]~ .= O" w 0" CY x

(5" z

Fig. 2-6. Stress state in a shale. Schematic of the stress state in a shale body underground, where crv' is
the effective (intergranular) stress in the vertical direction, cr~ is the horizontal effective stress, Ow is the
pore water stress and cr z is the total vertical stress component. The total horizontal stress component in the
x-direction Crx is equal to cr~ + ~r~. (Modified after Rieke and Chilingarian, 1974, fig. 50, p. 92.)

A useful expression in studying compaction is the ratio of the fluid stress to the total
stress, )~ (see Hottman and Johnson, 1965):
Ow pp
)~ -- = (2-36)
cr Pt
When stress is initially applied to the closed system, )~ has a value of 1 and the system
is overpressured. At final compaction equilibrium, when the load is carried entirely
by the skeletal structure (grains; spring), )~ is equal to 0. An example of the use of )~
is demonstrated in Figs. 2-5 and 2-7. At the final stages of compaction equilibrium,
the applied load is supported jointly by the skeletal structure and intergranular water
(hydrostatic) and the value of )~ is approximately equal to the normal pressure gradient,
i.e., 0.465. This value is typical of the normal pressure gradient on the U.S. Gulf Coast
(~0.465 psi/ft). The lithostatic (geostatic or overburden) pressure gradient is considered
to be about 1.0 psi/ft (0.231 kg cm -2 m -1) of depth. As discussed earlier, the hydrostatic
pressure will vary from locality to locality dependent upon the specific weight of the
water (salinity).