Page 160 - Origin and Prediction of Abnormal Formation Pressures

136 G.V. CHILINGAR, V.A. SEREBRYAKOV, S.A. KATZ AND J.O. ROBERTSON JR.

Using the above equation and the equations for the estimation of overburden pressure:
~y -- gprh (5-15)
and of pore pressure:

pp = gpwh (5-16)
it is possible to obtain an equation for the estimation of abnormally high pressure, ph
(Alexandrov, 1987):
ph gphh g(phre -- Pw )he
he
-- -- ( 5 - 1 7 )
where pr h and/9 he are the average densities of rocks at the estimated depth h and at the
equivalent depth he, respectively, and phwe is the average density of pore fluids to depth
he.
It is necessary to correct all parameter values for temperature, especially when the
resistivity data are used as a geophysical property to identify equivalent depths.

Method of normal compaction trend

In the method of normal compaction trend, the same assumptions are used as in
the equivalent depth method, i.e., lithologically identical rocks with equal values of
physical properties at different depths have the same effective stress. The normal
compaction trend is dependent on the variation of rock properties with depth of burial
at normal hydrostatic pressure. These properties can be determined using well-log
data, drilling data, core analysis data, etc. In particular, the physical properties of
shales depend primarily upon the degree of compaction. In nature, an exponential
relationship exists between the depth of burial and porosity, density, or resistivity of
normally compacted rocks (Fig. 5-7). When displayed on semilogarithmic plots, these
exponential dependencies are shown by straight lines. Deviations from the straight lines
indicate the upper boundary (top) of abnormal-pressure zones. For estimating abnormal
pressure, Pa, the following equation can be used (Dobrynin and Serebryakov, 1978):

g(Pr - Pw) Ah
P, -- Pn + log(xn/Xa) (5-18)
log(x2/xl ) + 0.435c~(x)GAh
where Pn is the normal hydrostatic pressure; Pr is the average density of rocks; Pw is the
average density of water; or(x) is the temperature coefficient for each physical property;
x2 and x~ are values of a certain geophysical parameter at depths h2 and hi; G is the
geothermal gradient for the interval (hi - h2); Ah = (h2 - hi); and Xn and Xa are the
values of a certain geophysical parameter used for the estimation of abnormal pressure
within the normal compaction trend and within the zone of abnormal pore pressure.
In the framework of this approach, it is not necessary to correct the values of
geophysical parameters for temperature effects, because the temperature coefficient and
geothermal gradient are included in Eq. 5-18. Using this equation, it is possible to
estimate abnormally high and abnormally low pressures. In the case of abnormally high
pressure, the second term in Eq. 5-18 is positive, whereas it is negative in the case of
abnormally low pressure.