Page 162 - Origin and Prediction of Abnormal Formation Pressures

138 G.V. CHILINGAR, V.A. SEREBRYAKOV, S.A. KATZ AND J.O. ROBERTSON JR.
Log X
m
E

t-

O
N
1

3
r
>
0

I.Li E
o
.Q 2
,<
o
o
N
l

Fig. 5-8. Schematic showing the dependence of the physical properties of shales on the effective pressure Pe
(Pe -- o" -- pp): 1 = depth hi" 2 = depth h z" 3 = depth h. (Modified after Dobrynin and Serebryakov, 1989,
fig. 57, p. 119.)

because they are continuous through the zones of both normal and abnormal pressure.
Thus, all values of a geophysical parameter in the zones of normal hydrostatic pressure
and abnormal pressure lie on a straight line (Fig. 5-8), because the physical properties
do not depend on depth, but rather on the effective stress (0" - pp).
The general equation for estimating the abnormal pressure using this method is:
log(x) -t- [0.435ot(x)(h - hi)G] - mx
Pa - 0" - (5-19)
?/x
where x is the value of a certain geophysical parameter at a depth of pressure estimation;
mx and n~ are the y-intercept and slope of the compressional curve, respectively; or(x)
is the temperature coefficient for each geophysical parameter used (plus or minus sign is
used depending on the physical property of the rock); and G is the geothermal gradient
for the depth interval (h - hI).
The parameters m~, and n~ are defined by the following equations:
log(xz/xl) • ot(x)(h2 - hl)(G/2.3)
nx = (5-20)
(0"2 -- P2) -- (0"1 -- Pl)