Page 159 - Origin and Prediction of Abnormal Formation Pressures

METHODS OF ESTIMATING AND PREDICTING ABNORMAL FORMATION PRESSURES 135
>,
0
0 Resistivity
t-

20 20
R R
i
CB)

h

Fig. 5-6. Determination of abnormally high (A) and abnormally low (B) pressures using the method
of equivalent depth. 1 -- Shale; 2 = sandstone; 3 = reservoir rock with anomalous pressures; 4 =
abnormal-pressure zone (pore pressure in shale); and 5 = crossed zone = zone not penetrated by drilling.
h is the depth of investigation and he the equivalent depth (see Eq. 5-17). (Modified after Dobrynin and
Serebryakov, 1989, fig. 55, p. 113.)

There are many methods for estimating and predicting abnormal pressure during
drilling. Most methods are empirical dependencies between the pore pressure and log
responses. There are three well-known analytical methods: (1) the method of equivalent
depth (Foster and Whalen, 1966), (2) the method of normal compaction trend (Dobrynin
and Serebryakov, 1978), and (3) the method of compressional curves (Dobrynin et al.,
1982).

Method of equivalent depth

The method of equivalent depth (Fig. 5-6) is based on the assumption that the same
shale with equal physical properties at different depths will have equal effective stress
(total overburden load, or, minus the pore pressure, pp)"

((7 -- Pp)l -- (0- -- Pp)2 (5-14)