Page 63 - Reservoir Geomechanics

47 Pore pressure at depth in sedimentary basins

stress, at any given depth and porosity can be measured directly through geophysical
logging or estimated from sonic velocity or resistivity data. Thus, if there is a unique
relation between porosity and the vertical effective stress, S v −P p , pore pressure can be
estimated because it is the only unknown.
It is helpful to consider a simple example. If a porosity of 0.17 was measured at
2km depth, one would infer approximately hydrostatic pore pressures in the shale at
that depth because the effective stress expected to result in a porosity of 0.17 for this
shale at that depth (∼26 MPa) is equivalent to the difference between the vertical stress
(∼46 MPa) and hydrostatic pore pressure (20 MPa). If, however, a porosity of 0.26 was
measured at the same depth, one would infer that there was anomalously low effective
stress (∼10 MPa) implying that the pore pressure was anomalously high (∼36 MPa),
or approximately 0.8S v .
Near the end of this chapter I review a number of important geologic reasons why the
quantitative application of this type of analysis must be used with caution. Qualitatively,
however, the simple concept of an expected compaction trend has utility for detecting
the onset of anomalously high pore pressure at depth. This is illustrated in Figure
2.14 (Burrus 1998). The increase in the vertical effective stress to depth C in Figure
2.14a (the maximum depth at which pore pressure is hydrostatic) is associated with a
uniform increase in the vertical effective stress and a corresponding decrease in porosity
as predicted using equation (2.4). The onset of overpressure at depths greater than C
is associated with a decrease in effective stress (the difference between the overburden
and pore pressure) as well as a reversal of the increase in effective stress with depth.
This corresponds to anomalously high porosity at depth due to the anomalously high
pressure. Note that the porosity at depth E in Figure 2.14bis the same as at the depth
A, even though it is buried much more deeply. The existence of anomalously high
pore pressure at depth E can be inferred from the marked deviation from the normal
compaction trend.
This concept can be formalized in a rather straightforward manner when attempting
sh
to evaluate shale pore pressure, P , from geophysical log data. M. Traugott (written
p
communication, 1999) has proposed the following equation for porosity measurements
derived from sonic velocity measurements

x
1 − φ ν

P sh = S v − P hydro (2.5)
p p
1 − φ n
where x is an empirical coefficient, φ v is the porosity from shale travel time, φ n is
hydro
the expected porosity from the normal trend, and P p is the equivalent hydrostatic
pore pressure at that depth. Because resistivity measurements can also be used to infer
porosity, Traugott has also proposed
hydro
1.2

P sh = Z S v − S v − P p R o (2.6)
p
Z Z Z R n