Page 61 - Reservoir Geomechanics

45 Pore pressure at depth in sedimentary basins

There are two circumstances in which estimation of pore pressure from geophysical
data is very important. First, is the estimation of pore pressure from seismic reflection
data in advance of drilling. This is obviously needed for the safe planning of wells
being drilled in areas of possible high pore pressure. Second, is estimation of the pore
pressure in shales even after wells are drilled, which tend to be so impermeable that
direct measurement is quite difficult. In the sections below, we discuss how geophysical
logging data (augmented by laboratory measurements on core, when available) are used
to estimate pore pressure in shales. In both cases, techniques which have proven to work
well in some areas have failed badly in others. We discuss the reasons why this appears
to be the case at the end of the chapter.
Most methods for estimating pore pressure from indirect geophysical measurements
are based on the fact that the porosity (φ)of shale is expected to decrease monotonically
as the vertical effective stress (S v −P p ) increases. The basis for this assumption is
laboratory observations such as that shown in Figure 2.13 (Finkbeiner, Zoback et al.
2001) which shows the reduction in porosity with effective stress for a shale sample
from SEI 330 field. It should be noted that these techniques are applied to shales (and
not sands or carbonates) because diagenetic processes tend to make the reduction of
porosity with effective confining pressures in sands and carbonates more complicated
than the simple exponential decrease illustrated in Figure 2.13.If one were attempting
to estimate pore pressure from seismic data before drilling in places like the Gulf of
Mexico, for example, one would first estimate pore pressure in shales using techniques
such as described below, and then map sand bodies and correct for the centroid effect,
as illustrated in Figure 2.12.
There are basically two types of direct compaction experiments used to obtain the
type of data shown in Figure 2.13: hydrostatic compression tests in which the applied
stress is a uniform confining pressure and an impermeable membrane separates the pore
volume of the rock from the confining pressure medium; and confined compaction tests
in which the sample is subjected to an axial load while enclosed in a rigid steel cylinder
that prevents lateral expansion of the sample. The data shown in Figure 2.13 were
collected with the latter type of apparatus because it was thought to be more analogous
to vertical loading in situ. Note that the application of moderate effective stresses results
in a marked porosity reduction. If we assume an overburden gradient of ∼23 MPa/km
(1 psi/ft), if hydrostatic pore pressure is encountered at depth, the vertical effective
stress would be expected to increase at a rate of 13 MPa/km. Correspondingly, shale
porosity would be expected to decrease from ∼0.38 near the surface to ∼0.11 at depths
of approaching 3km (∼10,000 ft). As indicated in the figure, in the case of moderate
and high overpressure (λ p = 0.65 and λ p = 0.8), respectively higher porosities would
be encountered at the same depth. It should also be noted that one can empirically
calibrate porosity as a function of effective stress data in areas with known overburden
and pore pressure.