Page 69 - Reservoir Geomechanics

53 Pore pressure at depth in sedimentary basins

Asomewhatmoredirectapproachtoestimationofporepressurefromseismicinterval
velocity data is based on empirical correlations between P-wave velocity, V p , S-wave
velocity, V s , both in units of km/s, mean effective stress, σ,in units of kbar (1 kbar =
100 MPa), porosity, φ, and clay content, C, based on point counts of thin sections
(0 ≤ C ≤ 1). The following formulae were derived by Eberhart-Phillips, Han et al.
(1989) for 64 different sandstones with varying amounts of shale from a comprehensive
suite of laboratory measurements of Han, Nur et al.(1986)
√
V p = 5.77 − 6.94φ − 1.73 C + 0.446(σ − e −16.7σ ) (2.11)
√
V s = 3.70 − 4.94φ − 1.57 C + 0.361(σ − e −16.7σ ) (2.12)

Whilethereareobviouslyanumberofrequiredparametersneededtoisolatetherelation-
ship between V p and V p /V s and effective stress, approaches to pore pressure prediction
using such relations have proven useful in many cases. Because of the non-uniqueness
of the relation between V p /V s and effective stress, porosity and clay content, an increase
of V p / V s could indicate a decrease in effective stress (increase in pore pressure), an
increase of clay content or some combination of the two.
When using the methodology outlined above, it is important to be aware of a number
of complicating factors. First, it is important to note that these methodologies apply
best to shales because in sands and carbonates variations in cementation and diagenesis
affect how they compact with depth such that relations such as equation (2.4) are
not applicable (Burrus 1998). Second, because the method assumes that all shales in
agiven section follow the same compaction trend, variations of shale lithology with
depth represent a decrease in effective stress, whereas they could result from a change of
lithology. Third, there are a variety of opinions about how pressure, or stress, in the lab
should be related to depth in the earth. Does hydrostatic confining pressure correspond
to the overburden stress, S v ;or does a laterally confined uniaxial compression test
correspond to S v ;or does hydrostatic confining pressure (or mean stress in a uniaxial
compression test) correspond to mean stress in the earth, thus requiring knowledge (or
estimates) of S hmin and S Hmax ? This is discussed at length by Harrold, Swarbrick et al.
(1999). The reasons underlying their concerns are discussed in Chapter 3 where we
consider porosity losses as a function of pressure and stress.
In addition to the complicating factors just mentioned (and perhaps more signif-
icantly), there are reasons why the types of pore pressure prediction methodologies
discussed above should not be used in certain geologic environments. Compaction-
based methods assume a prograde burial path in which effective stress monotonically
increases with burial and time. Hence, in such regions, the deviation from the expected
porosity at a given depth is evidence of anomalously high pore pressure. In regions with
a complex burial history and/or a history of pore pressure generation, the fundamental
assumption of a monotonic increase of effective stress with depth and time is incor-
rect. As pointed out by Burrus (1998) and schematically illustrated for a laboratory