Page 71 - Reservoir Geomechanics

55 Pore pressure at depth in sedimentary basins

test in Figure 2.18a, should effective stress decrease after loading, there is almost no
change in porosity along the unloading path. This is because the majority of the porosity
loss associated with compaction is associated with unrecoverable deformation. As can
be readily seen in Figure 2.18a, essentially constant porosity along the unloading path is
associated with a large range (∼16–40 MPa) of effective stresses, making it impossible
toestimateporepressureatagivendepthfromtheporosity–effective-stressrelationship.
That said, there tends to be a unique geophysical signature to unloading, expecially at
low effective stress (<15 MPa in Figure 2.18b) that helps identify cases when this
occurs (Bowers 1994). For example, induced micro-fracturing would tend to affect
measurements of sonic velocity or formation resistivity logs but not bulk measurements
such as density or neutron porosity.
There are a variety of geologic processes that could be responsible for such an
unloading effect. These include an increase in pore pressure after initial burial (associ-
ated with the various types of processes described above) or rapid uplift and erosion.
Areas with complicated burial and tectonic histories would also be areas where pre-
diction of pore pressure with the technique described above should be considered with
great caution. In such places the porosity reduction is more correctly predicted using
relationships derived from mean effective stress increases (S Hmax + S hmin + S v )/3 −
P p , see Chapter 3. Hence, in areas of significant tectonic compression, pore pressure
prediction methods require knowledge of all three principal stresses. A number of the
papers in Law, Ulmishek et al.(1998) and Huffman and Bowers (2002) discuss pore
pressure prediction in areas where compressive stress significantly complicate the use of
the relatively simple techniques outlined above.
Field data from the Mahakam delta (Figure 2.18b after Burrus 1998) show such an
effect. Note that porosity decreases (and density increases) uniformly with increasing
effective stress to ∼3km depth where there is normal compaction and hydrostatic pore
pressure. However, when overpressure is encountered at depth of >3 km, there is a
wide range of effective stresses (a wide range of overpressure) associated with almost
constant porosity. As the overburden stress at 4 km is expected to be ∼90 MPa, an
effective stress of ∼15 MPa implies a pore pressure of 75 MPa (λ = 0.83). However,
the porosity at this depth (∼12%), if interpreted using the loading curve, would imply
an effective stress of about 35 MPa. This would result in an under-prediction of pore
pressure by 20 MPa (∼3000 psi), a very appreciable amount. Again, simple compaction
curves are not adequate for pore pressure prediction in cases of high compressive stress
or when pore pressure increases appreciably after burial and initial compaction.