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ABNORMALLY-HIGH FORMATION PRESSURE 53

3.2.3. Drilling data

Overpressure can be estimated using drilling data. To predict pressure from the
drilling data, a fuzzy logic has been used by Aminzadeh et al. (1994). This method
was first applied to the data from South Caspian Basin. Drilling parameters such as
the bit weight, rate of penetration, and changes in the rate of penetration were used
for this purpose. The information obtained from pressure prediction is used to
choose the required drilling mud density. Also, lithology can be predicted from the
pressure data (Aminzadeh et al., 1994; Dunan, 1996; Lee, 2000).
Many factors contribute to the magnitude of abnormal formation pressure. These
include:
Compaction of the rocks with a change in porosity.
Mass transfer fluxes.
Temperature changes.
Diagenetic and catagenetic transformations.
Chemistry of intertstitial fluids.
Lithology and mineralogy.
Sand/shale ratio.
Distribution of porosity and permeability in associated sands and shales.
The abnormally-high formation pressures in the argillaceous sequences may
substantially affect the geological processes at depth. They evidently have played an
important role in folding, clay diapirism, mud volcanism, earthquakes, and
diagenesis–catagenesis. The models of these phenomena are described by the
Coulomb’s law and by the rheological models of various theoretical bodies.
According to the Coulomb’s law, resistance to shearing in shales is the first power
function of normal compressive stress. As abnormal pore pressure in shales
increases, the intergranular stress (effective stress) decreases, down to very low values
under certain conditions. Resistance to shearing, determined by friction, decreases
correspondingly. This leads to an intergranular sliding and facilitates, to a
considerable extent, the development of shearing. In such instances, plastic
argillaceous sequences become quite mobile at high pore pressure in shale and are
displaced. Depending on the subsurface environment and duration, this process
may lead to the development of folds, diapirs, mud volcanoes, or earthquakes. In
the South Caspian Basin and onshore of Azerbaijan, such geologic setup is
quite typical of thick Paleogene to Miocene argillaceous sequences with extremely
high, quasigeostatic values of AHFP, with shale pore pressure gradients of
0.020–0.023 MPa/m (Buryakovsky et al., 1986, 1995, 2001).
Development of abnormal pore pressures in shales of the South Caspian Basin
and onshore of Azerbaijan has been experimentally demonstrated by elastic
compression of hermetically sealed cores of Cenozoic shales. Fig. 3.12 shows
that the pore pressure (p ) in the core rises with increasing external confining
p
pressure (total overburden load, s) and then decreases as the confining pressure
decreases, but always remaining higher than in the case of increasing load, evidently
as a result of residual (irreversible) deformation of the rock (see Rieke and
Chilingarian, 1974).