Page 239 - Applied Petroleum Geomechanics

234 Applied Petroleum Geomechanics

generated by compaction disequilibrium, it is often recognized by higher-than-ex-
pected porosities at a given depth and the porosities deviated from the normal
porosity trend. Hydrocarbon generation is a process of the creation of mobile fluids
from an original solid immobile kerogen, which greatly increases fluid volume or
porosity and causes a great increase in pore pressure. Case studies show that hard
overpressures created from hydrocarbon generation are consistent with the increase
in vitrinite reflectance. In addition, a composite normal compaction trendline for pore
pressure prediction is proposed to honor smectite and illite transformation and pore
pressure generation. Abnormal pore pressures in several petroleum basins and shale
oil and gas plays are examined to illustrate the mechanisms of abnormal pressure
generations.

Keywords: Abnormal pore pressure; Compaction disequilibrium; Hydrocarbon
generation; Overpressure; Pore pressure gradient; Smectiteeillite transformation; Uplift
and unloading.

7.1 Normal and abnormal pore pressures
Pore pressure is the fluid pressure in pore spaces of the porous formation. It
varies from hydrostatic pressure (normal pore pressure) to severe over-
pressure, which can be more than two times of the hydrostatic pressure in a
subsurface formation. Overpressures exist in many geologic basins in the
world. If the abnormal pore pressure is not accurately predicted before
drilling or while drilling, drilling risks and incidents will increase greatly.
For example, in deepwater of the Gulf of Mexico, incidents associated with
pore pressure and wellbore instability accounted for 5.6% of drilling time in
non-subsalt wells and 12.6% of drilling time in the subsalt wells (York et al.,
2009). The abnormally high pore pressures have caused serious drilling
incidents, such as the kicks and well blowouts (Skalle and Podio, 1998;
Holand and Skalle, 2001). Therefore, pore pressure prediction is critically
important for drilling planning and operations in the oil and gas industry.
Abnormally high pressures also induced geologic hazards and disasters, such
as weakness in faults (e.g., Bird, 1995; Tobin and Saffer, 2009) and mud
volcanoes (Davies et al., 2007; Tingay et al., 2009).
7.1.1 Hydrostatic pressure and normal pore pressure

Normal pore pressure is the hydrostatic pressure caused by the column of
pore fluid from the surface to the interested depth. For the formations with
normal fluid pressure, the pore pressure gradient follows the hydrostatic
pressure gradient. The magnitude of hydrostatic pressure is proportional to