Page 270 - Applied Petroleum Geomechanics

Abnormal pore pressure mechanisms 261

the causes of abnormal pressures in the literature are referred to the
following reasons (Law and Spencer, 1998): compaction disequilibrium,
aquathermal expansion, hydrocarbon generation, mineral transformations,
tectonics, and osmosis. The most common cause of abnormally high
pressure is compaction disequilibrium; the age of the abnormally pressured
rocks is primarily Tertiary. In pre-Tertiary rocks, the main causes of
abnormal pressure include hydrocarbon generation, aquathermal
expansion, mineral transformations, and tectonic deformation; however,
hydrocarbon generation was cited as the most common cause. As these
deltaic sediments are buried deeper and experience higher temperatures,
hydrocarbon generation may supplant compaction disequilibrium as the
main cause of abnormally high pressure. In deltaic rock sequences where
the hydrocarbon source rock occurs stratigraphically below the compaction
disequilibrium-affected sediments, the generation of hydrocarbons from
these source rocks may result in the development of overpressure, which
could be physically transferred upward into the region of compaction
disequilibrium. This causes further increase of the overpressure generated by
compaction disequilibrium (Law and Spencer, 1998). In the following
sections, typical pore pressure profiles are analyzed in several petroleum
basins.

7.5.2 Abnormal pressure in the Macondo well of the Gulf of
Mexico

The Macondo well, MC 252-1 (BP) is an oil exploration well, located 133
miles SE of New Orleans in the Mississippi Canyon block 252, deepwater
Gulf of Mexico, USA. The mudline of the well was 5067 ft including the
air gap of 75 ft. On April 20, 2010, the Deepwater Horizon blowout of the
Macondo well occurred.
The Macondo pore pressure profile (Fig. 7.21) shows that the first
abnormal pore pressure indicator at 7500 ft TVD SS (below the sea level)
confirms the shallow onset of overpressure, and this is very common in
deepwater Gulf of Mexico (e.g., Flemings et al., 2002). From 7500 to
17,640 ft, pore pressure gradually increases and approximately parallels to
the overburden stress, with the maximum pore pressure gradient of
13.9 ppg. Thereafter, pore pressure decreases abruptly by 1200 psi over
370 ft as the main sandstone reservoir (M56) is approached (Pinkston,
2018). The M56, a Miocene-aged sandstone, is part of a larger hydraulically
connected aquifer and has a large pore pressure regression presented at the
Macondo (Pinkston, 2018). Case study in some Green Canyon wells show