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Abnormal pore pressure mechanisms 253

describe sonic velocity and effective stress relation, as shown in the
following equation (Zhang and Wieseneck, 2011):
V p ¼ 1467s 0:2577 (7.7)
e
where V p is the compressional velocity, in ft/s; and s e is the effective stress,
in psi.

7.3 Overpressures and smectiteeillite transformation

Several common mineral transformations in sediments involve the release
of bound water. The most common of these involves the dehydration of
smectite, a multilayered, mixed-layered clay commonly found in
mudrocks. Smectite represents a family of montmorillonite, beidellite,
saponite, and nontronite, and it is one of the frequently observed clay
mineral in nature. Smectite transforms to a new mineral, illite, involving the
release of water at certain temperatures; during this process, the bound
water in smectite is expelled into pore space, causing the increase of pore
volume and pore pressure because the bound water expands about
1.04e1.1 times as it turns into pore water. Experimental results (Hunnur,
2006) show that during smectite to illite transformation, an increase in pore
pressure of up to 3% was observed between day 3 and day 7 from the start
of the experiment. Other dehydration reactions include gypsum to anhy-
drite in evaporitic sediments and coalification.

7.3.1 Overpressure mechanism of smectite to illite
transformation

Evaluation of interlayer water loss (dehydration) by smectites under
diagenetic conditions indicates that smectites in compacting shales, where
the vertical effective stress is greater than zero, will expel one of the two
remaining water layers at temperatures of 67e81 C, a temperature range

coincident with the onset of smectite to illite transformation

(Colten-Bradley, 1987). The last water layer will be lost at 172e192 C.
The loss of water generated by dehydration from the minerals into pore
space causes fluid volume increase in low permeable shale due to smectite
to illite transformation, which will increase pore pressure, if the water
cannot be expelled to other formations.
The overall volume change accompanying the complex smectiteeillite
reaction is not fully understood. Calculations indicate a total increase in
volume of 4% occurring in three pulses of water release (Swarbrick and