Page 60 - Reservoir Geomechanics

44 Reservoir geomechanics

is a complex process (Hall 1993)but can lead to overall volume increases of both the
rock matrix and the pore water system. One component of this process is the phase
transition from montmorillonite to illite, which involves the expulsion of water from the
◦
crystal lattice of montmorillonite. The transition occurs at a temperature of about 100 C
in the Gulf of Mexico, which is often correlative with the depth at which overpressures
are observed to develop (Bruce 1984). The transition of anhydrite to gypsum is another
dehydration reaction that can lead to overpressure development, but only at relatively
shallow depths as the temperature at which this dehydration occurs is only about half
that of the smectite–illite transition.
The exact manner in which dehydration reactions may generate overpressure is quite
complicated.Forexample,inthecaseofthesmectite–illitetransition,theoverallvolume
change associated with the transition is poorly known and the phase transition may
work in conjunction with compaction disequilibrium (due to increased compressibil-
ity) and silica deposition (which lowers permeability). Nonetheless, a number of authors
(e.g. Alnes and Lilburn 1998)argue that dehydration reactions are an important mech-
anism for generating overpressure in some sedimentary basins around the world.
Hydrocarbon generation from the thermal maturation of kerogen in hydrocarbon
source rocks is associated with marked increases in the volume of pore fluid and thus
can also lead to overpressure generation. This is true of the generation of both oil and
gas from kerogen, although the latter process is obviously more important in terms of
changes in the volume of pore fluids. As discussed in detail by Swarbrick and Osborne
(1998), this mechanism appears to operate in some sedimentary basins where there
is an apparent correlation between the occurrence of overpressure and maturation. In
the North Sea, for example, the Kimmeridge clay is deeply buried and presently at
appropriate temperatures for the generation of oil or gas (Cayley 1987). However,
some younger formations (at depths well above the maturation temperatures) are also
overpressured (Gaarenstroom, Tromp et al. 1993), so other pore pressure mechanisms
are also operative in the area.

Estimating pore pressure at depth

Direct measurement of pore pressure in relatively permeable formations is straight-
forward using a variety of commercially available technologies conveyed either by
wireline (samplers that isolate formation pressure from annular pressure in a small area
at the wellbore wall) or pipe (packers and drill-stem testing tools that isolate sections
intervals of a formation). Similarly, mud weights are sometimes used to estimate pore
pressure in permeable formations as they tend to take drilling mud if the mud pressure
is significantly in excess of the pore pressure and produce fluids into the well if the
converse is true.