Page 162 - Fundamentals of Gas Shale Reservoirs

142 PORE PRESSuRE PREdIcTIOn FOR ShAlE FORmATIOnS uSInG WEll lOG dATA

Pressure/stress Transit time Density Pressure/stress

A
v A A
Depth
B B B Overburden stress
Overburden stress
C C C Pore pressure
Pore pressure
FIGURE 7.4 Graphic illustration of the response of wireline logs Depth VB
to overpressure generated by under‐compaction.
A B VB

across the charged interval increases or remains constant
(Fig. 7.4). moreover, the magnitude of pore pressure increase
due to the under‐compaction mechanism is less than or equal
to the increase in overburden stress (miller et al., 2002). In
other words, the under‐compaction mechanism cannot cause
a decrease in effective stress. All responses for the aforemen- FIGURE 7.5 Graphic illustration of overpressure generation by
tioned wireline logs are for the shale sequence, and it is unloading mechanisms, for example, the transformation of load‐
critical to differentiate between shales and other formations bearing grains or kerogen (black) into pore fluid (white).
prior to analyzing the logs response.

the transformation of the load‐bearing framework into pore
7.2.2 Unloading Mechanisms (Fluid Expansion)
fluids. The most significant unloading mechanisms presented
Overpressure in sedimentary basins can be generated by herein include hydrocarbon generation, clay diagenesis, and
unloading mechanisms. The process involves either expan- aqua‐thermal heating.
sion of the contained pore fluids or load transfer into pore
fluids with minimal change in porosity at rates that do not 7.2.2.1 Hydrocarbon Generation hydrocarbon genera-
allow the pore fluids to dissipate. Origins of fluid expansions tion processes represent an effective mechanism to generate
that are mentioned in the literature include hydrocarbon a large magnitude of overpressure. The processes include
generation, cracking of oil to gas, clay transformation, for cracking from oil into gas and the transformation of kerogen
example, smectite to illite, aqua‐thermal heating, and cemen- into gas or oil. The volume of the expanded pore fluids
tation and mineral participation (Osborne and Swarbrick, during hydrocarbon generation depends on the type of the
1997). Fluid expansion associated with the transformation of kerogen and the density of the hydrocarbon generated. As
the load‐bearing framework into pore fluids results in an mentioned by Swarbrick et al. (2002), out of the many fluid
increase in pore pressure when the expanded fluids have expansion mechanisms, the cracking from oil into gas pro-
been constrained by the rock matrix (Bowers, 1995; duces a high magnitude of overpressure as a result of fluid
Swarbrick et al., 2002. Overpressure caused by fluid expan- expansion. The high magnitude of overpressure generated
sion involves a decrease in the effective stress as depth by oil cracking into gas occurs in source rocks, and the
increases. This is due to an increase in volume of the pore generated gas is diluted into the connected pores. hansom
fluids and the transformation of matrix grains into pore and lee (2005) also mentioned in their numerical study that
fluids. As a result, some of the loads that were previously the cracking of oil into gas resulted in the generation of a
carried out by grains are transferred into pore fluids (miller high magnitude of overpressure. The transformation from
et al., 2002). Therefore, the reduction of effective stress kerogen into gas or oil involves the process of load transfer
resulting from overpressure generated by fluid expansion from the kerogen into the pore fluid in addition to the expan-
processes forces pore pressure to increase to a higher degree sion of pore fluids.
than the increase of pore pressure that is caused by the
under‐compaction process. The increase in pore pressure due 7.2.2.2 Clay Diagenesis clay diagenesis includes the
to the fluid expansion process is faster than the decrease in transformation of smectite to illite, kaolinite to illite,
effective stress and can be greater than the increase in over- and illitization of mixed‐layer clay (illite/smectite). The
burden stress (Fig. 7.5). transformation of smectite to illite is a broadly known clay
As mentioned earlier, overpressure generated by unload- transformation process that occurs in deeply buried shale
ing mechanisms involves the expansion of the pore fluids or formations (hower et al., 1976). In compacting shales under