Page 53 - Geology and Geochemistry of Oil and Gas

30 OIL- AND GAS-BEARING ROCKS
and Serebryakov (1989) determined that the lowermost limit of permeability in the
2
reservoirs was 10 15 –10 16 m , whereas the permeability in young argillaceous cap-
2
rocks was 10 20 m .
The same rocks react differently to different fluids. In some cases, rocks serve as
satisfactory or good conduits for water, but form barriers for oil or gas movement.
In some other situations rocks yield oil but stop gas movement, etc. This is deter-
mined by capillary forces, the magnitude of which depends on fluid and rock prop-
erties (fluid density, fluid viscosity, rock structure, rock wettability) and pore size
(capillary forces almost disappear when the pore diameter exceeds 0.5 mm). Ac-
cording to the Laplace’s law,

p ¼ 2d=r (2.12)
c
where p c ¼ the capillary pressure, d ¼ the surface tension, and r ¼ the surface cur-
vature radius (at a pore diameter o 0.1 mm). For fractures, the equation has the
following form:

p ¼ 2d cos a=w (2.13)
c
where w is the fracture opening (width) and a is the contact angle.
All aforementioned rock and fluid properties are strongly affected by the sub-
surface temperature and pressure and geochemical environment.
The writers propose the following definition of caprock: caprock is a rock that
prevents the flow of a given fluid at a certain temperature and pressure and geo-
chemical conditions. Therefore, the necessary properties of a rock to act as a seal will
be different for different fluids. The same rock with different fluids may or may not
have sealing properties up to a complete inversion (caprock - reservoir). I. M.
Mikhailov, B. I. Yakovlev and N. A. Eremenko (in: Eremenko, 1978) proposed the
following three types of caprocks.
Type I caprocks are typical for argillaceous sequences in a state of continuing
compaction; they are developed in areas of young subsidence of Earth’s crust, with
abnormally high pore water pressure. Sealing properties of these rocks are deter-
mined by the amount of capillary pressure at the contact of the reservoir and cap-
rock, the pore pressure of water saturating the caprock, initial pressure gradient of
water and the variation of hydraulic forces in the section. Sealing property,
or caprock breakthrough potential (P breakthrough ), may be presented as follows
(Fig. 2.6):
P breakthrough ¼ P b ¼ P pw þ P c þ G w ah (2.14)
where P pw ¼ the maximum pore water potential in caprock, P c ¼ the capillary
pressure of breakthrough into the caprock, G w ¼ the initial water pressure gradient
in caprock, h ¼ the seal thickness, and

1 P w:u:layer P w:l:layer
a ¼ þ 1
2 G w h