Page 253 - Applied Petroleum Geomechanics

246 Applied Petroleum Geomechanics

Pore pressure and stress (MPa)
0 20 40 60 80 100 120 140 160 180 200
0 0
Hydrostatic pressure
1000 Overburden stress 10
20
2000
30
3000
Depth (m) 4000 50

5000 Pp gas generaƟon 60
40 Time (M.Y.)
70
6000
80
7000
90
8000 100
Figure 7.9 Pore pressure build-up with depth and deposition time showing the pore
pressure generation trend developed in an isolated system when oil cracks to gas
(Carcione and Helle, 2002).

oil thermally cracks to 534.3 volumes of gas at standard temperature and
pressure during deep burial (Barker, 1990). This extremely high volume
will develop very high overpressures, if the reservoir is an effectively iso-
lated system. Fig. 7.9 displays the calculated pore pressure generated from
oil cracking into gas. It demonstrates clearly the potential for generating
very high overpressures, most of which are in the geologically unreasonable
range of greater than the overburden stress.
Calculations by Barker (1990) show that if the reservoir system remains
open (i.e., at hydrostatic pressure) and is initially filled with oil that is
subsequently cracked to gas, then roughly 75% of the gas will be lost or the
reservoir volume must effectively increase in size, for example, by moving
the gasewater contact downward. If the reservoir is sealed and totally filled
with oil, cracking of as little as 1% of the oil is enough to raise pressures to
1 psi/ft (22.6 MPa/km). When these pressures exceed the fracture gradient,
the induced fracturing will break the seals, causing gas loss and pressure
decrease. The induced overpressures from hydrocarbon generation also
create microfractures in the source rock. Laboratory measurements of
ultrasonic velocity and anisotropy in kerogen-rich black shales of varying