Page 266 - Applied Petroleum Geomechanics

Abnormal pore pressure mechanisms 257

2.8
Illi za on with unloading
2.7 Illi za on with loading
Density in shale (g/cm 3 ) 2.5 y = 0.9093x 0.1248
2.6

R² = 0.9386
2.4
2.3
Overpressure
2.2
Smectite normal pressure
Illite normal pressure
2.1
Illite unloading
Power (Loading)
2
0 1000 2000 3000 4000 5000 6000 7000
VES from MDT (psi)
Figure 7.17 Cross-plot of bulk density and vertical effective stress (VES in the figure) in
several wells showing illitization with loading and unloading.

behaviors of the shale diagenesis: illitization loading and unloading
(Fig. 7.17). From smectite shales to illite shales with normal pressure and
slight overpressure, the shales experience illitization as the loading (VES)
increases, i.e., bulk density increases as the loading increases. This mainly
appears at a shallow depth. Fig. 7.17 also indicates that a great shift occurs in
the overpressure data (most red squares) in the latter stages of illitization, in
which the overpressure data do not follow the stressedensity loading path.
This stage can be considered as the illitization with unloading, a similar
behavior reported in Lahann et al. (2001). From Fig. 7.17 the illitization
unloading curve corresponds to overpressures; however, the illitization
loading curve coincides to normal pressures. This implies that only the
illitization with unloading generates overpressures. The following correla-
tions can be used to describe the loading and unloading curves:
Illitization with loading: r ¼ 0:909s 0:1248
l
e
Illitization with unloading: r ¼ 1:71s 0:055
u
e
where r l and r u are the bulk densities in loading and unloading cases,
respectively; s e is the vertical effective stress.
7.3.4 Smectite and illite normal compaction trend and
overpressure

For the basins with smectite-dominated shales, the transformation from
smectite to illite may increase pore pressure. For pore pressure prediction,
the normal compaction trends in smectite and illite should be different, and
a composite NCT is needed. Transition from smectite to illite is primarily