Page 70 - Reservoir Geomechanics

a.

f

Porosity PRESSURE

BURIAL AT HYDROSTATIC

UNLOADING DUE
TO OVERPRESSURE
Effective stress

b. (10 3 psi)
1 2 3 4 5
1.8
50
1.9
MAHAKAM
2.0 GULF COAST 40
2.1
Density (g/cm 3 ) 2.2 NORMAL PRESSURE 30 Porosity

2.3
20
2.4
2.5
10
2.6
2.7 OVERPRESSURE
0
2.8
0 10 20 30 40
Effective stress (MPa)

Figure 2.18. (a) Schematic illustration of the small porosity recovery that occurs upon unloading in
weak sediments. Thus, if the current state of a reservoir represents a formation that has been
unloaded, a nearly constant porosity is observed over a wide range of effective stresses (i.e. pore
pressures). (b) Field data from the Mahakam Delta (Burrus 1998) which demonstrate that because
overpressure at depths >3kmdeveloped after burial, the relation between porosity and effective
stress during loading makes it difficult to infer pore pressure from porosity during unloading (see
text). AAPG C 1998 reprinted by permission of the AAPG whose permission is required for futher
use.