Page 299 - Reservoir Geomechanics

277 Stress fields

55 Normal faulting

50 S v

45
1982
40

DEPLETION
Stress (MPa) 35 1990

30
1998

25
= 0.6
20

15

10
10 15 20 25 30 35 40 45 50 55
Pore pressure (MPa)

Figure 9.6. Evolution of the least principal stress with depletion at the crest of the anticlinal
structure in the Valhall field of the North Sea (after Zoback and Zinke 2002). Note that the normal
faulting theory explains the measured least principal stress magnitudes (for a coefficient of friction
of 0.6) for the weak chalks in this field before, during and after depletion.

sand reservoirs separated by thick accumulations of shale (Alexander and Flemings
1995). Finkbeiner (1998) determined the magnitude of the least principal stress from
frac pack completions – hydraulic fractures made in reservoir sands for the purpose of
minimizing sand production. What makes these data important is that most published
least principal stress data for the Gulf of Mexico were compiled from leak-off tests
made at various depths in the region. These tests are usually made in shales (where
the casing is set). In many published studies, these data have been interpreted with
pore pressure measurements that were made in sands encountered at various depths in
the region. Hence, many published empirical studies juxtapose stress and pore pressure
measurements made in different wells and different lithologies. The advantage of using
frac pack data is that at any given depth, the values of both the least principal stress