Page 38 - Reservoir Geomechanics

22 Reservoir geomechanics

Figure 1.7. Stress map of the southernmost San Joaquin Valley from wellbore breakouts (after
Castillo and Zoback 1994). The state of stress in this area was severely affected by the 1952 Kern
county earthquake. AAPG C 1994 reprinted by permission of the AAPG whose permission is
required for futher use.

further to the north (Yowlumne, Yowlumne North, Paloma, and Rio Viejo) seem not to
be appreciably influenced by the 1952 earthquake as by the regional change in strike
of the San Andreas fault and associated folds and faults mentioned above. So even in
this geologically complex area, the observed pattern of stresses (which could not have
been predicted a priori), can be measured with routinely collected data and utilized to
address problems of hydraulic fracture propagation and wellbore stability. Localized
faulting-induced stress perturbations are also observed on the scale of observations in
single wells and boreholes as discussed in Chapter 11.
Drilling-induced tensile wall fractures (discussed in Chapters 6 and 7) also reveal
a consistent picture of stress orientation in the oil fields in the northern North Sea
(Figure 1.8, data after Grollimund and Zoback 2000). Once again, generally uniform
stresses are observed with minor rotations occurring over spatial scales of 40–100 km.
This area is a passive continental margin where stress magnitudes are currently affected
by glacial unloading and lithospheric flexure. It should be noted that while the state of
stress in western Europe is generally NNW–SSE compression and a strike-slip/normal