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In situ stress regimes with lithology-dependent and depletion effects 177

Fig. 5.10 indicates that in the critically stressed condition (before shear
failure) a stronger fault (m f ¼ 0.6) has a smaller effective minimum stress
(s h_s ), or higher maximum shear stress (s s_max ). However, a frictionally
0
weak fault (e.g., m f ¼ 0.2 in Fig. 5.10) needs a higher effective minimum
stress (s h_w ) (thus smaller Mohr circle and much lower maximum shear
0
stress, s w_max ) to keep the fault stability. This may potentially explain the
low inferred shear stresses along strike-slip faults, such as the San Andreas
(e.g., Hickman, 1991; Townend and Zoback, 2004). The link between the
frictional strength and the minimum stress has important implications for
slip behavior on natural faults. For instance, an unstable natural fault, to
keep the frictionally weak fault from slip, would require modification of the
minimum stress (increase of the minimum stress, as suggested by Zhang and
Zhang (2017)) or the fault gauge composition (as suggested by Ikari et al.,
2011).

5.5 Depletion and injection impacts
5.5.1 Depletion-reducing horizontal stresses
Data from hydrocarbon basins document the systematic relationship
where pore pressure depletion from oil and gas extraction in both con-
ventional and unconventional reservoirs is associated with a reduction in
both the minimum and maximum horizontal stresses (e.g., Salz, 1977;
Teufel et al., 1991; Segall and Fitzgerald, 1998; Lang et al., 2011; Dohmen
et al., 2013). Fig. 5.11 presents pore pressure and in situ stress profiles
before depletion and after 4285 psi of depletion in the Middle Bakken
unconventional oil reservoir at an equivalent depth of 10,000 ft (Dohmen
et al., 2014). It shows that depletion in pore pressure greatly reduces the
minimum and maximum horizontal stresses. Measurements from the
diagnostic fracture injection tests (DFIT) in the Middle Bakken and
Three Forks plays show that the minimum horizontal stress decreases
linearly with reservoir pressure (Fig. 5.12A). It has the following
correlation (Dohmen et al., 2017):

s h ¼ 0:79p p þ 2240 (5.22)
where s h is the minimum horizontal stress, in psi; and p p is the reservoir
pressure, in psi. The depletion-induced stress path (the slope of the line
in Fig. 5.12A)is c ¼ Ds h /Dp p ¼ 0.79.
By analyzing the instantaneous shut-in pressure data in a tight,
low-porosity, low-permeability sandstone of the Vicksburg formation,