Page 188 - Applied Petroleum Geomechanics

182 Applied Petroleum Geomechanics

stress conditions approach the shear failure envelope and appear critically
stressed. If the Mohr circle touches the MohreCoulomb failure envelope,
then slip on optimally oriented fractures will occur. The smallest Mohr
circle in Fig. 5.13 represents the initial effective stress state before
production with a pore pressure of 7539 psi, and with total stresses of
s h ¼ 8648 psi and s V ¼ 11,619 psi. The intermediate circle represents the
pore pressure of 5539 psi after 2000 psi of depletion. The largest circle has a
pore pressure of 4539 psi after 3000 psi of depletion. In either case shown in
Fig. 5.13, the Mohr circle grows and approaches critical stress with
continued depletion.

5.5.3 Injection and shear failures
After sufficient depletion pushes the reservoir rock to a critically stressed
state, a small volume of water injected into the fracture system should
trigger shear slip by increasing the pore pressure internal to the fractures and
lowering the effective stress that otherwise clamps them shut. Fig. 5.14 is a
Mohr circle illustrating the mechanism of shear failure induced by
repressurization (Dohmen et al., 2017). Although depletion takes years and
slowly changes the reservoir pressure, the pore pressure increase from either
direct well injection or from intersecting propagating hydraulic fractures
occurs over a short time period of hours. The rapid pore pressure increase
does not diffuse significantly into the matrix, and the injection does not
change the total stresses (horizontal stresses) immediately, but simply
reduces the effective stresses, moving the Mohr circle to the left. This effect
sets up the condition for shear failure on critically stressed, optimally

5000 Biot coeff = 0.75
injection 1500psi after 3000psi depletion
Shear stress (psi) 3000 No depletion deple on
Mohr-Coulomb envelope 0.5
4000
Depletion of 3000 psi

2000
Injec on
1000

0
0 2000 4000 6000 8000 10000
Effective stress (psi)
Figure 5.14 Mohr circle representation showing how repressurization impacts the
depleted reservoir (after drawdown of 3000 psi) with an injection of 1500 psi.