Seismic Event Due to Fluid Injection or Withdrawal - Induced Seismicity Potential in Energy Technologies - page 197

Seismic Event Due to
Fluid Injection or Withdrawal
To initiate a seismic event by activation of an existing fault, a critical condition involv-
ing the in situ state of stress and the pore pressure needs to be met. As discussed below,
this condition stems, at least for the simplest case of slip initiation along a preexisting fault,
from a combination of two fundamental concepts: (1) slip is initiated when the shear stress
acting on the fault overcomes the frictional resistance and (2) the frictional resistance is
given by the product of the friction coefficient times the normal effective stress, defined as
the normal stress across the fault reduced by the fluid pressure. This condition of slip initia-
tion, referred to as the Coulomb criterion, can then be translated as a limit condition on the
magnitude of the vertical and horizontal stress and of the pore pressure, which depends on
the inclination of the fault. The formation of a fault follows similar concepts but accounts
for an additional shear resistance due to cohesion; also the actual orientation of the created
fault corresponds to the inclination for which the condition of slip is first met.
Although the initial in situ stress state and pore pressure are often close to the limit
condition required to cause slip on an existing fault, not all perturbations in the stress and
pore pressure associated with fluid injection or extraction eventually trigger a seismic event.
First, the perturbation must be destabilizing in its nature; that is, it must bring the system
closer to critical conditions, irrespective of the magnitude of the perturbation. Indeed some
perturbations are stabilizing, meaning that they move the system farther away from criti-
cal conditions. The degree of destabilization can be assessed by a certain parameter m that
characterizes the nature of the stress and pore pressure perturbation (Figure G.1). Second,
if the perturbation is indeed destabilizing, the magnitude of the perturbation has to be large
enough to reach critical conditions. Finally, not all slip events are seismic, although most
are, as gouge-filled faults could respond in a ductile stable manner.
It is useful to contrast the case of fluid injection in reservoir rocks, where the fluid flows
and is stored in the pore network of the rock, from that in crystalline impermeable rocks,
where the injected fluid is essentially transmitted and stored in the fracture network. In the
permeable case, the pore pressure increases in the rock induce stress variation in the reservoir
and in the surrounding rock. In the impermeable case, the stress induced by injection is
negligible (except in situations where the fracture network is very dense), but fluid pres-
sure change can be transmitted over a large distance by fractures that offer little resistance
to flow. Although our analysis in this appendix refers to a finite-extent reservoir, solution
of the infinite case lies within the finite solution. For the purposes of understanding pore