Seismic Event Due to Fluid Injection or Withdrawal - Induced Seismicity Potential in Energy Technologies - page 199

and all the stress increase takes place in the horizontal direction, with increases that range
between 40 and 80 percent of the pore pressure increase.
The expansion of the reservoir as a whole also alters the stress state in the surrounding
rock, in particular inducing a decrease of the horizontal stress above and below a thin reservoir.
These stress variations could in principle also trigger normal faulting in these regions; however,
the combination of stress and pore pressure change caused by fluid injection is more likely to
trigger seismicity in the reservoir rather than outside. The reverse is true for fluid extraction.
FLUID INJECTION IN A FRACTURED IMPERMEABLE ROCK
Unlike fluid injection in permeable rocks, the injection of fluid in fractured imperme-
able rock is essentially inducing an increase of fluid pressure in the fractures, with negligible
concomitant changes in the stress. It is therefore a worst case compared to the permeable
rock case, where the increase of pore pressure is in part offset by an increase of the compres-
sive stress, which is a stabilizing factor. (In other words, factor m introduced in Figure G.1
is about equal to zero.) Because fractures can be very conductive and offer less storage
compared to a permeable rock, the pore pressure perturbations can travel on the order of
kilometers from the point of injection.
Coulomb Criterion and Effective Stress
For slip to take place on a fault, a critical condition involving the normal stress σ (the
force per unit area normal to the fault), the shear stress τ (the force per unit area parallel to
the fault), and the pressure ρ of the fluid on the fault plane, must be met (see Figure G.2
for a representation of σ and τ). This condition is embodied in the Coulomb criterion,
|τ| = μ(σ - ρ) + c , which depends on two parameters: the coefficient of friction μ, with values
typically in the narrow range from 0.6 to 0.8, and the cohesion c , equal to zero, however,
for a frictional fault.
The Coulomb criterion simply expresses that the condition for slip on the fault is
met when the magnitude of the “driving” shear stress, |τ|, is equal to the shear resistance
μ(σ - ρ) + c . The quantity (σ - ρ) is known as the effective stress, a concept initially intro-
duced by Terzaghi (1940) in the context of soil failure. It captures the counteracting influ-
ence of the fluid pressure ρ on the fault to the stabilizing effect of the compressive stress
σ acting across the fault.
As long as the shear resistance is larger than the shear stress magnitude, the fault is
stable. However, an increase of the shear stress magnitude or a decrease of the shear strength
would cause the fault to slip if the two quantities become equal. For example, an increase
of the fluid pressure induced by injection could be responsible for a drop of shear strength
large enough to reach the critical conditions.