Types and Causes of Induced Seismicity - Induced Seismicity Potential in Energy Technologies - page 25

dominates over the stress variation and, when the consequence of fluid injection with re-
gard to the induced seismicity is considered, the stress perturbations can often be ignored.
Disregarding the stress change in the rock caused by injection is a conservative approach
because these kinds of perturbations are usually of a stabilizing nature (see Appendix G
for a detailed explanation).
Pore pressure increases in the joints and faults are potentially destabilizing, since they

Figure (a) Injection of a finite volume of fluid inside the porous elastic sphere embedded in a large impermeable
elastic body induces a pore pressure increase Δρ inside the sphere as well as a stress perturbation Δσ inside and
outside the sphere, caused by the expansion Δ V of the sphere. (b) If the sphere is freed from its elastic surround- of the sphere. (b) If the sphere is freed from its elastic surround-

ing, it will expand by the amount Δ V * due to the pore pressure increase Δρ . (c) A confining stress Δσ * needs
to be applied on the free sphere to prevent the expansion Δ V * caused by Δρ . If the material in the surrounding
medium is much softer than the material in the sphere, then Δ V ≃ Δ V * and Δσ ≃ 0; if the medium is much stiffer,

then Δ V Δσ * . Δσ refers only to the radial stress in the exterior region. Note: Syringe based on a

concept from H.F. Wang (Wang, 2000).

≃ 0 and Δσ ≃