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Appendix G: Seismic Event Due to Fluid Injection or Withdrawal
Pages 219-224

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From page 219...
... 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.
From page 220...
... . But for very specific situations, the compressive stress increases in the vertical and in the horizontal directions are unequal, the stress ratio being a function of the shape of the reservoir and the contrast in elastic properties between the reservoir and the surrounding rocks (Rudnicki, 1999, 2002)
From page 221...
... FLUID INJECTION IN A FRACTURED IMPERMEABLE ROCK Unlike fluid injection in permeable rocks, the injection of fluid in fractured impermeable 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 compressive stress, which is a stabilizing factor.
From page 222...
... The normal and shear stress on the fault can actually be expressed in terms of the in situ vertical and horizontal stresses, σv and σh, through a relation that depends on the fault inclination β (Figure G.2)
From page 223...
... Only in the particular case of impermeable rocks, where flow of fluids only takes place in a fracture network, are the perturbations essentially only of a hydraulic nature. For example, injection of fluid in fractured impermeable rock causes mainly an increase of pore pressure Δρ leading to ΔΡ′ < 0 and ΔS = 0, which would cause the stress point in Figure G.3 to move horizontally (m = 0)
From page 224...
... 2002. Eshelby transformations, pore pressure and fluid mass changes, and subsidence.

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