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FIGURE G.3 Stress and pore pressure perturbations from an initial stable state leading to critical conditions. The vertical intercept represents the rock cohesive strength and is zero for a preexisting frictional fault. The slope mo of the slip criterion depends on the friction coefficient μ and on the fault inclination β. The sketch corresponds to the normal conditions when σ′v > σ′h.

The existence of a perturbation ΔS reflects the fact that injection or extraction of fluid in deep layers has consequences beyond simply increasing or decreasing the pore fluid pressure. As explained in Chapter 2, the propensity of permeable rocks to expand (contract) as a response to increase (decrease) of pore pressure induces stress change not only in the reservoir but also in the surrounding rocks. 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 ΔP′ < 0 and ΔS = 0, which would cause the stress point in Figure G.3 to move horizontally (m = 0) to the left.

So far the discussion has been focused on slip on a preexisting fault of known inclination β. The formation of a fault associated with the large-scale shear failure of the rock can be treated within the same framework, with the critical difference that the inclination of the created fault depends only on the friction coefficient μ. It also follows that in the representation of Figure G.3, the slope mo of the slip criterion (now usually referred to as the Mohr-Coulomb criterion) is exclusively a function of μ. The vertical intercept of the Mohr-Coulomb criterion with the S axis then embodies the cohesive shear strength of the rock.

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