Infoscience

Poster

Modelling of fluid injection into a frictional weakening dilatant fault

Fluid injection at a pressure below the local minimum principal total stress in a fault may (re)activate shear crack propagation (hydroshearing). Because of the presence of asperities along the fault's surfaces, the fault hydraulic width increase with the slip (up to a constant value). The question we want to address in this contribution is the fol- lowing: does the increment of hydraulic width (dilatancy) affect the shear crack propagation along the fault? does it play a role in the shear crack propagation of unstable faults? Garagash & Germanovich [1] showed that a fault subjected to locally elevated pore pressure associated with fluid injection hosts different limiting regimes depending on how far the initial stress state is from its strength level. Notably when a fault is stressed almost to its static strength level (critically loaded fault), a large slip zone is expected. Hence at the nucleation time, the pressurized region is within the slipping patch. On the other hand, for a marginally pressurized fault (i.e when the pore pressure is just enough to activate the slip), the slipping patch is much slower than the diffusive growth of the pressurized zone. In addition to this, they showed that the regime of propagation of such pressurized faults can be ultimately stable or unstable depending on whether the initial shear stress state is greater or lower than the fault residual strength. In the former case the shear crack propagates with a moderate velocity (quasi-static) as it is induced by fluid pressure diffusion (but it might turns into a dynamic instability followed by an arrest). In the latter case, the shear crack initially propagates quasi-statically; then, as slip accumulate along the fault, the quasi-static crack growth become unstable and the shear crack runs away. The effect of dilatancy leads to a local reduction of pore-pressure at the shear crack tip depending on the ability (viscosity-related) of the fluid to flow in the newly created void space, leading to a stabilizing effect [2].

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