Infoscience

Poster

Modelling of fluid injection into a frictional weakening dilatant fault

Understanding the mechanism of nucleation of dynamic rupture is an important issue in seismology. It is the key factor in determining the seismic potential of pre-existing faults under long-term loadings. Furthermore, the activation of Mode II fracture by means of fluid injection is the way to enhance the permeability of deep geothermal reservoirs (Enhanced Geothermal Systems) whose efficacy rely on the shear-induced dilation. Locally elevated pore pressure associated with fluid injection leads to a reduction of the fault frictional strength (product of the local normal effective stress and the slip-weakening friction coefficent) which may eventually falls below the background shear stress. As a result, a shear crack will start to propagate with an initially moderate velocity (quasi-static) as it is induced by fluid pressure diffusion. As slip accumulate, the quasi-static crack growth may become unstable due to the slip-weakening nature of friction, resulting in the nucleation of a dynamic rupture until residual frictional strength is reached (see Garagash & Germanovich, 2012). The size of such a dynamic rupture (associated with fluid injection) is intrinsically related to both the way the pore-pressure distribution evolves spatially and temporally along the fault and the initial background shear stress. Larger dynamic ruptures are actually obtained for lower overpressure that are spread over larger zones, while a dynamic rupture associated with larger (but more localized) peak overpressure reaches residual friction earlier. Moreover, for large values of overpressure (with respect to the initial effective stress state along the fault), the nucleation length is smaller for lower value of the background shear stress. In this contribution, we investigate the effect of the shear dilatancy of the fault on the diffusion of pore pressure. Dilatancy may locallly reduce pore-pressure depending on the ability of the fluid to flow in the newly created void space. Reduction in pore-pressure associated with dilatancy can result in increase of the fault shear resistance and thus can potentially arrest a dynamic rupture. We formulate a 2D model of fluid injection in a shear dilatant fault exhibiting slip weakening friction. The model couples elastic deformation, shear weakening Coulomb friction with dilatancy and fluid flow along the fault. We develop a numerical scheme based on boundary element (Displacement Discontinuity Method) for elastic deformation and a finite volume scheme for fluid flow. We verify our solver first on the non-dilatant case by comparing our results with the solution of Garagash & Germanovitch (2012). We then investigate the effect of shear-dilatancy and its feedback on the nucleation of dynamic rupture.

    Reference

    • EPFL-POSTER-223417

    Record created on 2016-11-25, modified on 2017-08-25

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