Hydraulic stimulation of pre-existing discontinuities in tight rocks

Hydraulic stimulation is an engineering technique whose aim is to enhance the permeability of fractured rock masses at depths ranging from one to five kilometers. It consists in the injection of fluid at sufficiently high pressure in order to shear pre-existing fractures and/or to create new fractures. The local reduction of effective stresses can indeed result into localized inelastic deformations along pre-existing discontinuities which upon dilation increase the overall permeability of the rock mass. Although this technique is used to extract deep geothermal energy from crystalline rocks, some fundamental hydro-mechanical mechanisms are not yet fully understood, especially in regards to the transition between aseismic and seismic slip. In this context, the present thesis investigates the interplay between pore pressure diffusion within pre-existing discontinuities and induced deformations including the possible nucleation of a dynamic rupture. This is achieved via the development of specific numerical algorithms that are extensively verified against existing analytical and semi-analytical solutions of fracture growth. Thanks to the use of a boundary integral representation for elasticity, fluid driven deformations localized on pre-existing discontinuities can be efficiently modelled, without involving an intensive discretization of the whole domain. We first present an in-depth study on the effect of dilatancy on the propagation of a fluid driven crack along a frictional weakening planar fault. The numerical results reveal that shear-induced dilatancy can effectively stabilize an otherwise unstable fault with respect to the nucleation of an unabated dynamic rupture. Although counter-intuitive, this is valid only for sufficiently large injection over-pressure. This important result is confirmed theoretically using linear elastic fracture mechanics (LEFM) under small-scale yielding approximation. The simulations further show that this stabilization still hold even for large increase of fault permeability associated with shear deformations. In a second part of this manuscript, a new boundary element solver for localized inelastic deformations along a large set of pre-existing planes is proposed and described. Upon validation against several analytical and semi-analytical solutions, a series of problems are addressed in order to illustrate the capabilities, accuracy and performance of this algorithm. It is then used to model hydraulic stimulation of fractured rock masses in the extreme cases of critically stressed and marginally pressurized conditions.