3D-hydromechanical Behavior of a Stimulated Fractured Rock Mass

Abstract

The EGS concept has the necessity to increase the local natural permeability of low permeability rock masses. Hydraulic stimulation is a widely used technique to increase the permeability. This technique was applied at various depths in the wells of the Soultz-sous-Forêts site since the beginning of the project. This has yielded an effective and irreversible increase in permeability, which is associated with microseismic events. The understanding of the mechanisms that lead to local increases in permeability is essential to the development of this technique.

Based on all the hydraulic stimulation tests that have been performed in the wells, a 3D hydro-mechanical model is proposed. The fracturation model selected around each well contains large deterministic fault zones (hectometric size), since they likely connect the well to the reservoir and potentially react during the hydraulic stimulation tests.

A 3D Distinct Element Method model was developed, with a specific hydro-mechanical coupling, where flow takes place only in the fractures and the rock matrix is impermeable and deformable. The constitutive law of the fault zones is a Mohr-Coulomb law for the mechanical part with an independent associated dilatancy law for the hydraulic part. Hydraulic stimulation tests and post-stimulation injection tests have been performed with this model. The results obtained clearly indicate how an irreversible increase in the permeability is caused by the shearing that occurs in the fault zones during the hydraulic stimulation tests. The location of this shearing in the fault zones results from the combination of the in-situ stresses, their orientations, and their interconnections. The magnitudes of the calculated shear displacements were found to be consistent with the observations along the wells and the information obtained from the analysis of the microseismic events.