The engineering of safe hydraulic stimulations for EGS development in hot crystalline rock masses

Abstract

Two types of hydraulic stimulation may be considered for the development of Enhanced Geothermal Systems (EGS), namely hydraulic fracturing and shear induced dilatancy. Only shear induced dilatancy has been found efficient for the shallow (750 m deep) granite test site at Le Mayet de Montagne.On this site, integration of focal mechanisms of induced seismic activity together with results from Hydraulic Tests on Pre-existing Fractures has provided a well constrained characterization of the complete regional stress field. Combination of this regional stress field characterization with focal mechanisms help map the pore pressure and its heterogeneity. This integrated inversion demonstrates that a Representative Elementary Volume (REV) of the order of 30x30x30 m3 may be defined for the absolute stress characterization, but cannot be used for the definition of effective stresses in this shallow depth environment.But the development by shear induced dilatancy of two EGS reservoirs at Soultz-sous-forêts have demonstrated the validity of contemplating large VER for characterizing effective stresses for depths greater than 2.8 km in this impervious granite massif. Stress measurements conducted on this site together with the stabilization of injection pressure at a given plateau, independent of the flow rate, have been found to be consistent with the development of fresh shear zones characterized by the Hoek and Brown criterion.Shear induced dilatancy has generated aseismic slips at Le Mayet de Montagne as well as for both the reservoirs developed at Soultz. This opens the way to developing safe shear induced dilatancy through appropriate injection protocols.