Influence of shear and deviatoric stress on the evolution of permeability in fractured rock

The evolution of permeability in fractured rock as a function of effective normal stress, shear displacement, and damage remains a complex issue. In this contribution, we report on experiments in which rock surfaces were subject to direct shear under controlled pore pressure and true triaxial stress conditions while permeability was monitored continuously via flow parallel to the shear direction. Shear tests were performed in a pressure vessel under drained conditions on samples of novaculite (Arkansas) and diorite (Coso geothermal field, California). The sample pairs were sheared to 18 mm of total displacement at 5 mu m/s at room temperature and at effective normal stresses on the shear plane ranging from 5 to 20 MPa. Permeability evolution was measured throughout shearing via flow of distilled water from an upstream reservoir discharging downstream of the sample at atmospheric pressure. For diorite and novaculite, initial (preshear) fracture permeability is 0.5-1 x 10(-14) m(2) and largely independent of the applied effective normal stresses. These permeabilities correspond to equivalent hydraulic apertures of 15-20 mu m. Because of the progressive formation of gouge during shear, the postshear permeability of the diorite fracture drops to a final steady value of 0.5 x 10(-17) m(2). The behavior is similar in novaculite but the final permeability of 0.5 x 10(-16) m(2) is obtained only at an effective normal stress of 20 MPa.