The permeability of granite deformed in the brittle regime to large strains: Implications for the permeability of fractured geothermal reservoirs

Efficient fluid flow and circulation are important for an economically viable geothermal reservoir. One type of underexplored reservoir for high-temperature geothermal exploitation is a crustal fault zone, where hot fluids from depths corresponding to the brittle-ductile transition are brought to the surface via crustal-scale, permeable fault zones. To better understand the evolving permeability of reservoir rock during deformation in the brittle regime—fault formation and sliding on the fault—we performed triaxial experiments on samples of well-characterised Lanhélin granite (France) in which we measured the permeability of the sample during deformation to large strains (up to an axial strain of about 0.1). We first thermally-stressed our samples to 700 °C to ensure their permeability was sufficiently high to measure on reasonable laboratory timescales. Experiments were performed on water-saturated samples (pore fluid pressure = 10 MPa), at effective pressures of 10, 30, and 50 MPa (corresponding to a maximum depth of about 3 km), and at ambient laboratory temperatures. Our data show that sample permeability decreased (by about an order of magnitude) prior to macroscopic shear failure, as the closure of pre-existing microcracks outweighed the formation of new microcracks during loading up to the peak stress. Sample permeability increased following fracture formation (by about a factor of two). Sliding on the fracture to large strains (corresponding to a fault displacement of ~7 mm) did not appreciably change the permeability of the sample, and therefore the permeability of the fracture did not fall below that of the host-rock. Although the permeability of the sample at the frictional sliding stress was lower at a higher effective pressure (by about an order of magnitude between 10 and 50 MPa), the evolution of sample permeability was qualitatively similar for effective pressures of 10−50 MPa. We now plan to use the results of this experimental study to inform numerical modelling designed to explore the influence of macroscopic fractures on fluid flow within a fractured geothermal reservoir.