Fluid flow through microcracked and fractured granitic reservoirs

The permeability of crustal rocks and rock-masses dictates the efficiency of hydrothermal circulation and therefore the productivity of geothermal resources. In this study, we find by monitoring P-wave velocity that thermal microcracking in Lanhélin granite (from Brittany, France) starts around 70 °C and accumulates up to the maximum imposed temperature of 700 °C. Porosity and permeability increase as thermal stressing temperature increases from room temperature to 700 °C. In-situ measurements also revealed a decrease of P-wave velocity of about 50% between room temperature and 450°C. The permeability of thermally stressed samples decreases by about one order of magnitude during triaxial loading to the peak stress, due to the closure of pre-existing microcracks, and then increases following the formation of a macroscopic fracture. Permeability then remains more-or-less constant as strain is accommodated by sliding on the resultant shear fracture. In all cases, the permeabilities of the samples at the end of the experiments is lower than at the start of the experiments, as it has been previously observed in other rock types such as porous sandstone of porosity larger than 10%. Our results show that the permeability of microcracked granite evolves differently to intact granite, for which permeability increases, during pre-failure deformation in the brittle regime. Such results have important implications for fluid flow in crustal fault systems and, in particular, for their potential for geothermal energy exploitation.