Fluid diffusion and pore-pressure distribution in microcracked rocks

Abstract

Pore pressure has a major influence on the effective stress and thus on the mechanical behaviour and the physical (elastic and transport) properties of microcracked rocks. In the field, in-situ measurements of pore-pressure is difficult outside of local measurements around boreholes. Yet, fluid migration is observed ubiquitously in the continental crust, whether in fault zones or in volcanic geothermal areas. In particular, pore pressure perturbations change the effective stress, which may lead to microseismic activity. This may also occur in conventional reservoirs, the storage of CO2 or deep geothermal energy extraction. In this study, we focus -in the laboratory- on the hydro-mechanical behavior of thermally treated Westerly granite and naturally microcracked Etna basalt samples (40 mm in diameter and 80 mm in length). The goal is to determine the pore pressure distribution and diffusion laws under different pore pressure gradients. First, classical (constant flow method) permeability measurements under small pore pressure gradient (1 MPa over the length of the sample) were carried out as a function of increasing confining pressures Pc (up to 70 MPa). The results show that permeability of samples varies exponentially with effective pressure, which is expected for cracks-porous rocks. The pressure sensitivity factor for permeability was then deduced to be of the order of 0.011~0.057 MPa-1. In a second step, permeability was measured at high (70 MPa) confining pressure, under large pore-pressure gradients (up to 60 MPa). During this part of the experiments, pore pressure was measured along the sample using newly developed fluid pressure sensors (with an absolute accuracy of +/-1MPa). Under small pore pressure gradient (2.5 MPa), our results show that the pore pressure varies linearly over the length of the sample, as expected from Darcy’s law and a constant permeability. However, with increasing pore pressure gradient (up to 60 MPa), the linearity is lost, as the permeability can no longer be assumed constant along the sample. To interpret our results, we solved the diffusion equation, assuming that permeability varies exponentially with effective pressure. For steady state flow conditions, our observations of the pore pressure distribution in the samples are consistent with the theoretical predictions. In particular, we show that the shape of the pore-pressure distribution at steady-sate does not depend on permeability itself, but rather on the permeability pressure sensitivity factor: the larger the latter, the more non-linear the pore pressure in the samples.