Résumé
Understanding the coupling between rock permeability, pore pressure, and fluid flow is crucial, as fluids play an important role in the Earth's crustal dynamics. We measured the distribution of fluid pressure during fluid-flow experiments on two typical crustal lithologies, granite and basalt. Our results demonstrate that the pore-pressure distribution transitions from a linear to a non-linear profile as the imposed pore-pressure gradient is increased (from 2.5 to 60 MPa) across the specimen. This non-linearity results from the effective pressure dependence of permeability, for which two analytical formulations were considered: an empirical exponential and a new micromechanics-based model. In both cases, the non-linearity of pore pressure distribution is predicted. Using a compilation of permeability versus Terzaghi's effective pressure data for granites and basalts, we show that our micromechanics-based model has the potential to predict the pore pressure distribution over the range of effective pressures expected within the brittle crust.
Fluids distributions and fluid migrations play an important role in the Earth's crustal dynamics and how fluids migrate through a rock will depend primarily on permeability. However, the permeability of crustal rocks may exhibit important pressure dependence, because cracks and fractures will increasingly close with increasing tectonic pressure. In this experimental study, we show that the couplings between increasing pressure, crack closure, and permeability reduction may result in non-linear pore pressure distributions on a rock specimen at the laboratory scale, which confirms for the first time pioneering theoretical and experimental works. Two simple analytical expressions of the pressure dependence of permeability predict this non-linearity. One empirical expression, most commonly used in the literature, takes the form of an exponential. The second one, a new model, based on crack micromechanics, was developed within this work and shown to outperform the exponential formulation at low Terzaghi's effective pressure.
Pore pressure was measured locally in rocks exhibiting pressure-dependent permeability We observed a transition from linear to nonlinear pore pressure distribution with increasing fluid pressure gradients A new, micromechanics-based, analytical model was developed for the pressure dependence of permeability in microcracked rocks