Acoustic wave velocity and permeability evolution during pressure cycles on a thermally cracked granite

Abstract

Compressional waves velocity V P and water permeability K were measured during long-term experiments in a high-pressure vessel (in the range 11–75 MPa for confining pressures and 10–70 MPa for pore pressures). Experiments were carried out on fractured granite specimens prepared by a controlled heating treatment at 510°C, which generated thermal cracks. A distinct effective pressure law was derived from the measurements within specific [ P C ; P P ] ranges, for each of these properties. Data analysis emphasizes the differences in V P and K evolution during pressure cycles. V P shows a regular increase during effective pressure increase; but some ‘crack lips sticking’ is thought to be responsible for the hysteresis found during the first stages of effective pressure decrease. K decreases exponentially with effective pressure increase, whereas during effective pressure decrease K values are systematically lower, which attest to irreversible changes in the connected crack network likely to be the result of irreversible crack closure of a subset of cracks. Such study of the simultaneous evolution of physical properties of the rocks as a function of the applied stresses is relevant for man-induced applications like radioactive waste disposal where perturbations of the environmental stresses may modify the rock structure, thus leading to changes in mechanical or transport properties of the host rock.