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

Compressional waves velocity V-P and water permeability K were measured during long-term experiments in a high-pressure vessel tin 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 degreesC, 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 in 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. (C) 2000 Elsevier Science Ltd. All rights reserved.