Closure of Micro Cracks in Rock Samples Under Confining Pressure

Abstract

AbstractMicro cracks widespreading in rocks have significant effects on their physical properties. In order to get a general idea on shape changes of micro cracks in rock samples, we measured P‐ and S‐wave velocities and linear strain in three orthogonal directions, under different confining pressures, by using several rocks with marked foliation and strong lattice preferred orientation. The effective Poisson's ratio, crack density, crack porosity and crack aspect ratio c/a (a is the length and c is the width of the “penny‐shaped” micro crack) of micro cracks are calculated from the measured data by O'Connell's model. The results show that the closure of micro cracks normal to the foliation plane is the fastest but the corresponding c/a value does not decrease, but increase, with the increase of confining pressure. It may be caused by the roughness and rugosity of the crack faces, which can make one crack into several smaller ones with significantly shorter lengths but widths not markedly smaller, during the progressive closure of micro cracks with increasing confining pressure. So, c/a ratio increases with confining pressure.