Characterization of crack distribution: fabric analysis versus ultrasonic inversion

We analyse the relation between rock fabric, expressed by the preferred ori entation of rock-forming minerals and microcracks, and elastic anisotropy o f crystalline rock from the KTB pilot well. Detailed analyses of mineralogi cal composition, textures and microcrack fabrics were performed. In additio n, ultrasonic velocity measurements of spherical samples in several directi ons were carried out at various confining pressures, and inverted in terms of the complete set of 21 elastic constants. By comparing the elastic tenso rs of the rocks at the final confining pressure (at which most of the micro cracks are closed) with those at a lower pressure level, it is possible to separate the anisotropy induced by microcracks from that caused by mineral alignment. In contrast to previous work, no a priori knowledge of the type of anisotropy (triclinic, monoclinic, orthotropic etc.), or of the spatial orientation of the symmetry elements (planes, axes) of the cracked rock or of the intact rock is assumed. Furthermore, no restrictive assumptions on t he orientation distribution function and the shape of the cracks are needed . The results show that the elastic anisotropy characteristics, whether they are related to the microcracks or to the rock-forming minerals, are clearly correlated with the directly observed rock fabrics. We show that the symme try directions of the mineral fabric and of microcrack fabric agree. A furt her result is that the microcrack-induced anisotropy dominates the other ca uses of anisotropy at confining pressures smaller than a few tens of megapa scals, the situation being reversed at higher pressures. The laboratory dat a are quantitatively compared with sonic log data from the KTB well, showin g the influence of pore fluids, effective pressure and crack density reduct ion on the anisotropy in situ.