Inversion of elastic wave velocities in granitic rocks from the Utah FORGE geothermal project for the anisotropic orientation distribution of microcracks

ABSTRACT Based on ultrasonic measurements on granitic rock samples from well 58–32 at the Utah FORGE site under confining pressures ranging from 5 to 70 MPa, the anisotropy of P- and S-waves was found to decrease with increasing confining pressure. Although quantitative analysis of the mineral content, performed using mineral point counts on thin sections, provided insight into the anisotropy at high confining stress, it could not predict the stress-dependent anisotropy magnitude. Using X-ray computed tomography, microcracks in the samples were found to have a preferred orientation. The contribution of microcracks to the observed anisotropy was quantified by inverting the measured anisotropic P- and S-velocities for the orientation distribution of microcracks. The measurements and their theoretical interpretation suggested that increasing fluid pressure during hydraulic fracturing in enhanced geothermal systems might lead to higher seismic anisotropy due to the formation and growth of microcracks, cracks, and fractures. It was expected that the proposed inversion approach would prove useful in monitoring production-induced reservoir changes resulting from opening and closing of natural and hydraulic fractures due to fluid injection and production.