Elastic wave velocities in a granitic geothermal reservoir

Geothermal resources have potential for providing cost-effective and sustainable energy. Monitoring of production-induced changes in geothermal reservoirs using seismic waves requires understanding of the elastic properties of the rock and how they change due to injection of fluids and opening and closing of natural and hydraulic fractures. P- and S-wave velocities measured in a granitic geothermal reservoir using sonic logging are systematically lower than those predicted using the composition of the rock. Cracks may occur in granitic rocks from tectonic stresses and from the thermal expansion mismatch between differently oriented anisotropic crystals. An isotropic orientation distribution of microcracks causes a significant reduction in both the P- and S-velocities, consistent with the observed sonic P- and S-velocities. Vertical fractures cause a difference in the velocity of vertically propagating shear waves polarized parallel and perpendicular to the fractures. An assumption that the lower measured velocities are caused by the presence of vertical fractures is inconsistent with the sonic data. This is because vertical fractures cause a decrease in slow S-wave velocity that greatly exceeds the decrease in P-wave velocity, in contrast to the observed data. The growth of vertical fractures in the geothermal reservoir may be monitored using the difference in velocity of the fast and slow shear waves, while the change in P-velocity in a crossplot of measured P- and slow S-velocities is useful for estimating the ratio of the normal-to-shear compliance of the fractures.