MECHANICAL AND POROELASTIC PROPERTIES OF LITHOLOGIC UNITS WITHIN THE SAN EMIDIO GEOTHERMAL SYSTEM, NEVADA, UNITED STATES

The primary objective of the WHOLESCALE project is to simulate the temporal evolution and spatial distribution of stress in and around the geothermal reservoir at San Emidio, Nevada, United States. To constrain stress modeling efforts, laboratory measurements of static and dynamic elastic stiffness were performed using oriented rock samples collected from outcrops located near the San Emidio geothermal field. In this work, we sought to understand (1) the static and dynamic elastic properties of the rock formations, (2) the existence or absence of anisotropy or heterogeneity-controlled behavior at the millimeter to centimeter scales, (3) whether the elastic properties are stress dependent, and (4) whether there exists any stress induced anisotropy under reasonable net mean stress variations. To evaluate the existence of anisotropy or heterogeneity, we measured ultrasonic velocities, Vp and Vs, at 45-degree increments around the circumference of oriented cylindrical specimens for each rock type. Combining the three-dimensional velocity data with geological and textural descriptions, we addressed whether the material properties are heterogeneity-controlled and/or anisotropic at the plug scale. Static and dynamic elastic moduli were also measured at the net mean stresses of interest for each of the rock types obtained. Dynamic stress dependence and whether each material analyzed contained stress induced anisotropy were evaluated via stepped hydrostatic ultrasonic velocity measurements. The work presented herein was funded in part by the Office of Energy Efficiency and Renewable Energy (EERE), U.S. Department of Energy, under Award Numbers DE-EE0007698 and DE-EE0009032