Measurements of Elastic Moduli and Stress Dependence of Geothermal Rocks

Abstract

ABSTRACT: In the WHOLESCALE project we aim 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, we perform laboratory measurements of static and dynamic elastic stiffnesses using oriented rock samples collected from outcrops located near the San Emidio geothermal field. In this paper, we seek 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 measure 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 address whether the materials 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 the material contained stress induced anisotropy was evaluated via stepped hydrostatic ultrasonic velocity measurements. 1. INTRODUCTION The geothermal field at San Emidio has provided more than two decades of data collection, with power first being produced in 1987 (Feigl et al., 2022). Since 1987, three DOE-sponsored projects have been awarded to study the geothermal resource at San Emidio. The first DOE-sponsored project (DOE award DE-EE0002847) conducted exploratory drilling between 2010 and 2014 (Feigl et al., 2020). The second and third DOE-sponsored projects (DOE awards DE-EE0007698 and DE-EE0009032) are currently underway to characterize the geothermal resource at San Emidio and to further investigate stress in the geothermal system. One of these DOE-sponsored projects is called WHOLESCALE. As such, the WHOLESCALE acronym stands for "Water and Hole Observations Leverage Effective Stress Calculations and Lessen Expenses".