Lab-scale Structural Analysis and Poroelastic Measurements of Rocks from the San Emidio Geothermal Field, Nevada, U.S

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 the stress modeling efforts, we perform laboratory measurements of elastic stiffnesses and effective pressure coefficients using oriented rock samples collected from outcrops located near the San Emidio geothermal field. To help contextualize lab-scale measurements through a field-scale lens, it is important to understand whether lab-scale rock deformation is controlled by structural anisotropy and/or heterogeneity. To that end, 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 descrptions, we address whether the velocity is controlled by heterogeneity and/or anisotropy at the plug scale. To better model stress in subsurface volumes of a geothermal field, it is also important to obtain accurate estimates of elastic stiffnesses and effective stress coefficients at the laboratory scale; particularly the Biot coefficients which are the effective stress coefficients for volumetric strain in an elastic porous solid. Using the information from the velocity structure and textural descrptions, we physically measure the associated stiffnesses and Biot coefficients to help constrain material behavior predictions within the stress model. In this paper, we provide a snapshot of the work in progress, including the highlights listed in the Conclusions below. The work presented herein has been 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.