Overview and Preliminary Results from the WHOLESCALE Project at San Emidio, Nevada, U.S

Abstract

The WHOLESCALE acronym stands for Water & Hole Observations Leverage Effective Stress Calculations and Lessen Expenses. The goal of the WHOLESCALE project is to simulate the spatial distribution and temporal evolution of stress in the geothermal system at San Emidio in Nevada, United States. To reach this goal, the WHOLESCALE team is developing a methodology that will incorporate and interpret data from four methods of measurement into a multi-physics model that couples thermal, hydrological, and mechanical (T H M) processes over spatial scales ranging from the diameter of a borehole (~0.1 m) to the extent of the entire field (~10 km) and temporal scales ranging from the duration of a microseismic event (~1 second) to the typical lifetime of a producing field (3 decades). The data sets include observations from geology, seismology, drilling, geodesy, and hydrology. The WHOLESCALE team is taking advantage of the perturbations created by changes in pumping operations to infer temporal changes in the state of stress in the geothermal system. This rheological experiment will apply the key idea that increasing pore-fluid pressure reduces the effective normal stress acting across preexisting faults. The work plan includes: (1) measuring rock-mechanical material properties in the laboratory, (2) manipulating the stress field via hydraulic and thermal methods, (3) measuring the resulting response by geophysical methods, and (4) calculating the stress, strain, pressure, and temperature in the geothermal system using an open-source, numerical simulator named GEOS. To interpolate and interpret these rich data sets, GEOS will use the finite-element method to solve the coupled differential equations governing the physics of a fractured, poroelastic medium under stress. The study site at San Emidio includes a volume with length of ~6 km, width ~5 km, and depth ~2 km. In this paper, we provide a snapshot of 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.