Overview of Results and Lessons Learned from the PoroTomo project at Brady Hot Springs, Nevada: Poroelastic Tomography by Adjoint Inverse Modeling of Data from Seismology, Geodesy, and Hydrology

In the geothermal field at Brady Hot Springs, Nevada, subsidence occurs over an elliptical area that is 4 km by 1.5 km. Characterizing such structures in terms of their mechanical properties is essential for successful operations of Enhanced Geothermal Systems (EGS). The goal of the PoroTomo project is to assess an integrated technology for characterizing and monitoring changes in the rock-mechanical properties of an EGS reservoir in three dimensions with a spatial resolution better than 50 meters. In March 2016, we deployed the integrated technology in a 1500-by-500-by-400-meter volume at Brady Hot Springs. The data set includes: active seismic sources, fiber-optic cables for Distributed Acoustic Sensing (DAS) and Distributed Temperature Sensing (DTS) arranged vertically in a borehole to 400 m depth as well as horizontally in a trench 8700 m in length and 0.5 m in depth, 246 seismometers on the surface, 3 pressure sensors in observation wells, continuous geodetic measurements at three GPS stations, and 7 InSAR acquisitions. The deployment consisted of four distinct time intervals ("stages"). Between each measurement interval, the hydrological conditions were intentionally manipulated by modifying the rates of pumping in the injection and production wells. To account for the mechanical behavior of both the rock matrix and the fluids within its fractures and pores, we are developing numerical models for the 3-dimensional distribution of the material properties. Overall, the data sets collected at Brady Hot Springs support a conceptual model of highly permeable conduits along faults channeling fluids from shallow aquifers to the deep geothermal reservoir tapped by the production wells. 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 Number DE-EE0006760. The web site for the PoroTomo project is: http://geoscience.wisc.edu/feigl/porotomo/