Electrical Resistivity Tomography Based Monitoring of Stress Perturbations to Optimize Placement of High-Precision Strain Meters
Authors: Johnson T. C. , Burghardt J. , Hammond G. E. , Karra S. , Jaysaval P. , Rosso K. , Hyman J. D. 
Published: August 2024

ABSTRACT:
The Center for Understanding Subsurface Signals and Permeability is a new U.S. Department of Energy Earthshot Center focused on understanding and predicting the long-term evolution of permeability in enhanced geothermal systems. The center will use a highly instrumented testbed within the Sanford Underground Research Facility to conduct field scale experiments that elucidate and test capabilities to simulate geochemical-geomechanical interactions and permeability evolution. Here we demonstrate initial developments using previously collected electrical resistivity tomography (ERT) monitoring data with high-performance multi-physics modelling advancements to inform the optimal location of two new monitoring boreholes. Specifically, ERT monitoring data collected during shear stimulation testing shows marked responses to changes in stress during borehole pressurization. We demonstrate how the same response is being simulated, ultimately to train a machine-learning algorithm to estimate rock properties and enable enhanced prediction of stress and strain responses anticipated during future testing campaigns.

1. INTRODUCTION
The Center for Understanding Subsurface Signals and Permeability (CUSSP) is a new U.S. Department of Energy – Office of Science funded Enhanced Geothermal System (EGS) Earthshot center focused on elucidating the complex feedbacks between physical and chemical processes and permeability evolution in EGS systems. CUSSP plans to enhance the highly instrumented testbed on the 4100 level of the Sanford Underground Research Facility, developed under the EGS Collab project, to conduct closely monitored and accurately simulated physiochemical experiments that ultimately demonstrate understanding, predictability, and control of system permeability over time.
Multi-physics signal integration is a key aspect of CUSSP. CUSSP will develop and exploit electrical resistivity tomography (ERT) for its ability to inform on stress-induced changes in porosity, which when combined with strain monitoring will help deconvolute relationships between pressure distribution and deformation. The 4100-level testbed will be augmented with new monitoring boreholes containing ERT electrodes and high precision multi-component strain meters. In this paper, we demonstrate how prior pressure, flow, and 4D ERT monitoring data collected under the EGS Collab project during shear stimulation attempts are being used to inform optimal placement of the new monitoring boreholes. The shear stimulation experiments, meant to increase fracture permeability, involved pressurizing isolated sections of an injection borehole below the minimum principal stress to open and shear slip a potential flowable natural fracture. Although each stimulation attempt failed to achieve shear-slip, time-lapse ERT collected during each attempt was inverted to reveal zones of decreased electrical conductivity forming around the isolated and pressured stimulation interval. Because there was no flow into the system, the zone of decreased electrical conductivity could only have been caused by the compressive stress-bulb that developed around pressurized interval, and the corresponding decrease in porosity within the stress-bulb footprint. These results conclusively demonstrate the field-scale sensitivity of electrical conductivity to stress that have been previously demonstrated at the laboratory scale.