4D Proxy Imaging of Fracture Dilation and Stress Shadowing Using Electrical Resistivity Tomography During High Pressure Injections Into a Dense Rock Formation

Abstract

Understanding the state and evolution of subsurface stress is critical for developing effective strategies for extracting geothermal and fossil energy resources, sequestering carbon in geologic repositories, managing the risk of induced seismicity, and protecting the environment (U.S. Department of Energy, 2015). Subsurface energy production and carbon sequestration objectives require adequate understanding and control of subsurface fluid flow. The hydrogeologic properties that govern fluid flow are altered by significant changes in stress state (Ito & Hayashi, 2003;Min et al., 2004;Zoback & Byerlee, 1975). For example, during reservoir stimulation injections, deformations caused by changes in fluid pressure can cause existing fractures to open, close, or shear even at locations away from, and without direct hydraulic connection to the injected fluid. Interactions between stress and hydraulic properties often result in complex and evolving fluid flow fields both during and after fluid injections and extractions. Understanding these site-specific interactions over time will enable improved understanding and control of stress-sensitive fractured rock flow systems.

4D Electrical Resistivity Imaging of Stress Perturbations Induced During High‐Pressure Shear Stimulation Tests

“…Decreases in BEC mirror the anticipated effective stress perturbation, in that the largest magnitudes occur at, and decay with distance from, the pressurized interval. The relationship between stress and BEC can be described as follows (Brace, 1975; Johnson et al., 2021; Kaselow & Shapiro, 2004):…”

4D Electrical Resistivity Imaging of Stress Perturbations Induced During High-Pressure Shear Stimulation Tests

“…Decreases in BEC mirror the anticipated effective stress perturbation, in that the largest magnitudes occur at, and decay with distance from, the pressurized interval. The relationship between stress and BEC can be described as follows (Brace, 1975;Johnson et al, 2021;Kaselow & Shapiro, 2004): that either the pore water compresses within the pore space, or the pore water migrates out of the pore space. Although meausurements of host rock compressibility (or its inverse, bulk modulus) aren't available, we assume that, like other dense crystalline rocks, rock compressibility is less than that of water at the same temperature and pressure.…”

Relationship Between Permeability and Resistivity of Sheared Rock Fractures: The Role of Tortuosity and Flow Path Percolation

“…The presence and reactivation of mesoscale fractures and the associated changes in permeability are important for developing enhanced geothermal systems (EGS). In practice, resistivity observations in EGS projects detect resistivity changes (approximately a few percent) (Didana et al., 2017; Johnson et al., 2021; Peacock et al., 2013) that are associated with the hydraulic stimulation of preexisting fault systems. Under the assumption that α takes a value between 1 and 3 in Equation , these resistivity changes correspond to a few to several tens of percent increases in permeability.…”