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

Fluid flow through fractured rock systems is governed in large part by the distribution, interconnectivity, and size of fracture apertures. In‐situ stress is one of the primary factors controlling fracture aperture, and one that is altered significantly during high‐pressure fluid injections or extractions. Interactions between stress, pore pressure, aperture, and fluid flow can result in complex and evolving poroelastic behavior with significant implications regarding the predictability and risk involved with developing and managing deep subsurface reservoirs (geothermal, fossil energy, and geologic carbon sequestration). In saturated rocks, bulk electrical conductivity is sensitive to both primary and secondary porosity (i.e., matrix porosity and fractures), and therefore to fracture aperture size and distribution. We demonstrate the use of time‐lapse 3D electrical resistivity tomography for remotely monitoring stress induced changes in aperture distribution during high pressure injections into a dense fractured rock system at a scale of tens of meters. Results reveal a complex and continuously evolving stress field involving aperture dilations in the natural fracture system and aperture contractions in adjacent zones of shadow stress. Results provide information about the spatiotemporal changes in the system behavior and point to the potential of electrical imaging for autonomously and remotely monitoring evolving stress conditions by proxy through changes in bulk electrical conductivity. Plain Language Summary: The state and evolution of the stress exerted on rocks has a governing influence on how fluids migrate through deep, fractured rock systems. Dynamic changes in stress during subsurface fluid injections and extractions are difficult to observe, which limits the ability to optimize the management of subsurface reservoirs. We demonstrate how time‐lapse 3D (i.e., 4D) electrical geophysical imaging can be used to monitor how fractures expand and contract in response to stresses induced during high‐pressure injections into a deep fractured rock formation. Results point to the potential of autonomous electrical imaging for providing actionable feedback information during reservoir operations, enabling enhanced understanding and control. Key Points: Time‐Lapse 3D ERT imaging is used to monitor stress‐induced fracture dilation and contraction during high pressure injectionsHigh temporal resolution reveals fine details of complex dynamic poroelastic system behavior during and after high pressure injectionsResults demonstrate the potential of ERT for remote, proxy monitoring of changes in stress and fracture aperture in fractured‐rock systems [ABSTRACT FROM AUTHOR]