Thermo-hydro-mechanical-chemical controls on fracture and fault responses to fluid injection in Enhanced Geothermal Systems: Current understanding and future directions

The large-scale deployment of Enhanced Geothermal Systems (EGS) is hindered by induced seismicity, as well as by limited and unsustained reservoir permeability enhancement. This review synthesizes recent advances in our understanding of the way fracture/fault stability and permeability evolve together, based on laboratory experiments, numerical simulations, and field observations. While frameworks such as rate-and-state friction and displacement- and velocity-dependent aperture models have improved our understanding, significant gaps remain, particularly in capturing the coupled evolution of friction and permeability in geological discontinuities (fractures and faults) under realistic geothermal conditions. In this context, we have reviewed recent advances and outlined future endeavors in key factors influencing discontinuity behavior, including proppant use, mineral infill effects, thermal effects, fracture branching effects, normal constraint effects, and stick-split (i.e., episodic hydro-fracturing). We also highlighted the need for improved cross-scale analysis linking laboratory-, mine-, and field-data. Emerging technologies, such as distributed fiber-optic sensing and machine learning, offer new pathways to monitor, process, and interpret complex datasets, enabling better prediction and management of fracture and fault behavior under in-situ conditions. By proposing future research directions and emphasizing the integration of thermo-hydro-mechanical-chemical (THMC) coupling and cross-scale approaches, this review aims to support the development of safer and more effective geothermal energy systems.