Positive and Negative THMC Feedback Mechanisms for Fracture Flow Channeling in Enhanced Geothermal Systems

Abstract

Geothermal energy stored in hot dry rocks (HDRs) has been characterized as an essential constituent of the global effort to achieve carbon neutrality and combat climate change. Enhanced geothermal system (EGS) is a promising technique to extract geothermal energy from HDRs, but the long-term thermal performance is largely jeopardized by flow/thermal short-circuiting, a phenomenon that most injected fluid concentrates in a few major flow channels and has been widely reported in field EGS practices. To understand the underlying physical and chemical mechanisms of flow short-circuiting, we first perform a comprehensive literature review to summarize the main causes of flow short-circuiting. Based on previous studies, we develop a thermo-hydro-mechanical-chemical (THMC) coupled model to simulate the complex physical and chemical processes associated with heat extraction from EGS reservoirs. With the model, we comprehensively quantify the effect of fluid pressure, poroelastic process, thermoelastic process and water-rock reactions on fracture aperture evolution, as well as the corresponding influences on fracture flow channeling behavior and long-term thermal performance. Specifically, we reveal various positive and negative feedback loops between THMC processes and fracture flow channeling. According to the model results, we further discuss potential techniques for flow channeling mitigation that can be employed in field applications to improve heat extraction efficiency from EGS reservoirs.