Exploring the role of fracture networks in enhanced geothermal systems: Insights from integrated thermal-hydraulic-mechanical-chemical and wellbore dynamics simulations

Abstract

Hot dry rock (HDR) fracturing is a critical stage in the development of enhanced geothermal systems (EGS), and the pattern of an engineered fracture network plays a crucial role in cumulative heat recovery. However, current studies often lack completeness and accuracy when exploring the effects of various fracture networks, overlooking key factors such as chemical reactions, wellbore dynamics, and/or rock mechanical behaviors. This study develops combined thermal-hydraulic-mechanical-chemical (THMC) and wellbore heat loss models, for the first time, to evaluate EGS heat recovery under different vertical-fracture and shear-fracture networks. The results reveal an over 3.9 % variance in heat recovery between THMC and other coupled models, while wellbore heat loss accounts for approximately 7.7 % of the thermal power production, underscoring the significance of incorporating both complex reservoir mechanisms and wellbore heat loss in EGS assessments. In addition, heat recovery improves with increased fracture spacing and number but decreased conductivity. Among vertical-fracture networks, an interrupted complex vertical-fracture system achieves the highest electricity generation of 1119.0 GWh over 20 years of operation. Meanwhile, shear-fracture networks often perform better in heat extraction than vertical-fracture systems, with the case featuring more shear fractures and higher permeability showing the highest electricity output of 1136.7 GWh. Importantly, increasing a fracture number contributes an additional 20.2 GWh, compared to only a 2.2 GWh gain from higher permeability, highlighting the fracture number as the dominant factor in shear-fracture systems. However, due to the higher injection pressure requirements, shear fracturing is best suited for reservoirs with abundant natural fractures. Otherwise, an interrupted complex fracture system is the preferred alternative. This study significantly improves the understanding of EGS performance across different fracture patterns, offering valuable insights to operators for improved decision-making in EGS development.