Experimental Study on Transient Hydraulic and Heat Transfer Characteristics of Granite Fractures under Thermo-Hydro-Mechanical-Chemical Coupling

The hydraulic and heat transfer evolution of granite fractures under thermo-hydro-mechano-chemical coupling represent a pivotal challenge for the sustainable and stable operation of Enhanced Geothermal Systems. In this study, transient fracture flow and heat transfer experiments were conducted under high-temperature and high-pressure conditions. Concurrently, an analytical model for the average fracture surface temperature was developed to achieve a quantitative characterization of the hydraulic and thermal characteristics. The results indicate that normal stress reduced the equivalent hydraulic aperture by 4∼6 µm, dominating the macroscopic closure of the fracture. Meanwhile, it triggered pressure solution and micro-fracturing, which macroscopically smoothed the surface-as evidenced by a 0.1∼0.4 decrease in the Joint Roughness Coefficient while enhancing its microscopic complexity. Under elevated temperatures, the concentrations of N a + , K + , and C a 2 + escalated rapidly to 1.2∼1.6 mmol/L within the first 8 hours, accompanied by a pH rebound from weakly acidic back to neutral, reflecting distinct mineral dissolution and chemical buffering mechanisms. An increased flow rate enhanced the local roughness via hydraulic erosion, whereas the acidic injectate intensified mineral dissolution along with secondary precipitation, ultimately driving the microscopic remodeling of local apertures. Multi-scale analysis demonstrates that high temperatures and chemical activation mitigated the mesoscopic thermal resistance through localized preferential dissolution. Conversely, hydraulic and mechanical constraints locked the hydraulic aperture, resulting in a distinct decoupling between thermal and hydraulic performances. This study elucidates the co-evolutionary mechanisms of fractures under THMC coupling, providing a rigorous quantitative framework for heat transfer prediction and engineering optimization in geothermal reservoirs.