Experimental study on real-time seepage-heat transfer characteristics of fractured granite during the cyclic liquid nitrogen injecting the hot dry rock reservoir

To investigate the evolution of seepage-heat transfer characteristics in fractured granite reservoirs during liquid nitrogen (LN2) injection, the variation laws of these characteristics were acquired in real time using a high-temperature triaxial fractured seepage and heat transfer test system. Meanwhile, the fracture morphologies before and after the impact of LN2 were imported into the COMSOL computational platform to determine the influence mechanism of fracture morphology changes on the seepage and heat transfer characteristics of fractures. Physical experimental results show LN2 cooling significantly affects fracture surface roughness, with stronger impacts at higher temperatures (e.g., surface roughness increase by 2.089, fractal dimension D from 2.0766 to 2.116 at 400 °C). More cooling cycles increase hydraulic aperture and permeability at 400 °C by 12.74 % and 27.1 %. LN2 impact can significantly increase the heat transfer coefficient of fractures (increase by 23.34 % at 400 °C with 1 time cooling). Numerical experimental results shows that after LN2 impact, the connectivity of fractures is significantly enhanced, and the seepage characteristics transitions from localized high-velocity flow to uniform flow across fracture surfaces, increasing both the heat exchange area and time, the heat transfer efficiency is improved.