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 (LN 2 ) 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 LN 2 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 LN 2 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 %. LN 2 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 LN 2 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.