Experimental Study on Seepage and Heat Transfer Patterns of Hot Water in Fractured Coal.

This study investigates the thermo-fluid-solid coupling mechanism during hot water injection into coal for enhanced coalbed methane recovery. Using a self-developed multifield coupled experimental system, we conducted heat injection tests on both raw and hydraulically fractured coal samples. The results show that hydraulic fracturing effectively reconstructs the pore-fracture network, increasing porosity from 0.926% to 8.651% (an approximately 9-fold increase) and connectivity from 71.55% to 98.55%. This engineered fracture network provides the essential pathways for fluid migration, while injection pressure and temperature act as external drivers that regulate flow and heat transfer. The heat transfer mechanism is dominated by forced convection, with conduction playing a secondary role. Replacing pore air with hot water increases the equivalent thermal conductivity from 0.34 W/(m K) to 0.359 W/(m K), enhancing heat diffusion into the coal matrix. Forced convection, driven by high-pressure hot water, governs rapid and deep heating and is primarily controlled by flow rate and temperature. Therefore, enhancing coal permeability through hydraulic fracturing is the key to intensifying convective heat exchange. Heat transfer rates exhibit strong positive correlations with permeability, injection pressure, and injection temperature, which collectively determine heating efficiency and thermal influence zones. This work provides a theoretical framework that elucidates the synergistic effects of fracture structure modification and injection parameter optimization, offering a scientific foundation for designing efficient hot water stimulation strategies in coalbed methane reservoirs.