On the role of non-equilibrium heat transfer in the filled fracture-rock matrix system

Understanding heat transfer processes in fractured porous media is critical for characterizing thermal behaviors and improving the operation efficiency of geothermal energy. The field observations have shown that many fractures are filled with sediment infilling rather than being fully open, but heat transfer in such a filled fracture-rock matrix system has received less attention to date. Since multiple domains are involved in such a system and the fracture is highly permeable even with sediment infilling, local thermal equilibrium may not be reached instantaneously. In this study, a fully coupled analytical model is proposed and developed to investigate the role of transient local heat transfer in the filled fracture-rock matrix system and analyze the spatiotemporal thermal distributions in each domain. The results indicate that multiple thermal properties of the rock matrix, such as average volumetric thermal capacity and thermal conductivity, have significant impacts on the spatiotemporal thermal distribution in the fracture. The temperatures of the pore water and sediment infilling in the fracture are sensitive to flow velocity, thermal conductivity of the rock matrix, and volumetric thermal capacities of the fluid and rock matrix. The thermal transfer coefficient has moderate interaction effects with other parameters according to the sensitivity analysis.