Numerical investigation of pore-scale heat conduction in rock matrix blocks subject to time-dependent fracture thermal conditions

Geothermal energy extraction from fractured reservoirs is strongly controlled by heat transfer between the rock matrix and fractures. Although fractures act as the primary flow pathways, the low-permeability matrix often dominates the thermal response of the system. In this study, transient heat conduction within heterogeneous rock matrix blocks is investigated at the pore scale using binarized micro-CT images of sandstone and carbonate rocks. Fractures are not explicitly resolved in the computational domain; instead, they are represented by imposing time-dependent thermal boundary conditions on the external surfaces of the matrix blocks. The heat conduction equation is solved using an explicit finite difference method on the voxel grid, with temperature-dependent thermal properties assigned to solid grains and water-filled pores. The effects of mineralogy, porosity, and the spatial distribution of pores and grains on heat diffusion and the matrix–fracture heat transfer shape factor are examined. In addition, the influence of block size and proximity to the injection well, represented by constant and linearly varying fracture temperature boundary conditions, is analyzed. The results demonstrate that pore-scale heterogeneity of the rock matrix plays a significant role in controlling heat transfer in fractured geothermal reservoirs.