Enhanced Geothermal Systems (EGS) represent a key technological pathway for developing hot dry rock (HDR) geothermal resources. The core concept involves the use of hydraulic fracturing to create deep artificial fracture networks that enhance heat exchange efficiency ^[1]. However, due to the inherent heterogeneity of geological formations, fracturing fluids tend to preferentially flow through major fractures or dominant pathways, leading to phenomena such as “channeling” or “short-circuiting,” which hinder the formation of complex fracture networks ^[2].
To address this issue, diverting fracturing techniques have been developed by introducing temporary plugging agents into the fracturing fluid. These agents temporarily block dominant channels, redirecting the fluid into secondary fracture zones and thereby promoting fracture complexity. Previous studies have shown that factors such as fluid viscosity, injection rate, and formation temperature significantly influence the plugging behavior of particles, with particle size identified as a critical parameter determining plugging efficiency ^[3,4]. Nevertheless, most existing models assume that the plugging particles are inert, thermally stable, and non-deformable, overlooking the particle size evolution caused by thermally induced degradation under high-temperature HDR conditions.
To fill the aforementioned research gap, this study first conducted thermal degradation experiments on spherical temporary plugging particles at 150 °C and established an empirical correlation for their thermally induced degradation behavior. The degradation process was found to consist of two distinct stages: a heating-induced degradation stage and an isothermal degradation stage. In the first stage, particles gradually degrade while their temperature continues to rise; in the second stage, the temperature remains nearly constant while degradation proceeds continuously, resembling a melting process. Based on these characteristics, a thermal degradation model for the plugging particles was developed. A user-defined function (UDF) was implemented in Fluent through secondary development to simulate the particle degradation behavior, and the model’s accuracy was validated against experimental results.
Furthermore, the degradation model was integrated into a CFD–DEM coupled framework to simulate the transport and particle size evolution of plugging agents within fractures in hot dry rock formations. Simulation results showed that during continuous injection, the average particle size decreased by approximately 1.83%, while the leading particles experienced a maximum degradation of up to 28.5%. In addition, a quantitative relationship was established between the degradation rate and dimensionless parameters—including the Stefan number (Ste), Biot number (Bi), Fourier number (Fo), and temperature ratio—as well as particle characteristic size. These findings provide a theoretical basis for the design of plugging materials and the optimization of plugging strategies in high-temperature geothermal reservoirs.