Proppant transport in rough fracture networks using supercritical CO₂

Proppant transport within fractures is one of the most critical tasks in oil, gas and geothermal reservoir stimulation, as it largely determines the ultimate performance of the operating well. Proppant transport in rough fracture networks is still a relatively new area of research and the associated transport mechanisms are still unclear. In this study, representative parameters of rough fracture surfaces formed by supercritical CO₂ fracturing were used to generate a rough fracture network model based on a spectral synthesis method. Computational fluid dynamics (CFD) coupled with the discrete element method (DEM) was used to study proppant transport in this rough fracture network. To reveal the turning transport mechanism of proppants into branching fractures at the intersections of rough fracture networks, a comparison was made with the behavior within smooth fracture networks, and the effect of key pumping parameters on the proppant placement in a secondary fracture was analyzed. The results show that the transport behavior of proppant in rough fracture networks is very different from that of the one in the smooth fracture networks. The turning transport mechanisms of proppant into secondary fractures in rough fracture networks are gravity-driven sliding, high velocity fluid suspension, and fracture structure induction. Under the same injection conditions, supercritical CO₂ with high flow Reynolds number still has a weaker ability to transport proppant into secondary fractures than water. Thickening of the supercritical CO₂ needs to be increased beyond a certain value to have a significant effect on proppant carrying, and under the temperature and pressure conditions of this paper, it needs to be increased more than 20 times (about 0.94 mPa s). Increasing the injection velocity and decreasing the proppant concentration facilitates the entry of proppant into the branching fractures, which in turn results in a larger stimulated reservoir volume. The results help to understand the proppant transport and placement process in rough fracture networks formed by reservoir stimulation, and provide a theoretical reference for the optimization of proppant pumping parameters in hydraulic fracturing.