The Mechanism of Proppant Transport during Flowback in Rough Fracture for Supercritical CO₂ Fracturing

Supercritical CO2 fracturing technology has shown great potential for enhancing the production in unconventional reservoirs. The uncertainty of proppant transport during flowback is a critical problem in fracturing operations, which can lead to adverse consequences such as partial flow channels blockage or even production interruption. Despite its significance, comprehensive studies of proppant transport during flowback are still limited. It is essential to clarify the mechanism of proppant transport during flowback in rough fractures for supercritical CO2 fracturing. A realistic rough fracture model is reconstructed, and computational fluid dynamics (CFD) simulations are conducted to track proppant movement during flowback. The whole process of supercritical CO2 fracturing is described, including the pumping stage, the fracture closure stage, and the subsequent flowback stage. The effects of flowback rate, proppant size, temperature, and fracture width on the support effect are discussed. The results show that flowback stage can be subdivided into two stages: translation push and tumbling creep. The motion of proppants initiates with translation push and subsequently shifts to tumbling creep. Proppant motion is predominantly controlled by the tumbling creep throughout the flowback stage. To obtain a stable sand bed, a wider initial fracture width before the flowback stage is required. The proppant sand bed with a higher fluid temperature or fracture width is more stable than the sand bed with a lower fluid temperature. The equilibrium sand bed length decreases due to enhanced drag force with the increase of flowback rate. In addition, large size proppant tailings can improve the stability of the proppant sand bed, while ensuring high fracture conductivity and increasing postfracturing productivity. These insights significantly enhance the understanding of the proppant transport mechanism during flowback, offering important guidance for supercritical CO2 fracturing processes.