A new simulation approach for fracture fluid leak off during pre-CO2 fracturing in shale reservoirs

To explore the impact of pre-CO2 fracturing fluids on shale oil production, this study proposes the filtration behavior of the fracturing fluids within the reservoir and their effect on pressure in the fractured zone. Using a secondary development approach for numerical simulators, the model modifies the dynamic permeability of the affected area based on the fluid dynamics and sweep range of the fracturing fluids. This model characterizes the leak-off performance of the fracturing fluids in relation to reservoir pressure in shale reservoirs. The study explores the filtration patterns and reservoir pressure variations during the injection of CO2 and slickwater in the pre-CO2 fracturing process. Results show that using dynamic permeability to describe the leak off of fracturing fluid yields a larger sweep range, more uniform diffusion, and a more effective overall energy increase in the reservoir. Analysis of influencing factors indicates that the post-CO2 mode significantly enhances energy in the fracture-controlled area after fracturing. Specifically, when CO2 displacement is 2 m3/min, liquid volume is 120 t/m, and shut-in time is 4 days, both sweep area and energy increase slow down, with pressure distribution concentrated around the fracture. Block application results demonstrate that a higher pressure drop gradient leads to a higher CO2 flowback rate, suggesting that controlling production speed is necessary to reduce CO2 flowback. This research provides valuable theoretical guidance for the optimal design of CO2 fracturing in shale reservoirs.