Analysis of the experimental effect and parameter optimization of well shut-in energy storage in the post-fracturing shale oil reservoir

To address the challenges such as rapid production decline and short natural flow cycle in shale oil wells, complementary energy is needful in the early stage of production to slow down the fast depletion of formation pressure after well startup. Energy storage experiment during the shut-in period of shale reservoir layers was conducted. We compared the energy storage effects of four different types of fracturing fluids: pure CO₂, CO₂ pre-fracturing, CO₂ post-fracturing, and pure slippery water. And thereby, the pre-fracturing load of CO₂ and well shut-in time were optimized. The laboratory findings show that pure CO₂ presents the best energy storage effect and the shortest pressure equilibrium time due to its large expansion coefficient and low interfacial tension. The energy storage effect of slippery water is exactly the opposite. Taking into account of factors such as economy and energy storage efficiency, the optimal CO₂ pre-fracturing load range is from 20% to 30%. Based on the optimization criterion of energy storage pressure balance, it is recommended that the optimal shut in time for a single section of CO₂ multistage fracturing well ranges from 5 to 8.3 hours, and the optimal shut in time for a single section of slippery hydraulic multistage fracturing well is 12.8 hours or so. This study guided the determination of on-site fracturing construction plans and post-fracturing production systems. The research results provide certain technical support for energy storage fracturing construction in similar shale oil reservoirs.