Prediction and Optimization of Shale Oil CO₂ Pre-Fracturing Energy Enhancement Effect

Rapid production decline and insufficient formation energy are major challenges in shale oil development. To address these issues, this study focuses on CO₂ pre-fracturing technology. A productivity model based on the dynamic drainage area (DDA) concept was introduced, treating unsteady flow as a series of pseudo-steady-state flow stages. By coupling the productivity equation, material balance equation, and pressure propagation distance (DOI) equation, the pressure, saturation, and production at each time step were solved using the Newton-Raphson iteration method. This approach enables rapid and accurate prediction of production performance for shale oil wells subjected to CO₂ prepad fracturing. The model demonstrated high engineering application value, with errors of less than 1% compared to commercial numerical simulation software. Using the Jimsar shale oil reservoir as a case study, the CO₂ prepad slug size was optimized based on the model. The results indicate that a CO₂ mole fraction of 30% (corresponding to 0.1 pv) yields the optimal enhanced oil recovery effect, balancing both crude oil production increase and CO₂ sequestration efficiency. When the CO₂ mole fraction is below 30%, the production improvement is limited; whereas exceeding 30% leads to a sharp rise in gas production and a significant decline in both displacement efficiency and sequestration efficiency. This research provides theoretical and practical guidance for optimizing CO₂ prepad slug design in shale oil development.