Abstract: In the development of shale oil, polymer fracturing fluid is one of the key technologies for increasing oil and gas production. However, the molecular weight of the polymer significantly affects its fracturing effectiveness. Excessively high molecular weight may exacerbate polymer retention and aggregation within the reservoir, while overly low molecular weight may result in insufficient fracturing effects. To address this issue, this study aims to investigate the influence of polymer molecular weight and concentration on the retention and aggregation behavior of fracturing fluid in shale oil reservoirs, with the goal of optimizing fracturing fluid formulations, minimizing reservoir damage, and improving fracturing efficiency. Through experimental research and technical analysis, polymer solutions with different molecular weights and concentrations were selected for displacement experiments on shale cores. Micro-CT scanners and scanning electron microscopy (SEM) were employed to observe and compare the cores before and after displacement, analyzing the retention and migration characteristics of polymers within the cores. The results showed that when the polymer molecular weight was below 1 754×10⁴ and the concentration was below 2 g/L 1 and 3, where the concentration was 1 g/L—the retention effect was relatively weak. This was manifested as a porosity loss of less than 8.9%, a permeability reduction of less than 15%, and a retention index below 0.35. When the molecular weight exceeded 1 754×10⁴ and the concentration was relatively high, the retention effect was significantly enhanced, with a retention index exceeding 0.70, especially prone to accumulation near fracture entrances, affecting oil and gas flow. Additionally, the study found that polymer retention in cores was jointly affected by molecular weight and concentration, with molecular weight being the main factor. By determining an appropriate range of polymer molecular weight, this study provides a scientific basis for reducing retention and aggregation of fracturing fluid in reservoirs, thereby mitigating reservoir damage and enhancing fracturing performance. The findings offer a scientific foundation for optimizing fracturing fluid formulations in shale oil development and serve as a useful reference for technological improvements in related fields.

Key words: Shale, Polymer Fracturing Fluid, Retention Characteristics, Micro-CT, Scanning Electron Microscope