Abstract: The shale oil reservoirs in Qianjiang Sag are of abundant reserves, and their pores are rich in a variety of soluble minerals. In the development with CO₂ - pure water stimulation, the supercritical characteristics of CO₂ can be utilized for oil displacement and the CO₂ sequestration in the target reservoirs also can be improved, which effectively reduce CO₂ emissions. However, the key to implementation of this method lies in whether the CO₂- pure water system can enter the shale micro-nano pores and effectively drive the crude oil in the pores. Experiments of stimulation with CO₂, CO₂- pure water and CO₂- formation water were designed, the stimulation characteristics of CO₂ injection, CO₂- pure water injection and CO₂- formation water injection were clarified based on NMR method, and the law and mechanism of crude oil production in different pores were summarized. The experimental results showed that the stimulation efficiency of CO₂- pure water injection was 8.62 percentage points higher than that of CO₂ injection, and 12.66 percentage points higher than that of CO₂- formation water injection, and the acidic fluid formed by the combination of CO₂- pure water could dissolve the soluble minerals on the pore surface, improve the pore connectivity and enhance the reservoir permeability, and effectively increase the production of crude oil in pores smaller than 0.01 μm and 0.01 to 0.10 μm. After multiple cycles of stimulation, the volumetric solubility of ionic substances in the produced liquid was decreased significantly, indicating that the subsequent injection of pure water failed to expand the swept volume, but only improved the production of crude oil. This study proves that the CO₂- pure water injection can effectively improve the recovery rate of shale reservoirs, and provides a new idea for the effective development of onshore shale oil.

Key words: shale oil, development mechanism of CO₂- pure water stimulation, nuclear magnetic resonance, micro-nano pores, exploitation characteristics