Objective In the Subei Basin, the depletion-drive development of shale oil following reservoir volume fracturing faces challenges such as rapid production decline and low oil recovery. To address these issues, this study explored an experimental method of CO₂ + formation water (also referred to as carbonated water) huff and puff for enhanced oil recovery (EOR) by combining the felsic, clayey, and organic-rich characteristics of shale oil reservoirs in the Subei Basin, along with CO₂ high pressure quality exchange technology.

Methods Through multiple physical simulation experiments in the laboratory, combined with nuclear magnetic resonance (NMR) experiment technology, this study conducted dynamic and static experiments on cores, including high-temperature and high-pressure imbibition, dissolution, and carbonated water huff and puff, to verify the feasibility of carbonated water huff and puff for shale oil recovery. Variations in 2D NMR T₁-T₂ spectra were analyzed to determine the characteristics and patterns of the production of different types of crude oil during huff and puff.

Results and Conclusions  By creating acidic conditions, CO₂ huff and puff following imbibition could dissolve minerals such as calcites and dolomites in shale oil reservoirs, thus improving the microscopic pore-throat structures in the reservoirs, as well as the porosity and permeability of cores. Accordingly, the late-stage shale oil recovery was enhanced. Comparison of CO₂ and carbonated water huff and puff revealed that the carbonated water huff and puff increased the injection-production pressure difference, enhancing cumulative recovery by 6.7%. In the first several rounds, CO₂ huff and puff achieved rapid and effective production of movable oil and light hydrocarbons, enhancing oil recovery more significantly. In contrast, in the late stage, the carbonated water huff and puff expanded the influence of CO₂ through dissolution and diffused CO₂ into more matrix pores. Consequently, the conversion between heavy hydrocarbons (or adsorbed oil) and light hydrocarbons was gradually achieved through mass transfer, resulting in significantly improved huff-n-puff performance. Therefore, to achieve the optimal shale oil recovery, it is advisable to adopt two rounds of CO₂ huff and puff in the early stage, followed by multiple rounds of carbonated water huff and puff in the late stage, when designing huff-n-puff schemes in the field. Overall, the results of this study reveal the mechanisms by which the carbonated water huff and puff enhance crude oil production, confirming the feasibility of this method for EOR of shale oil. This study provides a significant experimental basis and new directions for developing EOR technologies for shale oil reservoirs in the Subei Basin and even in eastern China.