Laboratory experimental research on production characteristics with CO₂ injection in shale oil

Depletion development of shale oil reservoirs after fracturing tends to face challenges including rapid production decline, rapid formation energy attenuation, and low recovery efficiency. Therefore, it is of great significance to explore effective enhanced oil recovery technologies. CO₂ injection is an important method to enhance oil and gas recovery, but its performance in actual applications and mechanisms remains unclear, limiting the optimization of development parameters. To address this issue, an intelligent core holder was innovatively developed, enabling integrated steps of evacuation, pressurized saturation, and displacement. A new high-temperature vacuuming method with multi-stage pressurized oil saturation was designed, addressing the technical difficulty of oil saturation in large-diameter cores. This enabled integrated physical simulation experiments of cores, including core evacuation, pressurized saturation, and displacement. The saturation process, diffusion law during soaking, component changes of produced fluids, and multi-cycle huff and puff characteristics were systematically analyzed. The results showed that multi-cycle CO₂ huff and puff effectively improved the recovery efficiency of shale oil, with the total huff and puff efficiency reaching 38.02% after six cycles. The first two cycles showed the highest efficiency(28.12%). Gas chromatographic analysis indicated that the relative content of medium-heavy components of C₁₆ to C₂₇ in produced fluids exhibited an increasing trend, confirming that CO₂ preferentially extracted light components at an early stage and activated heavy components at a later stage. With increasing injection cycles, oil saturation decreased while the diffusion capacity of CO₂ in the matrix continued to improve. Based on these experimental results, a staged CO₂ injection is recommended in field applications. High gas injection volume should be adopted in the first two cycles to quickly establish seepage channels. After the third cycle, it is necessary to increase gas injection pressure or add additives to improve extraction effects. This study provides experimental support and optimization guidance for CO₂ huff and puff development in shale oil reservoirs.