CO₂ Huff-n-Puff Process to Enhance Shale Oil Recovery via Large-Diameter Core Experiments

CO2 huff-n-puff (HnP) technology, known for its low reservoir sensitivity, is considered one of the most effective methods for enhancing oil recovery in tight reservoirs such as shales. In previous studies, most research has been conducted using standard small size (φ25 × 50 mm) shale cores, which are dense and oil-poor, differing from real reservoir conditions. In order to deeply investigate the CO2 HnP characteristics, a series of large-diameter core (φ100 × 200 mm) experiments were conducted with recombined live oil under simulated reservoir conditions (69 °C, 19 MPa), and the CO2 HnP performance was analyzed under various parameters. Meanwhile, gas chromatography–mass spectrometry (GC–MS) was used to analyze the characteristics of oil and gas production and the short-term CO2 sequestration potential was also assessed. The results showed that increasing injection pressure from 17 to 24 MPa enhanced oil recovery from 2.81% to 5.72%, with diminishing returns. The gas/oil ratio (GOR) remained relatively stable, while the oil recovery efficiency peaked and then declined with higher CO2 injection volumes. The produced oil tended to become lighter. Lower production pressures and longer soaking times improved recovery and CO2 storage efficiency but gains diminished over time. After six cycles, total recovery reached 25.40%, but per-cycle recovery decreased, GOR and gas production increased, and the CO2 injection volume rose, reducing the overall efficiency. Under the experimental conditions, the optimal CO2 HnP and short-term CO2 storage efficiency were achieved with an injection pressure of 22 MPa, a production pressure of 12 MPa, a soaking time of 16 h, and 3 cycles of HnP. This study provides further support for developing accurate and efficient shale oil development strategies for the target reservoir.