Mechanism of CO₂ flooding in shale reservoirs – insights from nanofluids

The development potential of CO₂-enhanced shale oil recovery is significant, but shale reservoirs have developed nanoscale pores, often accompanied by fissures and micro/nanoscale fractures. This characteristic makes the micro-nanoscale CO₂ flooding mechanism unclear. In this study, the minimum miscible pressure (MMP) of CO₂ and <i>n</i>-octane was determined from a microscopic perspective using the nanofluidic method. Based on this, the displacement behavior of CO₂ in three types of micro-nano networks was investigated, and the degree of inter-fracture matrix utilization was studied for the first time using visualization techniques. It was found that compared to immiscible flooding, the stronger the heterogeneity, the more pronounced the improvement in recovery efficiency by miscible flooding. In addition, transfer and diffusion in the nanofracture network system are intense, and the displacement process can be divided into three stages: pressure-driven flow, matrix-fracture co-production, and matrix oil production. This study applies a novel nanofluidic method to extend the lower limit of the microscopic visualization experimental pore scale to 30 nm, filling the gap in experimental research on shale microscale flow and contributing to the understanding of the mechanism of CO₂-enhanced recovery in shale reservoirs. Additionally, it provides necessary references for microscopic flow simulation.