Analyzing Factors Influencing CO2 Huff-and-Puff in Shale Oil with Diffusion Inclusion: An Integrated Experimental and Numerical Investigation

CO 2 huff-and-puff (HNP) leverages CO 2 diffusion to effectively enhance shale oil recovery while concurrently enabling CO 2 sequestration. Nevertheless, systematic assessments of the controlling factors in CO 2 HNP remain limited. In this work, the diffusion coefficient of CO 2 in shale cores was measured using the pressure-decay method, and the variation of CO 2 diffusion distance with time was obtained. Nuclear magnetic resonance (NMR) scanning was applied to determine recovery contributions across pore-size classes. Furthermore, diffusion effects were incorporated into numerical simulations, and orthogonal experimental analysis was employed to investigate the influence of multiple factors on the performance of CO 2 HNP in shale oil reservoirs. The results showed that under the experimental conditions, the diffusion coefficient of CO 2 in shale cores was 1.488 × 10 − 10 m 2 / s . The diffusion distance increased monotonically with diffusion time, reaching 1.13 cm after 10 h. NMR measurements conducted before and after the diffusion experiment indicated an overall recovery of 33.37%, with recoveries of 29.53%, 35.61%, 40.77%, and 53.70% for micropores, small pores, medium pores, and macropores, respectively. Orthogonal analysis demonstrated that the importance ranking for the oil exchange ratio and CO 2 storage efficiency was consistent: cycle injection volume had the greatest influence, followed by HNP timing, soaking time, and injection rate. For oil increment, the cycle injection volume and HNP timing remained the dominant factors, whereas the influence of injection rate exceeded that of soaking time. This study provides valuable insights and practical guidance for the research and field application of CO 2 HNP in shale oil reservoirs.