Influence of pore heterogeneity of shale with different lithofacies on CO2 storage: Experiments and molecular dynamics simulation

Abstract

CO₂ storage in shale reservoirs has become increasingly important in the context of the problems posed by global warming. However, a clear understanding of the impact of pore heterogeneity of shale with different lithofacies on CO₂ storage capacity is lacking. In this study, we examined the Paleogene shale in the Nanpu Sag of the Bohai Bay Basin as an example of a reservoir for CO₂ storage. First, we used experimental methods such as X-ray diffraction (XRD) analysis, Total organic carbon (TOC) content determination, Rock-Eval pyrolysis analysis, vitrinite reflectance analysis, CO₂ adsorption, N₂ adsorption, and High-pressure mercury intrusion (HPMI) to study the effect of pore heterogeneity of shale with different lithofacies on CO₂ storage. Subsequently, Molecular dynamics (MD) simulations were performed to simulate the isothermal adsorption of CH₄ and CO₂, verifying the rationality of CO₂ storage with enhanced gas recovery. Results reveal that the research area mainly contains clayey shale, mixed shale, and felsic shale. Micropore heterogeneity decreases in the order of mixed shale > felsic shale > clayey shale, micropore connectivity in the order of clayey shale > felsic shale > mixed shale, mesopore heterogeneity in the order of mixed shale > felsic shale > clayey shale, and mesopore connectivity in the order of clayey shale > felsic shale > mixed shale. The micropores in the shale in this area have higher heterogeneity and considerably lower connectivity than the mesopores. An increase in clay minerals, carbonate minerals, and pore heterogeneity is beneficial for CO₂ storage. In contrast, increases in TOC, quartz, potassium feldspar, plagioclase, and pore connectivity are not conducive to CO₂ storage. Overall, pore connectivity exerts the greatest impact on CO₂ storage. Compared with micropores, the heterogeneity of mesopores exerts a greater impact on CO₂ storage. For low-permeability shale, organic-medium mixed shale has low connectivity and strong heterogeneity, making it more suitable for CO₂ storage. The results of isothermal adsorption suggest a stronger affinity of the shale for CO₂ than for CH₄. Thus, the injected CO₂ can displace CH₄ adsorbed in the pores, thereby enabling effective CO₂ storage. This study holds guiding significance for implementing subsequent CO₂ storage projects.