Evaluation of CO2 phase connectivity for the sealing efficiency in low-permeability shale caprocks: Insights from pore heterogeneity and irreducible water saturation

CO₂ phase connectivity plays a crucial role in the spatial distribution of CO₂ and its flow mobility within the porous medium, significantly influencing the sealing efficiency of low-permeability shale caprocks. Traditional methods rely on geometric topology to calculate pore connectivity, which tends to overestimate CO₂ phase connectivity due to neglecting the mechanism of hydrodynamic resistance. This study conducted a stepwise increase in injection pressure displacement experiment (SIPD) combined with nuclear magnetic resonance (NMR) to detail CO₂ trapping and flow mobility within multiscale pores, informing the inversion analysis of CO₂ phase connectivity. First, a novel method for quantitatively characterizing CO₂ phase connectivity was proposed, incorporating viscous resistance, capillary resistance, and entrance resistance. Subsequently, the SIPD-NMR experiment was conducted to obtain the pore size distribution, irreducible water saturation, and relative permeability. Finally, the relationship between CO₂ phase connectivity and relative permeability was investigated. The above results indicate that the mesopores exhibit the highest fluid mobility due to the lowest irreducible water saturation. The fractal dimension reflects the heterogeneity of multiscale pores, increasing CO₂ entrance resistance and thereby reducing the CO₂ phase connectivity. Higher irreducible water saturation enhances viscous and capillary resistances during two-phase flow displacement, negatively impacting CO₂ phase connectivity. The sudden increase in CO₂ phase connectivity is closely related to permeability evolution, threatening the sealing efficiency of shale caprocks.