Lithofacies-dependent pore structure heterogeneity governing microscopic oil mobilization during supercritical CO2 huff-and-puff in shale

This study systematically analyzes the pore properties and multifractal features of three typical shale lithofacies in the Kongdian Formation of Bohai Bay Basin, East China. The results show that laminated felsic shale exhibits superior macropore development (accounting for 64%) and the lowest D - 10 - D 10 heterogeneity parameter, confirming its most homogeneous pore structure. In contrast, laminated mixed shale demonstrates a higher H value than laminated dolomitic shale, indicating superior pore connectivity. The effects of injection pressure and lithofacies on shale oil mobility were also investigated by integrating CO 2 huff-and-puff experiments with nuclear magnetic resonance (NMR) analysis, in order to elucidate the mobilization mechanisms and controlling factors of CO 2 in pore-fracture systems of lacustrine Kongdian shale. A high-pressure/high-temperature (HTHP) CO 2 flooding apparatus was combined with NMR T 2 spectrum-based quantification of fluid occurrence spaces. Multiple huff-and-puff experiments were conducted using n -dodecane to simulate oil. Key findings include: (1) CO 2 injection pressure critically governs recovery efficiency, with supercritical (SC) conditions (>10 MPa) markedly enhancing fluid extraction from matrix pores (reflected by P1 and P2 peaks of NMR T 2 spectra). (2) Multi-cycle SC-CO 2 huff-and-puff tests (15 MPa, above the measured minimum miscibility pressure of 14 MPa) show that felsic and mixed shale lithofacies, characterized by higher meso-macro porosity, exhibit a higher total recovery than dolomitic shale. This study confirms that the capability of SC-CO 2 to overcome capillary resistance and achieve miscibility with crude oil, constitutes the key mechanism for mobilizing n -dodecane from matrix pores, while lithological variations regulate CO 2 diffusion and displacement through pore structure heterogeneity. These findings provide theoretical guidance for optimizing CO 2 huff-and-puff parameters and target zone selection in the enhanced oil recovery (EOR) from shale reservoirs. Furthermore, this study innovatively combines multifractal analyses with NMR to reveal the lithofacies-dependent pore structure heterogeneity and quantitatively clarify its governing role in microscopic mechanisms of oil mobilization during scCO 2 huff-and-puff cycles. This integrated multi-scale approach provides a novel theoretical and practical framework for optimizing CO 2 EOR in lacustrine shale reservoirs.