Abstract: Understanding the transmission behaviors of supercritical CO₂ and CH₄ in shale nanopores is fundamental for studying the supercritical CO₂ injection capacity of shale reserves and the temporal and spatial distribution of injected supercritical CO₂. Understanding these behaviors is also essential for improving the recovery rate of shale reserves. In the study, the transmission capacities of supercritical CO₂ and CH₄ in shale nanopores were compared and the difference in their transmission capacities was analyzed for the underlying reason. A core flow experiment was performed on organic-matter-rich shale matrix samples of the Longmaxi Formation (permeability smaller than 100 nD) by varying the pressure as well as the CO₂ and CH₄ concentrations in the CO₂/CH₄ mixture at the inlet and outlet of the core. The experimental results show that the pressure transmission rate of supercritical CO₂ in shale nanopores was clearly lower than that of CH₄. At the end of the experiment, the CH₄ content by percentage in the CO₂/CH₄ mixture significantly decreased at the inlet of the rock sample, demonstrating that the transmission capacity of supercritical CO₂ in shale nanopores is significantly lower than that of CH₄. This can be explained by certain properties of supercritical CO₂, such as its higher absorption capacity and lower diffusibility and permeability as well as its immiscible displacement and piston-like displacement behaviors owing to its super-high density characteristics. Based on this understanding, some fractured sections were selected for supercritical CO₂ injection, and other fractured sections were reserved for production. Compared with the single-well injection-soaking-production design, this configuration facilitates better displacement of free-state methane in shale nanopores.

Key words: supercritical CO₂, methane, transport, nanopores, shale gas, recovery rate