页岩纳米孔内超临界CO2、CH4传输行为实验研究

了解超临界CO₂、CH₄在页岩纳米孔内传输行为，是研究页岩储层超临界CO₂注入能力、注入后时空分布以及提高页岩气藏采收率的基础。选用渗透率小于100 nD的龙马溪组富有机质页岩基块岩样，利用岩芯流动实验装置，通过监测岩芯入/出口端气体压力与混合气体（CO₂、CH₄）浓度变化，对比了页岩纳米孔内超临界CO₂、CH₄传输能力，分析了传输能力差异的原因。结果表明：页岩纳米孔内超临界CO₂压力传递速率明显小于CH₄，实验结束后岩样入口端二元混合气体组分中CH₄百分含量显著降低，证实页岩纳米孔内超临界CO₂传输能力显著低于CH₄，主要原因是超临界CO₂吸附能力更强、扩散-渗流能力更小以及超高密度特性表现出的非混相、活塞式驱替行为。基于以上认识，选择某些压裂段注入超临界CO₂，而其他压裂段作为生产段，比单井"吞吐"方式（即注入-焖井-开井生产）更有利于驱替置换页岩纳米孔内游离态甲烷。

Experimental Study on the Transmission Behaviors of Supercritical CO2 and CH4 in Shale Nanopores

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.