页岩纳米孔内超临界CO_2、CH_4传输行为实验研究

Experimental Study on the Transmission Behaviors of Supercritical CO_2 and CH_4 in Shale Nanopores

了解超临界CO_2、CH_4在页岩纳米孔内传输行为,是研究页岩储层超临界CO_2注入能力、注入后时空分布及提高页岩气藏采收率的基础。选用渗透率小于100nD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(CO_2、CH_4)浓度变化,对比了页岩纳米孔内超临界CO_2、CH_4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CO_2压力传递速率明显小于CH_4,实验结束后岩样口端二元混合气体组分中CH_4百分含量显著降低,证实页岩纳米孔内超临界CO_2传输能力显著低于CH_4,主要原因是超临界CO_2吸附能力更强、扩散一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。基于以上识,选择某些压裂段注入超临界CO_2,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)更不利于驱替置换页岩纳米孔内游离态甲烷。

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.