页岩气多组分竞争吸附机理研究

The competitive adsorption behavior of multi-components in shale gas reservoir is not only the key scientific topic for shale gas development, but also the fundamental theory for greenhouse gas sequestration. In this research plan, we intend to gain insights into the competitive adsorption mechanisms of multi-components in shale nanopores by a combination of experiment and molecular simulation. The experimental platform used for multi-components adsorption is set up, and the adsorption dynamic equilibrium tests of multi-components on shale samples and clay samples under different temperature, pressure, mole ratio and moisture content are carried out. The molecular models of organic matter and clay mineral in shale are constructed based on analytic experimental data, and the competitive adsorption behaviors of multi-components on molecular models are simulated under various temperature, pressure, mole ratio, moisture contents and pore sizes. Then the adsorption state of multi-components under supercritical conditions is determined, the adsorption energy and adsorption heat of multi-components are calculated, and the competitive adsorption mechanisms of shale gas components are proclaimed. Based on the competitive adsorption characteristics of multi-components under supercritical conditions, the supercritical adsorption isotherm model is developed considering the dominating factors of adsorption equilibrium. Then the adsorption equilibrium of multi-components is predicted under different temperature and mole ratio, so as to analyze the competitive adsorption behaviors of supercritical multi-components in depth. Based on the above research, we hope to improve the competitive adsorption mechanisms of multi-components in shale gas reservoir and to provide theoretical support for carbon dioxide sequestration and recovery enhancement of shale gas.

页岩气为甲烷、二氧化碳等多组分的混合物，储层条件下以超临界状态赋存于页岩含水纳米级孔隙中。针对目前超临界多组分页岩气在含水纳米级孔隙中吸附机理认识不清的问题，本项目拟通过室内物理模拟实验和分子模拟相结合的方法开展研究。通过搭建超临界多组分页岩气吸附实验平台，在不同条件下开展多组分气体在页岩及纯粘土矿物中的吸附平衡测试；构建页岩有机质及粘土矿物分子模型，模拟多组分气体在不同条件下的竞争吸附行为，确定超临界条件下气体的吸附状态，分析气体的吸附能和吸附热，揭示页岩孔隙超临界多组分气体的竞争吸附机理；基于页岩超临界多组分气体竞争吸附特性，结合吸附平衡主控因素，建立超临界多组分气体竞争吸附预测模型，开展不同条件下的多组分吸附平衡预测，深入剖析页岩储层超临界多组分气体的竞争吸附行为。通过上述研究，完善页岩储层多组分气体竞争吸附机理，为注二氧化碳提高页岩气藏采收率提供理论支撑。

页岩气藏衰竭式开发采收率普遍低于30%，注二氧化碳是页岩气藏极具潜力的提高采收率方式。在注二氧化碳开采中，页岩纳米孔隙中的水分处于与甲烷和二氧化碳并存的环境。在储层高温高压的环境下，超临界多组分气体在页岩纳米级孔隙中的竞争吸附机理认识不清，多组分竞争吸附预测模型有待建立。本项目结合室内实验测试、分子模拟以及理论模型方法开展了多项工作：搭建多组分气体吸附实验平台，测试了多组分气体在页岩及黏土矿物上的吸附平衡；通过表征和构建代表性的页岩纳米孔隙模型，借助分子模拟方法揭示了多组分气体的竞争吸附行为和机理；通过建立简化局部密度函数（SLD）圆管孔模型和基于扩展Langmuir方程的多组分气体竞争吸附预测模型，完善了甲烷和二氧化碳在含水页岩孔隙上的竞争吸附理论。研究结果表明，水分可显著降低页岩和黏土中的甲烷吸附量，忽略水蒸气压力将大幅低估甲烷实际吸附量。二氧化碳在页岩上表现出优先吸附性，温度升高会促进二氧化碳优先吸附。二氧化碳在ⅢA型有机质中的埋存能力和吸附选择性最强，这可作为页岩气藏注二氧化碳优选开采层位的重要参考。对于高成熟的页岩储层，适当提高有机质内水分含量，可改善二氧化碳置换甲烷的效果。储层中二氧化碳含量的升高会促进干酪根的膨胀，形成更大的孔隙空间和更多渗流通道，有利于提高二氧化碳埋存量和页岩气产量。考虑气体吸附诱导的干酪根膨胀，能使页岩气地质储量评价和二氧化碳埋存潜力评估更加准确。SLD圆管孔模型能较好地拟合页岩气等温吸附曲线，可以预测气体在更宽温度范围内的吸附。受孔隙对气体排斥力的影响，孔径大于0.42nm时，二氧化碳优先吸附，孔径小于0.42nm时，甲烷优先吸附。忽略吸附相体积与多组分气体的竞争吸附效应将显著高估页岩储层的含气量。页岩储层多组分气体的竞争吸附机理研究，可为页岩气藏注二氧化碳提高采收率技术提供理论指导，对页岩气储量的准确评估意义重大。