低渗煤层CO2驱替CH4煤储层渗透率动态变化规律及变形机制

Injecting carbon dioxide to displace methane is one of main increasing production measures to coal reservoir. The difference both crude pore system and component variance make the coal adsorption characteristic more complex in the process of sorption. Previous study mainly focused on coal adsorption features of different adsorption gas and effect factors, and considered scarcely any the influencing permeability change mechanics of sorption inducing matrix of coal swelling strain. Injecting carbon dioxide to displace methane about different deformation degree coal in deep-seated coal seam is the lack of effective theoretical support..Based on studying areas of the south of Qinshui Basin, Jiaozuo, using the associated methods of experiments and theoretical analysis, according drilling data, well logging data, coal wall observation data in coal seam tunnel, the coal structure of different deformation degree will be described and discriminated quantitatively. The temperature field and ground strain field will be simulated with mathematical method according to geological condition in research areas, and adsorption-strain-seepage experiments about multicomponent gaseous mixture will be finished in future study, the regular of between adsorption and swelling strain, swelling strain and porosity, porosity and permeability can be obtained under variant testing conditions. The mutual controlling action mode for permeability will be discussed on different deformation degree coal, temperature, pressure and stress. The hot-solid-liquid couple model about different coal structure can be built on multi-phase medium system to explain the adsorption strain-seepage mechanics in the process of injecting carbon dioxide into coal to release methane in theory. It will provide theoretical guidance for parameters optimization of injecting carbon dioxide into different coal structure in deep-seated coal seam.

注CO2驱替甲烷是煤层气开采的主要措施之一，煤的物质组成和孔隙系统的差异性决定了煤吸附气体特性的复杂性，以往研究多集中于煤对不同气体吸附特征及其影响因素研究，很少考虑煤体吸附/解吸气体引起的煤体膨胀/收缩变形对渗透率变化的作用机理，低渗煤层不同变形程度煤体注CO2采气工艺缺乏行之有效的理论支撑。.本项目以沁水盆地南部、焦作等主采煤层为研究对象，采用室内实验与理论分析相结合的方法，根据钻井、测井及井下观察，对不同变形程度煤体进行数字描述与表征；对研究区温度场和应力场开展物理模拟，进行多组分混合气体的吸附－变形－渗流实验，研究多参数条件下吸附与变形、变形与孔隙率、渗透率的变化规律，探讨CO2置换CH4时不同变形程度煤体、地应力、温度场及压力场对渗透率的控制作用，建立多相介质热－固－流耦合理论模型，阐明CO2置换CH4煤体变形渗流机理，对低渗煤层注CO2开采煤层气工艺参参数优化提供理论指导。

低渗煤层渗透率低的问题限制了煤层气开发产能提高和煤层气开发技术的广泛应用，CO2驱替甲烷是低渗煤层煤层气高效开采的主要技术措施之一。本项目对低渗煤层的煤体变形程度、CO2驱替甲烷煤体吸附/解吸特征、吸附/解吸引起的煤岩膨胀/收缩变形特征、驱替过程中气体渗透率动态变化等进行了试验研究和理论分析。提出一种新的煤体结构与渗透率定量判识方法，搭建了煤岩孔渗吸附模拟装置，建立了低渗煤层注CO2驱替甲烷的渗透率动态变化理论模型，耦合温度场、应力场、煤体结构等参数初步分析了CO2驱替甲烷煤体变形-渗流控制机理。CO2驱替甲烷试验显示，注气压力增大，置换体积比先增大后减小，注气效率持续增大；地应力增大，煤体所受有效应力增大而导致驱替效率下降。非饱和煤层注CO2驱替的效果比饱气煤层更好，置换体积比更大；温度对CO2驱替甲烷影响更明显，低温更有利于甲烷解吸；吸附平衡压力增大，单位压降下解吸率略有下降；吸附变形试验显示，在相同条件下，CO2驱替甲烷时渗透率先减小后增大，随着注气压力稳定，渗透率趋于稳定；低围压（6-8MPa）驱替渗透率呈“下降-平稳-下降”规律，高围压（12-14MPa）呈“平稳下降-平稳增大”，液态CO2驱替时渗透率大于气态CO2。本项研究为探讨CO2封存与注CO2驱替甲烷效率奠定了坚实的工作基础，同时为注气提高煤层气采收率提供了重要的理论依据，所建立的注CO2驱替甲烷渗透率动态变化模型为煤储层渗透率预测提供了理论参考与借鉴。