煤层气注热开采中蒸汽-热-煤岩的互馈机制研究

Temperature can accelerate desorption of CBM (coalbed methane). Steam is a good medium of carrying heat, and it will become an effective means for improving the efficiency of mining CBM through steam injection. But the interaction mechanism of steam and coal rocks to be studied further. In this project, the self-developed experimental apparatus of seepage of injecting steam into coal seam is used to conduct the experiments of seepage of injecting steam into coal seam. And CT scanner is used to study the change of the pore structure before and after injecting steam and the distribution feature of the water saturation. And the relationship of the steam injection efficiency, the coal rock features and the injection parameters will be obtained. On the basis of the experiments, the basic theories of heat transfer, mass transfer, force conservation and vapor phase transition in porous media are used to establish the model of Three-phase and two-component with Vapor phase transition, which is used to develop the numerical simulation system of mining CBM of injecting steam. And the seepage and the dynamic evolution process of thermal diffusion of steam and CBM in coal seam can be simulated and analyzed. By combining the numerical simulations with the experiments, the phase change process and the thermal diffusion of the steam of different injection mining conditions are studied, and the main factors that affect the flow of CBM and steam in coal rock will be found. And the interaction mechanism of steam-thermal-CBM in coal seam will be revealed. This project can provide theoretical basis and technical support for mining CBM of injecting heat.

温度能够加快煤层气解吸，蒸汽是一种较好的携热介质，注蒸汽将是一种提高煤层气开采效率的有效途径，但蒸汽与煤岩的相互作用机制还需深入研究。利用自主研制的煤岩注蒸汽径向渗流装置和CT扫描，开展煤岩注蒸汽实验研究，分析注蒸汽前后煤岩的孔隙和裂隙结构变化、比表面积及水饱和度分布特征，获得蒸汽的有效注入与煤岩特性及注气参数的内在关系；在实验研究基础上应用多孔介质中的传热、传质、力的守恒和蒸汽相变原理建立含有蒸汽相变过程的多物理场耦合三相二组分模型，研发注蒸汽开采煤层气数值模拟系统；应用数值模拟系统对蒸汽、煤层气在煤岩中的渗流及热扩散动态演化过程进行数值模拟分析，研究不同注汽条件下蒸汽在煤岩中的相变特征及其热扩散效应，得出影响煤层气、蒸汽在煤岩中运移的主要因素，揭示蒸汽-热-煤岩的互馈机制。该项目研究将为煤层气注热开采提供一定的理论基础和技术支撑。

本课题利用自主研制的煤岩渗透率渗流实验系统开展了不同温度和有效应力对煤的渗透率影响实验研究，结果表明煤的气体渗透率随有效应力升高呈先减小后增大的变化规律；在相同气体压力下，煤样气体渗透率随温度升高逐渐减小。采用高压气体吸附仪及外设的恒温水浴装置进行了不同恒温环境下的煤层气吸附/解吸实验研究，结果表明随着温度的升高，气体压力对煤层气吸附的影响更加显著；采用修正的Langmuir方程对实验得到的煤吸附煤层气进行曲线拟合，其拟合曲线与实验结果的吻合度比Langmuir方程拟合更高；煤对煤层气的吸附/解吸滞后量会随着温度的升高而减小；分别对无烟煤、焦煤、气煤和气肥煤开展了不同温度下煤对煤层气吸附/解吸影响的实验研究，发现煤层气吸附量随煤的变质程度升高而变大；甲烷的解吸滞后性与煤的变质程度呈负相关，即煤的变质程度越高，越容易达到动态吸附/解吸平衡。基于传热、传质和应力守恒原理及蒸汽相变理论，建立了考虑蒸汽相变影响的煤层气注热开采热-流-固耦合三相二组分模型，模型不仅考虑到蒸汽热对煤层气开采的有利因素，而且还能反映蒸汽相变凝结水对煤层气注热开采的不利作用；采用有限容积法对建立的煤层气注热开采模型进行了离散，利用牛顿-拉夫逊迭代和GMRES(广义极小残余算法)相结合离散方程进行了求解，并通过FORTAN语言编程实现模型的求解过程。利用研发的注热开采煤层气数值模拟系统，分别对注250℃蒸汽、250℃热水和直接抽采3种注采方案进行数值模拟，分析了蒸汽在煤层中的流动特性和传热过程及其对煤层气解吸渗流的影响，发现虽然注蒸汽质量低于注热水，但由于蒸汽相变潜热影响，导致注蒸汽后煤层井底附近温度高于注热水，而且注蒸汽后煤层气的累计抽采量得到明显提高，但提高的幅度随抽采时间增加而逐渐降低；统计了累计抽采100天时注蒸汽后煤层气累计抽采量比直接抽采提高了17.5%，但是注热水后累计抽采量反而降低了9.8%，表明注热水不利于煤层气的开采。