多级孔裂隙介质约束下的煤层气扩散-渗流联动效应研究

Pores and fractures are the sites for coalbed methane (CBM) enrichment in CBM reservoir, which are also the pathways for gas migration. Currently, taking the pores and fractures as a multilevel porous media, the research for interaction effect between diffusion and seepage in the CBM production is still rare. This project will focus on the pores and fractures of CBM reservoirs from Qinshui Basin, Eastern Ordos Basin and Southern Junngar Basin, which have the highest CBM exploration and development, the most abundant CBM Wells and the CBM research hotspots. In this project, the CBM reservoir will be regarded as a multilevel porous media, and the interaction effect for gas diffusion-seepage in CBM production will be simulated and investigated. The experiments, physical simulation and mathematical modeling will be adopted to study the interaction effect for gas diffusion-seepage in multilevel porous media during CBM production. The details include: firstly, the investigation of the difference of multilevel porous media in varied rank coals and the establishment of multilevel porous model for different rank coals; secondly, the revelation of the differential contribution of matrix pores and fractures on permeability; additionally, the establishment of CBM diffusion in multilevel pores; finally the investigation on the mechanism of the interaction effect of CBM diffusion-seepage in multilevel porous media. Therefore, this project would benefit the improvement of "restriction of single CBM well production due to reservoir condition", the enrichment of CBM geological development theory and the theoretical basis for improving the CBM recovery.

煤储层孔裂隙是煤层气富集场所，也是煤层气开采过程中的气体运移通道。目前，视煤储层多级孔裂隙介质为研究对象，针对煤层气生产过程中的气体扩散-渗流联动效应研究还鲜见报道。本项目以我国煤层气勘探开发程度最高、煤层气井资料最为丰富、研究最热点的沁水盆地、鄂尔多斯东缘和准噶尔盆地南缘为依托，视煤储层多级孔裂隙介质为一整体，以煤层气开采过程中的扩散-渗流联动效应为研究对象，采用室内实验、物理模拟和数学分析相结合的方法，就多级孔裂隙介质中煤层气扩散-渗流联动效应这一核心问题，拟分析不同煤阶煤储层孔裂隙结构的多级模式和成因差异；力求查明孔隙和裂隙对渗透性的差异性贡献；建立多级孔裂隙介质模式下的煤层气扩散模型；阐明多级孔裂隙介质约束下的煤层气扩散-渗流联动效应过程及机理。项目研究成果能够促进“煤层气单井产量的储层制约”问题的解决，丰富煤层气开发地质理论，为提高煤层气采收率提供理论依据。

本项目研究基于沁水盆地、鄂尔多斯东缘和准噶尔盆地南缘地区煤层气开发中的科学问题“不同煤阶煤储层孔裂隙结构的多级性模式及差异”和“煤层气扩散-渗流临界态的流动过程、介质空间和控制机理”，以“识别不同煤阶煤储层多级孔裂隙结构发育模式“、“ 阐述多级孔裂隙介质中的煤层气扩散特征及控气机理”、“揭示煤储层孔裂隙复杂程度和渗透率的关系”和“揭示煤层气储层扩散-渗流联动效应”为主要研究内容，开展了煤储层低温氮、压汞、NMR、等温吸附、聚焦离子束扫描电镜法(FIB-SEM)、μ-CT技术、核磁冻融法（NMRC）等多项分析，研究阐明了不同煤阶煤储层孔裂隙结构特征，阐述了多级孔裂隙介质中煤层气扩散特征及控制机理，揭示了煤层气储层基质孔隙和裂隙的渗透率差异性贡献，并探讨了煤层气储层中的扩散-渗流之间的关系。低-中成熟度的烟煤中发育不规则的椭圆形（约占20%）、楔形和狭缝型（约占50%）、短线状气孔（约占20%），而高阶烟煤和无烟煤中则发育大量变质气孔，占比约70%；吸附孔主要分为三类：细颈瓶孔或墨水瓶孔（H1类，25%）、平行板状孔及尖劈型孔（H2类，25%）和一端封闭的平行板状孔及狭缝型孔（H3类，50%）；渗流孔的孔隙分布类型也可分为三种：（I~III类），依次代表连通性好、连通性一般、连通性差的孔隙结构。微孔和大孔含量以及分维数Dc3和Dc4决定着孔隙的连通性，从而对孔隙的渗透性影响大。扩散模型以多孔扩散模型为优，煤储层中微孔、过渡孔和大孔的比表面占比与扩散阶段有很好的对应性。阐明了煤储层中甲烷吸附-扩散控制机理，甲烷的扩散系数随煤阶增大呈现先减小后增加的“U”趋势；水分的存在明显降低了煤层气的扩散能力；扩散系数随粒度的增大而减小，随温度的增高而增大，随着压力的增加呈现降低的趋势。富含A+B型微裂隙的煤通常具有良好的连通性和高渗透性。渗透率对可动流体孔隙度和ST的关系均表现出高相关系数的指数函数。低煤阶煤样渗透率随着流速的增加呈现增加后减少的趋势；中高煤阶煤样渗透率随流速的增加逐渐减小。压汞置换法在煤储层渗透率及甲烷扩散系数数值计算中有很好的适用性。甲烷扩散率与煤储层渗透率之间呈现幂函数关系。