低阶煤储层物性演化轨迹及其煤化作用机制

The formation and evolution of low-rank coals generally cover a series of actions, such as the biochemical process, the filling, dehydration, cementing and hydrocarbon generation of gel materials and the transformation of petrographic composition, resulting in the complexity of physical evolution and uncertainty of evolution mechanism for low-rank coal reservoirs. Since current studies on physical property evolution of low-rank coal reservoirs and its effects on the changes of reservoir spaces and gas mass transfer mode cannot meet technical requirements of exploration and development for low-rank coalbed methane, further research on the development and evolution of effective reservoir spaces controlled by coalification and systematic reveal on the evolution mechanism of physical property are urgently needed. Taking the low-rank coals in Erlian Basin as the research object, the characteristics of physical development and the constitution of pore-fracture system will be analyzed by detailed description and special test. Based on the physical and numerical simulation, the continuous distribution model of pore-fracture system and three-dimensional spatial model of coal physical property evolution will be constructed, under which the variations of gas-water storage spaces, movable fluid spaces and effective reservoir spaces induced by the material consumption and transformation during coalification can be discussed. Combining the analysis on key geological processes, the evolution trajectory of physical properties for low-rank coal reservoirs and its coalification mechanism will be unveiled, which may provide scientific basis for the optimization and stimulation of coal reservoirs and working system establishment of coalbed methane wells.

低阶煤的形成和演化涵盖了生物化学作用、凝胶物质充填、脱水、胶结和生烃作用、煤岩成分转化作用等，造成了低阶煤储层物性演化复杂，演化作用机制不明。对低阶煤储层物性演化及其引起的储渗空间和气体传质方式变化研究远不能满足低煤阶煤层气勘探开发技术要求，精细阐明煤化作用制约下的低阶煤储层有效储渗空间发育和演化特征，揭示物性演化作用机制成为亟待探索的科学问题。以二连盆地低阶煤储层为研究对象，通过精细描述和专项测试，分析低阶煤储层物性显现特征和孔裂隙系统构成；采用物理模拟、数值模拟技术构建煤储层孔裂隙连续分布模型和煤储层物性三维空间演化模型，探讨煤化作用制约下的物质消耗、转化所引起的煤体气、水储集空间、可动流体空间、有效孔渗空间变化特征；结合储层物性演化关键地质作用过程分析，揭示低阶煤储层物性演化轨迹及其煤化作用机制，为有利煤储层优选、储层改造和排采制度的建立提供科学依据。

低阶煤储层储渗空间精细表征及其演化轨迹对煤层气储集、渗流、产出具有重要的指示意义，决定了有利储层优选、资源可采性评价、储层改造方案和排采制度的建立。精细阐明煤化作用（主要含凝胶化作用、沥青化作用和镜煤化作用关键作用事件）制约下的低阶煤储层有效储渗空间发育和演化特征，揭示物性演化作用机制成为亟待探索的科学问题。本项目在野外地质调查、资料收集、样品采集、煤岩描述等基础工作基础上，选择重点盆地低阶煤样品开展了煤岩煤质、压汞、液氮、孔渗、扫描电镜、X射线衍射、核磁共振等实验，精细刻画煤储层物性显现特征及孔裂隙系统构成，建立了低阶煤储层孔隙连续分布模型，刻画了第一次煤化作用跃变前后储层物性三维空间演化轨迹，探讨了其对气体储集和渗流的影响；结合褐煤样品热脱水物理模拟实验，阐释了煤化作用与储层物性转化的内在关系；进一步提取低阶煤中镜煤和暗煤条带开展物性分析，探讨了镜煤化作用对不同煤岩成分样品物质构成和孔裂隙系统的作用机制。研究表明：低阶煤样品大、中孔比例较高，为游离气提供了广阔的存储空间，气体产出相对较易；第一次煤化作用跃变致使低阶煤样品渗流孔比例在逐渐减少，吸附孔比例在逐渐增加，且孔隙连通性变差；微波加热具有改善煤孔裂隙分布的作用，微波功率的增加极大地促进了孔隙的扩大、互联和整合；镜煤化作用导致煤中镜质组含量增高，渗流孔发育程度增高，同时孔体积和比表面积整体呈现减少的趋势。