页岩孔隙-流体系统演化对页岩气赋存状态的影响机理

Growing attentions have been paid to the nanometer to shale gas, an important unconventional gas resource, around the world during the last several years. Compared with conventional hydrocarbon reservoirs, shale gas has complicated and special pore space and gas storage mechanism. The evolution of pore-fluid system have an important effect on the evolution of shale gas occurrence which is the key scientific question for shale gas accumulation. In this project, the Lower Paleozoic marine shale in peripheral regions of Sichuan basin and different maturity shale of the Upper Cretaceous Eagle Ford shale from the south Texas in the United States are selected as a research object. Using field emission scanning electron microscopy (FE-SEM), scanning transmission X-ray microscopy (STXM)-based carbon X-ray absorption near edge structure (C-XANES) spectroscopy, the morphological characteristics and chemical bond characteristics of different organic matter are identified to discuss the formation mechanism of different organic pores. Low pressure nitrogen gas physisorption and high temperature and high pressure methane adsorption are conducted using different maturity Eagle Ford shale with Soxhlet-extracted in pentane and dichloromethane to study pore structure and fluid storage in shale with increasing maturity and then to real the coupling effects of pore structure and different organic fluid on methane adsorption. Combined with burial history and pore-fluid system evolution, the storage evolution model of shale gas will be constructed.

页岩气作为重要的非常规油气资源受到国内外的广泛关注。与常规油气相比，页岩气具有更复杂的储集空间及特殊气体赋存机理。页岩孔隙-流体系统演化影响页岩气赋存状态演化过程，是研究页岩气成藏过程的核心科学问题之一。本项目选取四川盆地周缘下古生界页岩及美国德州不同成熟阶段Eagle Ford页岩为研究对象，运用场发射扫描电镜、扫描透射X射线显微镜、碳原子-X射线近边结构吸收光谱，识别不同类型有机质形态学特征和化学键特征，探讨不同类型有机质中孔隙发育特征及成因机理；利用不同有机质溶剂索氏抽提处理后的不同成熟阶段Eagle Ford页岩样品开展低温低压氮气吸附实验和高温高压甲烷吸附实验，研究页岩储层孔隙演化过程及其中流体赋存特征，探讨页岩演化过程中孔隙结构和不同流体对甲烷吸附的影响机理，结合地层埋藏史、沉积成岩史、有机质生排烃史等，建立页岩孔隙-流体及气体赋存演化综合模式。

富有机质页岩在热演化过程中，干酪根不断降解生成不同的烃类，部分烃类流体残留在泥页岩孔隙内，影响页岩孔隙的发育和甲烷气体的吸附作用，进而控制页岩气成藏动态演化过程。本项目选取中国重庆地区高成熟度龙马溪组页岩和美国德州不同成熟度Eagle Ford页岩，开展了富有机质页岩孔隙演化及流体赋存特征研究，初步取得以下成果：（1）龙马溪组页岩中有机质存在不同赋存类型：原始沉积孤立有机质、有机质-黏土矿物复合体、有机质-重矿物复合体以及次生运移沥青有机质。有机质是页岩储层孔隙发育的最重要的控制因素，黏土矿物主要影响页岩中介孔和宏孔的发育，脆性矿物有利于宏孔和微裂缝的发育。陆源碎屑物质的稀释作用是盆缘地区龙马溪组页岩有机质富集程度的控制因素。（2）基于Langmuir方程，以甲烷等温吸附实验数据为基础，建立温度压力耦合控制下的扩展Lagmuir方程，计算储层条件下页岩吸附甲烷能力。基于地层埋藏史及热演化生烃史，将南方海相页岩气赋存状态演化过程划分为4个阶段：①早期生物游离气阶段；②热解吸附气阶段；③热解游离气阶段；④游离气吸附阶段。（3）Eagle Ford页岩中的水主要赋存于大孔隙中，而残留烃类主要赋存于小孔隙中。随埋藏深度和成熟度增加，大孔逐渐降低，小孔不断增加，平均孔径逐渐降低，孔容和比表面积起始阶段迅速减小，进入生油高峰后比表面积迅速增加，孔容缓慢增加。大孔隙的降低一方面是由于压实作用导致的粒间孔压缩，另一方面是由于干酪根生成的沥青或烃类堵塞孔隙，小孔隙的增加主要源于有机质生烃作用生成的孔隙。页岩中残留烃的存在大大降低页岩中甲烷的吸附能力。(4)综合中国南方海相高-过成熟度龙马溪页岩及美国德州南部的Eagle Ford不同成熟度页岩的孔隙结构及甲烷吸附的研究成果总结了页岩孔隙-流体系统演化的综合模式。发表SCI收录期刊论文3篇。