海相页岩干酪根分子结构与孔隙网络关联特征及其微观吸附机理

Adsorption is one of the key factors in shale gas accumulation, which is directly controlled by molecular structure and pore network of kerogen in shale. Taking the marine shale of Longmaxi Formation in Southeast Chongqing as the example, this project focuses on the inner correlation between molecular structure and pore network and its microscopic the adsorption mechanism. By taking the step of sample collection, experimental test, molecular simulation, cross validation and theoretical analysis, the molecular structure of kerogen was gained by comprehensive analysis of X-ray diffraction, fourier transform infrared spectroscopy and high-resolution transmission electron microscopy. Pore morphology and pore structure were finely characterized by field-emission scanning electron microscopy as well as low-temperature nitrogen and carbon dioxide adsorption. Based on the molecular structure parameters from experimental results, the three-dimensional molecular structure model of kerogen was constructed, and the pore structure characteristics of three-dimensional molecular model were obtained by molecular probe method. Meanwhile, the correlation analyses of the experimental structure parameters and the molecular simulation results were combined to reveal the inner correlation between molecular structure and pore network. The adsorption characteristics of kerogen were obtained by high pressure isothermal adsorption experiment. The methane adsorption properties of kerogen was simulated using three-dimensional molecular structure model by the Monte Carlo method. Moreover, the simulation results were cross-validated with the measured adsorption results and then the microscopic control mechanism of methane adsorption by kerogen was revealed through molecular level and nanoscale scale.

吸附性是页岩气成藏的核心之一，直接受控于页岩干酪根分子结构及其纳米级孔隙网络，项目围绕干酪根微观结构关联特征及其吸附机理这一科学问题，以渝东南龙马溪组海相页岩为研究对象，以样品采集-实验测试-分子模拟-交叉验证-理论分析为研究思路，借助X射线衍射、傅立叶红外光谱和高分辨率透射电镜等技术，联合表征干酪根分子结构；通过场发射扫描电镜、低温液氮和二氧化碳吸附测试，实现形貌描述-结构定量相结合的孔隙网络精细表征；基于实验获得的分子结构参数，构建干酪根三维分子结构模型，通过分子探针技术获取三维模型中纳米级孔隙分布特征，同时结合实测参数相关性分析，揭示分子结构与纳米级孔隙网络结构的内在关联特征；通过高压等温吸附实验，获取干酪根吸附性能，利用巨正则蒙特卡洛法模拟甲烷在干酪根三维分子结构模型中吸附行为，并与实测吸附结果交叉验证，进而从分子层面、纳米尺度揭示干酪根微观结构对甲烷吸附的微观控制机理。

吸附性是页岩气成藏的核心之一，直接受控于页岩干酪根分子结构及其纳米级孔隙网络，项目围绕干酪根微观结构关联特征及其吸附机理这一科学问题，展开系统研究。研究表明笔石生物对龙马溪组有机质贡献突出，且有机质内发育大量纳米级孔隙。借助13C NMR、HRTEM、XPS等实验对页岩有机质干酪根结构进行系统表征，发现龙马溪组页岩干酪根分子结构以芳香层片为主，范围介于0.51至0.8之间，芳香结构主要以苯、萘、蒽、菲四种形式存在；脂肪碳结构包含甲基、亚甲基、含氧结构等，干酪根脂肪链以短链形式存在，羰基碳区以羰基、羧基形式存在。基于13C NMR数据分峰拟合定量分析不同结构的含量，并建立了干酪根二维平均分子结构，优化得到最优三维结构，再通过平均分子模型建立超晶胞结构。基于超晶胞结构研究发现干酪根内部孔隙发育，连通孔隙主要形成于芳香片层之间，与芳香结构的无序程度及芳向片层的大小有关，芳香片层堆积的无序度越高，越有利于孔隙的发育，芳香片层尺寸越大，无序堆积产生的孔隙也越大。不连通的孔隙一般发育在各支链官能团的不契合堆积空间以及方向片层的空缺位。不同的官能团对孔隙的贡献不同，在不考虑官能团间的相互作用基础上，甲基+亚甲基组合对孔隙的贡献最大，羟基的贡献最小；在官能团两两相互影响的情况下，对孔隙的增长均有正向贡献，增幅在32%～45%，其中甲基与苯的贡献最大，甲基与甲基+亚甲基的贡献最小。不同官能团对甲烷的吸附性能也不同，综合吸附热与原子之间的最短距离得到不同官能团对甲烷吸附性能的排序：芳香环>甲基>羧基>羰基>羟基>甲基、亚甲基组合。干酪根复杂的分子结构造成了复杂的孔隙结构，甲烷吸附随压力增加在不同孔隙中表现出由单层吸附向多层吸附的趋势。本项目研究成果对揭示页岩气在纳米孔隙中分子级吸附机理提供了理论依据。