海相页岩储层黏土矿物甲烷吸附贡献及其动态模型

Adsorption mechanism is the key factor in the accumulation of shale gas reservoir. Clay minerals and organic matter are the main geological carriers of adsorption. However, the adsorption contribution and mechanism of clay minerals are not clear when the clay minerals are in different minerals composition, content and diagenetic process, so it has become the main theoretical bottleneck of constructing shale gas fine resource evaluation model. This project focuses on studying the adsorption characteristics of the clay minerals in marine shale reservoir. The thermal evolution simulations will be carried out to get the adsorption characteristics in diagenetic stages, by choosing the samples of the pure clay minerals, the sapropelic coal with I type kerogen and the low mature marine shale. The characteristics of the intermediated products in the thermal simulation evolution and the pure clay minerals would be represented by their molecule structure, pore structure and the capacity of the adsorption. Molecular dynamics method and Monte Carlo method will be employed to obtain the dynamics parameters including adsorption site of methane, adsorption sequence, density distribution, main pore diameter, and competitive adsorption characteristics. The results would illustrate the methane molecular-level adsorption mechanism of clay minerals with pore structure in various diagenetic stages. It will also reveal the contribution and the dynamic evolution mechanism of clay minerals in marine shale reservoir. Moreover, the adsorption model of the clay minerals in marine shale reservoir would be built.

吸附性是页岩气成藏的核心地质要素之一，黏土矿物和有机质均是甲烷吸附的主要地质载体，但黏土矿物组成、含量及对不同成岩阶段的吸附贡献和演化机制尚不明确，成为构建页岩气资源精细评价模型的理论瓶颈。项目围绕这一科学问题，以我国海相页岩储层为对象，采用不同种类纯相黏土矿物与I型干酪根（腐泥煤）配比的方法，结合低成熟海相页岩样品，开展热演化物理模拟，获得不同模拟成岩阶段产物样品；从结构、孔隙、吸附等方面表征模拟产物及不同纯相黏土矿物的基本特征，采用分子动力学、蒙特卡洛等方法获得吸附甲烷的吸附位、吸附次序、密度分布等动力学参数，探讨不同成岩阶段孔隙结构与黏土矿物种类的分子级吸附机制，揭示海相页岩黏土矿物吸附贡献及其动态演化规律，建立海相页岩储层黏土矿物吸附模型。

吸附性是页岩气成藏的关键，黏土矿物作为页岩主要成分，在页岩储层的吸附储集方面具有重要作用，但黏土矿物对甲烷的分子级吸附机理、黏土矿物的相态转化及其孔隙演化对甲烷吸附性能的影响机理尚未解决。项目围绕这一核心科学问题，主要取得以下创新性成果：对于经历成熟过程后的复杂形态孔隙，形态越复杂其吸附性对压力越敏感，孔隙壁面凹槽空间的存在可以提升孔隙甲烷吸附量，狭缝型孔隙甲烷吸附能力远大于圆柱型孔隙，最利于甲烷吸附的孔径尺寸在2 nm左右。页岩中主要吸附载体的单组分模拟表明，超临界条件下石墨、黏土、干酪根单位面积上对甲烷的吸附过剩量均在同一数量级。相同温压条件下石墨吸附过剩量整体大于黏土，但随着压力的增加，过剩量差异逐渐缩小。孔隙结构控制的页岩表面积发育程度是影响超临界条件下单位岩石吸附过剩量的主要因素。随着成岩作用的加强，黏土矿物间的转换相当显著并且在孔隙结构和吸附能力方面具有继承性。孔隙结构和吸附性能演化主要受黏土矿物（主要是伊蒙混层和伊利石）和有机质共同控制。伊蒙混层向伊利石的转化有利于微孔和介孔的发育，而宏孔的演化主要与绿泥石有关。此外，微孔和介孔的发育在成熟-高成熟阶段还与有机质密切相关。由浅至深，五峰-龙马溪组有机质孔隙贡献增大（0~84.51%），黏土矿物孔隙贡献减小（3.48%~93.55%），脆性矿物孔隙贡献变化不明显。甲烷吸附能力的变化趋势与伊利石含量几乎一致，而与有机质含量大致相反。微孔和介孔对吸附能力均有控制作用，且在过成熟阶段介孔对吸附能力的控制作用更加明显。研究成果为构建页岩气资源精细评价模型和揭示页岩气赋存机理提供了理论依据。