中国科学院力学研究所机构知识库

中国坐拥世界第一大页岩气资源, 页岩气可采资源潜力为 25.1 万亿立方米.尽管页岩气具有储量巨大、低碳清 洁等优势, 但是我国页岩气的赋存特点导致了其开发难度巨大. 我国页岩地层极深 (~3 km), 页岩孔隙结构复杂、孔喉直径非常小, 渗透率极低 (<1 μD), 并且 70% 的页岩气以吸附气的方式储集在页岩中, 必须进行水力压裂才能进行商业化开采, 而 “甜点区” 水资源紧缺. 因此, 通过传统的压裂、加热、减压、化学试剂注入等方法开采, 效率极低、成本极高, 并且会导致生态、地质和安全问题, 这直接导致了目前我国页岩气的采收率极低、发展缓慢的现状. 目前国内外的研究者们已经在其中的某些方面开展了相关的研究工作, 并取得了一定的研究进展. 然而这些工作大多是基于连续介质的模型上通过不同方式的修正进行展开的. 要研究清楚气体在纳米孔中的吸附/解吸和驱替的规律与机理, 需要从量子力学和统计力学出发, 真正考虑到系统的原子、分子尺度结构, 并且结合实验和模拟多种手段, 才能获得较为本质的认识.

本学位论文在此背景下, 主要围绕 CH₄ 在微纳孔隙中的吸附、解吸及驱替动力学展开研究, 针对多孔介质中毛细凝聚和吸附相变、驱替运移两大关键科学问题, 通过第一性原理及分子动力学 (MD) 跨尺度模拟, 统计力学和准连续介质理论分析以及实验观测相结合的方法, 开展研究工作.

基于分子吸附过程中空间点群的变化, 厘清了吸附过程中对称性破缺机理并确立了高对称性位点; 通过范德华修正的密度泛函理论 (DFT-D2) 计算不同分子在石墨烯上的吸附位点及吸附能. 基于 MD 计算了多种分子在石墨烯表面的三维势能分布及不同分子在石墨烯表面对应吸附位点的吸附能. 阐明了 CH₄ 在石墨烯缝隙中的吸附特性及其作为页岩模拟模型的依据. 通过理论分析研究了 CH₄ 与石墨烯的相互作用势, 探讨了石墨烯层数对相互作用势的影响.

建立了等效吸附应力模型和应变-吸附理论. 结合粒子间相互作用势, 提出了一种简单而有效的方法来描述或计算吸附剂在吸附和解吸过程中的应力场. 从统计力学配分函数出发, 得到了吸附量与基底表面性质的关系, 结合 Cauchy-Born 准则, 厘清了基底应变和吸附能对吸附量的影响及其量化关系, 为后续设计出更为实用的吸附模型提供理论指导.

通过 MD 模拟, 厘清了驱替过程中需跨越能垒的动力学过程及微观机理. 从热动平衡熵判据出发, 通过分析不同直线型、非直线型分子在游离态及吸附态时构形熵的变化, 揭示了超临界流体驱替天然气过程中自发、熵变、放热等详细过程, 进而解释不同超临界流体驱替效率的差异. 明确了超临界流体驱替受限 CH₄ 的动力学过程及驱替效率, 为业界筛选驱替介质.

从实验出发, 通过 FE-SEM 观察、图像分析、盒计数法, 获得了在地下 3000 米深的多块页岩样品的原位孔隙结构及其分形特征. 基于图像识别, 建立多孔介质模型, 首次实现了深部页岩气的原位模拟. 阐明了超额吸附等温线交叉现象的微观机理并建立了高压 Langmuir 吸附模型. 厘清了解吸迟滞现象的微观机理及其存在性条件.

本论文探索了页岩气在微纳孔隙中的吸附、解吸及驱替动力学的微观机理, 为解决页岩气开采中存在的基本难题提供理论基础, 为推进工业界形成新的非常规能源开发核心技术, 提供指导和预测.

China has the world's largest shale gas resources, with a potential of 25.1 trillion cubic meters of recoverable shale gas resources. Although shale gas has the advantages of huge reserves and cleanliness of low carbon, the occurrence characteristics of shale gas in China have led to great difficulties in its development. The shale layer in China is extremely deep (~3 km), the shale pore structure is complex, the pore throat diameter is very small, the permeability is extremely low (<1 μD), and 70% of the shale gas is stored in the shale by adsorption. Hydraulic fracturing is required for commercial exploitation, while the “dessert areas” are in short supply of water. Therefore, traditional fracturing, heating, decompression and chemical reagent injection are very inefficient, costly, and will lead to ecological, geological and safety problems, which directly leads to the current situation of extremely low rate and slow development of shale gas in China. At present, researchers at home and abroad have carried out relevant research work in some aspects and made some progress. However, most of these works is based on the continuum model and modified in different ways. To study the mechanism of gas adsorption/desorption and displacement in nanopores, it is necessary to take into account the atomic and molecular scale structures of the system from the perspective of quantum mechanics and statistical mechanics, and combine various methods of experiment and simulation to obtain a more essential understanding.

In this context, this dissertation mainly focuses on the adsorption, desorption and displacement dynamics of methane (CH₄) in nanopores. It is aimed at two key academic issues: the capillary condensation and adsorption phase transition; the displacement and migration in porous media. The first principle and molecular dynamics (MD) cross-scale simulation, statistical mechanics and quasi-continuous medium theory analysis and experimental observation are combined to carry out the research work.

Based on the change of space point group in the process of molecular adsorption, the mechanism of symmetry breaking in the process of adsorption is clarified and the high symmetry sites are established. The adsorption sites and adsorption energies of different molecules on graphene were calculated by van der Waals modified density functional theory (DFT-D2). The three-dimensional potential energy distribution of various molecules on graphene surface and the adsorption energy of different molecules at the corresponding adsorption sites on graphene surface were calculated by MD simulations. The adsorption characteristics of CH₄ in graphene slit and its feasibility as a shale simulation model are clarified. The interaction potential between CH₄ and graphene is studied by theoretical analysis, and the influence of graphene layer number on the interaction potential is discussed.

The equivalent adsorption stress model and strain-adsorption theory are established. A simple and effective method is proposed to describe or calculate the stress field of adsorbent in the process of adsorption and desorption based on the interaction potential between particles. Based on the partition function of statistical mechanics, the relationship between the adsorption capacity and the properties of the substrate surface is obtained. Combining with the Cauchy-Born rule, the influence of the strain and the adsorption energy on the adsorption capacity and its quantitative relationship are clarified, which provided theoretical guidance for the subsequent design of a more practical attachment model.

The dynamics process and micro-mechanism of crossing energy barrier in displacement process are clarified by MD simulation. Based on the criterion of thermodynamic equilibrium entropy, the changes of configuration entropy of different linear and non-linear molecules in free and adsorbed states are analyzed. The detailed spontaneous, entropy change and exothermic process of supercritical fluid displacing natural gas are revealed, and then the displacement of CH₄ by different supercritical fluids is explained. The dynamics process and displacement efficiency of supercritical fluid displacement confined CH₄ are clarified, and the displacement media are screened for the industry.

Based on the experiments, the in situ pore structure and the fractal characteristics of several shale samples drilled at 3000 m depth are analyzed using scanning electron microscopy (SEM) and image analysis. On the basis of image recognition, the porous medium model is established, and the in situ simulation of deep shale gas is realized for the first time. The micro-mechanism of the cross phenomenon of excess adsorption isotherms is clarified and the high-pressure Langmuir adsorption model is established. The microscopic mechanism of hysteresis loop and its existing conditions are clarified.

This dissertation explores the micro-mechanism of adsorption, desorption and displacement dynamics of shale gas in nanopores. It provides theoretical basis for solving the basic problems in shale gas exploitation. Our study may provide guidance and prediction for promoting the formation of new unconventional energy development core technology in industry.