煤层气是从煤层中开采的天然气，主要成分为甲烷。煤层基质含有大量的纳米级孔隙，具有低孔隙度和低渗透率的特点。实际地质条件下，煤层气具有多储存形式和多尺度流动的特征。本文基于巨正则蒙特卡洛和分子动力学方法，并通过 Python 语言自编程加入力场、原子模型和边界条件等模块，对流体流动进行精确计算。从微观角度揭示了煤纳米孔隙中甲烷吸附、注气驱替、单相流动以及气水两相流动机理。获得的主要结论如下：

明晰了甲烷在煤纳米孔隙中的吸附行为。发现煤表面与甲烷存在分子间相互作用力，煤纳米孔隙中吸附态甲烷含量大于游离态甲烷含量。低压时煤中的小孔隙吸附能力更强，能吸附更多的甲烷。

阐明了煤纳米孔隙中二氧化碳驱替甲烷的作用机制。发现随着孔径的增大，孔隙对二氧化碳封存量逐渐增加，对甲烷的吸附能力逐渐降低。随着地质深度的增加，甲烷的吸附量逐渐增大，二氧化碳的封存量明显减小。纳米孔隙中注入二氧化碳能够驱替出孔隙中吸附态甲烷，提高甲烷采收率。

明确了煤纳米孔隙中甲烷流动的微尺度效应。甲烷在流动过程中会吸附于煤孔隙壁面，增大煤孔径，壁面范德华力对游离态甲烷影响减弱，甲烷流动速度增大，孔隙内出现大量游离态甲烷，甲烷吸附层均表现为两个对称的双峰分布。大孔径中甲烷黏度较低，流动性好，Hagen-Poiseuille 方程更适用于较大孔径中的甲烷流动。升高温度，甲烷分子热运动增强，吸附层密度降低，甲烷流动速度增加，煤孔隙壁上吸附态甲烷解吸为游离态甲烷，甲烷流量增大。增大压力，孔隙内甲烷数量逐渐增多，甲烷分子间强烈的相互碰撞使得甲烷流动阻力增大，流速减小。

厘清了煤纳米孔隙中甲烷和水两相流动规律。发现含水量较低时，孔隙中部产生水桥，形成塞流。随着含水量的增加产生水-气-水层状结构，形成层流。在煤纳米孔隙中，水膜主要改变甲烷与煤壁的相互作用，甲烷气体的流动主要为滑移流动。当水膜存在时，甲烷流量与外力作用呈线性关系。当水膜消失时，甲烷流量与外力作用呈非线性关系。

结合矿井条件和吸附理论，开展分子模拟的工程应用，对平顶山天安煤业八矿煤层甲烷含量进行预测。发现计算结果与现场瓦斯含量实测值较为吻合，研究对预防煤与瓦斯突出及瓦斯治理具有指导意义。

Coal bed methane is a natural gas extracted from coal seams and is mainly composed of methane. Nano pores are prevalent in the coal bed matrix, which is characterized by low porosity and poor permeability. Under actual geological conditions, CBM is characterized by diverse storage forms and multi-scale seepage. In this work, based on the giant regular Monte Carlo and molecular dynamics methods, and through the Python language self-programming with force field, atomic model and boundary conditions and other modules, accurate calculations of fluid flow are achieved. The mechanisms of methane adsorption, gas injection displacement, single-phase flow and gas-water two-phase flow in coal nanopores are revealed from the microscopic perspective. The main conclusions are as follows:

The adsorption behaviour of methane in coal nanopores was clarified. It is found that there exists intermolecular interaction forces between the coal surface and methane, and the adsorbed methane content in coal nanopores is greater than the free methane content. The small pores in the coal has a higher adsorption capacity at low pressure and is able to adsorb more methane. The injection of carbon dioxide into the nanopores can displace the methane adsorbed in the pores and improve recovery.

The mechanism of carbon dioxide displacement of methane in coal nanopores was elucidated. It is found that with the increase of pore size, the carbon dioxide storage increases, and the adsorption capacity of methane decreases. With the increase of geological depth, the adsorption capacity of methane increases, and the storage capacity of carbon dioxide decreases obviously. Injecting carbon dioxide into nano-pores can displace the adsorbed methane from the pores and improve the methane recovery.

The microscale effect of methane flow in coal nanopores is clarified. Methane adsorbs to the walls of coal pores during the flow process, increasing the pore size of the coal, the effect of Van der Waals forces on the free methane at the walls is reduced, the methane flow rateincreases, a large amount of free methane appears in the pores, and the methane adsorption layers all show two symmetrical bimodal distributions. The Hagen-Poiseuille equation is more applicable to methane flow in larger pore sizes, where methane viscosity is lower and mobility is better. By increasing the temperature, the thermal movement of methane molecules increases, the density of the adsorbed layer decreases, the methane flow rate increases, the adsorbed methane on the coal pore wall desorbs to free methane, and the methane flow rate increases. Increase the pressure, the amount of methane in the pore gradually increases, the strong collision between methane molecules makes the methane flow resistance increase, the flow rate decreases.

The two-phase flow law of methane and water in coal nano-pore was clarified. It is found that when the water content is low, a water bridge is generated in the middle of pore, and plug flow is formed. With the increase of water content, the water-gas-water layered structure is generated, and laminar flow is formed. In the coal nanopores, the interaction between methane and coal wall is mainly changed by water film, and the flow of methane gas is mainly slip flow. In the presence of a water film, the methane flow rate is linear to the external force. When the water film disappears, the methane flow has a nonlinear relationship with external force.

Combining mine conditions and adsorption theory, the engineering application of molecular simulation was studied, and the methane content of the coal seam in Pingdingshan Tianan Coal Mine 8 was predicted. The results show that the calculated results are in good agreement with the actual measured values of gas content in the field, which is of great significance to the prevention of coal and gas outburst and gas control.

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