表征页岩孔隙结构是评价页岩含气性的重要一环，低温氮吸附实验是表征页岩纳米孔孔径分布和比表面积的重要方法。现有孔径分布计算方法建立在规则孔隙形状假设的基础上，用于分析具有复杂连通纳米孔隙的页岩孔径分布存在诸多局限，由脱附支计算得到的孔径分布结果存在“假峰”；常用的比表面积计算方法容易高估微孔充填特征显著的页岩的比表面积。针对上述问题，本文采用巨正则蒙特卡洛-分子动力学混合模拟方法，研究了连通纳米孔隙中的氮气冷凝和蒸发机制，并将获得的微观特征及认识用于页岩低温氮吸附曲线的定量解释和页岩纳米孔隙孔径分布和比表面积计算的优化。主要的研究内容如下：

构建了连通纳米孔的分子模拟体系，模拟了低温条件的氮气吸附与脱附过程。通过圆柱孔的模拟获得了吸附曲线出现迟滞现象的临界孔径，揭示了封闭端对冷凝和蒸发机制的影响。对墨水瓶孔结构的喉道尺寸、孔腔尺寸和形状影响进行讨论，得到了墨水瓶孔蒸发过程中存在的孔阻塞和空化机制，探究了发生空化的喉道临界尺寸；针对冷凝过程，综合孔腔尺寸和形状，提出了冷凝孔腔参数，实现了对孔腔冷凝压力的预测。考虑纳米孔连接结构，通过构建多喉道连接孔单元，对连接体系各部分的冷凝和蒸发机制进行了分析，明确了狭窄喉道对内部体系蒸发过程的阻塞作用，发现了导致空化压力滞后的多级空化现象。

将分子模拟获得的认识应用于页岩纳米孔表征方法的优化。基于墨水瓶孔连接结构开始发生空化和多级空化结束的相对压力与页岩氮气吸附实验曲线脱附支出现突变和迟滞环闭合的相对压力的对应关系，提出由脱附支曲线得到的孔径分布结果存在“假峰”这一现象的一种定量解释，实现孔径分布计算分支和计算核心的优选。基于冷凝孔腔参数，发展了一种不考虑孔型假设的页岩孔隙比表面积计算方法。利用脱附支曲线可反映页岩介孔孔隙阻塞的特点，提出了页岩纳米孔隙渗透率贡献参数，建立了氮吸附实验曲线和介孔连通性之间的关系。

本文通过分子模拟方法对低温氮吸附实验曲线进行了详细的定量解释，为更准确地表征页岩纳米孔隙孔径分布和比表面积提供了依据。

Characterization of shale pore structure is an important step to evaluate shale gas-bearing capacity, and low-temperature nitrogen adsorption experiment is an important method to characterize pore size distribution and specific surface area of shale. The existing calculation methods of pore size distribution are based on the assumption of regular pore shape, there are many limitations in analyzing the pore size distribution of shale with complex interconnected nano-pores: the pore size distribution calculated by desorption has an “artefact peak”; It is easy to overestimate the specific surface area of shale with obvious pore-filling characteristics by the common calculation method of specific surface area. In order to solve these problems, the mechanism of nitrogen condensation and evaporation at low temperature in interconnected nano-pores was studied by using the giant regular Monte Carlo-Molecular dynamics hybrid simulation method, the obtained microscopic characteristics and understanding are used for quantitative interpretation of low-temperature nitrogen adsorption curves in shale and optimization of pore size distribution and specific surface area calculation in shale nanopores. The main research contents are as follows:

A molecular simulation system was constructed to simulate the adsorption and desorption of nitrogen at low temperature. The critical pore size of hysteresis in the adsorption curve was obtained by the simulation of the cylindrical pore, and the effect of the closed end on the condensation and evaporation mechanism was revealed. The influence of throat size, cavity size and shape on the pore structure of ink bottle was discussed, and the mechanism of pore blocking and cavitation in the evaporation process of ink bottle pore was obtained, and the critical size of the throat where cavitation occurs was explored. According to the condensation process and the size, and shape of the cavity, the parameters of the cavity are proposed, and the prediction of the cavity condensation pressure is completed. Considering the nanopore structure, the condensation and evaporation mechanism of each part of the connecting system was analyzed by constructing a multi-throat connecting pore unit, and the blocking effect of the narrow throat on the evaporation process of the internal system was clarified, a multi-stage cavitation phenomenon is found, which causes the cavitation pressure to lag.

The knowledge gained from molecular simulation was applied to optimize the characterization method of shale nanopore. A quantitative explanation for the phenomenon of the " artefact peak " in the pore size distribution results obtained from the desorption branch is proposed based on the corresponding relationship between the relative pressure at the beginning of cavitation and the end of multi-stage cavitation in the ink bottle pore structure and the relative pressure at the abrupt change and hysteresis loop closure of the desorption branch in the shale nitrogen adsorption experiment. It leads to the optimization of the pore size distribution calculation branch and calculation kernel. A method for calculating the specific surface area of shale pores without considering the assumption of pore type has been developed based on the parameters of the condensation pore cavity. The desorption branch curve can reflect the characteristics of shale mesoporous pore blockage, and the contribution parameter of shale nanopore permeability is proposed. The relationship between the nitrogen adsorption experimental curve and mesoporous connectivity is established.

In this paper, the experimental curve of nitrogen adsorption at low temperature is explained quantitatively by molecular simulation, which provides a basis for more accurate characterization of pore size distribution and specific surface area of shale nano-pores.