页岩有机质——石英复合孔隙中甲烷吸附与流动行为的分子模拟研究

Molecular Simulation of Methane Adsorption and Transport in Shale Organic Matter—Quartz Composite Nanopores

摘要: 吸附气是页岩气赋存在页岩储层中的主要方式之一，而且吸附气是页岩气后期产量的主要来源，页岩储层甲烷高温高压条件下的赋存特征是准确评估页岩气储量的关键。依据四川盆地五峰组——龙一₁亚段页岩无机地球化学特征，构建了石英、干酪根和复合孔组成的有机质——石英复合孔隙体系分子模型。通过巨正则蒙特卡洛(GCMC)、分子动力学(MD)和非平衡分子动力学(EFMD)模拟方法，研究了甲烷分子在复合孔隙体系中的微观赋存特征及其流动扩散特征。研究结果显示，随着压力的升高，赋存于孔隙空间中的甲烷分子增多，同时其扩散能力受到限制(压力越大，扩散系数越小)，且赋存的甲烷分子数量随着孔径的增大而增多；孔隙壁面附近存在明显密度峰值，峰值随着压力的升高而增大；采用外力法模拟施加压降，外力的增大会提升甲烷在孔隙中的流速，且赋存空间(孔径)越大，甲烷在孔隙中的流速越大。在开采过程中制造较大的压差，更有利于甲烷从页岩储层中脱离出来。

Abstract: Adsorption of gas is a primary method of shale gas storage in shale reservoirs. Furthermore, adsorbed gas constitutes the predominant source of shale gas production in the late stage. The accurate assessment of shale gas reserves is contingent upon a comprehensive understanding of the characteristics of methane storage in shale reservoirs under conditions of high temperature and high pressure. A molecular model of the organic matter-quartz composite pore system composed of quartz, casein and composite pores was constructed on the basis of the inorganic geochemical characteristics of shale in the Wufeng Formation (Longyi 1 subsection of the Sichuan Basin). The micro-existence characteristics of methane molecules in the composite pore system, and their flow-diffusion characteristics, were investigated by means of a range of simulation methods, including giant canonical Monte Carlo (GCMC), molecular dynamics (MD) and nonequilibrium molecular dynamics (EFMD). The results demonstrate that the number of methane molecules in the pore space increases with an increase in pressure, while their diffusion ability is limited (the higher the pressure, the smaller the diffusion coefficient), and the number of methane molecules increases with an increase in pore diameter. It is evident that there is a conspicuous density peak in proximity to the pore wall, which exhibits an increase in magnitude with an escalation in pressure. The pressure drop is simulated by the method of applying an external force, and an augmentation in the external force will result in an enhancement of the flow rate of methane in the pore space. Concurrently, the flow rate of the stored space will be increased by the external force. The pressure drop is simulated using the external force method, and an increase in external force will enhance the flow rate of methane in the pore space. Furthermore, it has been demonstrated that a larger endowment space (pore diameter) will result in a larger flow rate of methane in the pore space. It has been demonstrated that creating a larger pressure drop during the extraction process is more favourable for methane to detach from the shale reservoir.