碳纳米孔隙中甲烷输运性质的分子动力学模拟

发布时间：2018-03-07 23:11

本文选题：分子动力学　切入点：蒙特卡洛模拟　出处：《西南石油大学》2017年硕士论文　论文类型：学位论文

【摘要】：纳米孔隙中流体的输运性质是页岩气等致密油气藏开发、多孔纳米材料研发的基础科学问题。由于气体在纳米级孔隙中流动受到尺寸效应的影响,常规的流体力学理论已经无法适用。本文基于分子动力学理论和软件,在前人研究的均一壁面基础上,模拟了碳纳米孔隙中甲烷的输运过程,分析了甲烷在超临界状态下的滑移流以及过渡流;通过在不同壁面结构的石墨多孔介质模型中,模拟了甲烷在该多孔介质中的分布及扩散过程,揭示了甲烷在纳米孔隙裂缝中与固体壁面粒子相互作用的微观机理;最后,通过建立Ⅰ类和Ⅱ类干酪根有机质的分子模型,模拟了不同温度下甲烷在干酪根基质单元中的吸附和扩散行为,计算了所产生的吸附热和扩散活化能。论文的主要内容和结论为:(1)通过甲烷在两个不同孔径的石墨孔隙中的分子动力学模型,发现,石墨孔隙的开孔方向变化会导致甲烷与孔隙壁面的势能作用发生变化,并影响甲烷的流动性质;在过渡流中,甲烷轴向速度的径向分布波动剧烈,石墨六晶格面是一种非常光滑的壁面,甲烷在这种壁面的孔隙中的流动速度快,且边界滑移长度大;甲烷的平均速度随着微观孔隙尺度的增加而增加,且轴向速度的径向分布波动变缓。垂直于石墨晶格面的孔隙中,甲烷流动的平均速度低于平行于石墨晶格面孔隙中的甲烷流速。在滑移流中,孔隙方向垂直于石墨晶格面时,速度分布与切向动量协调系数ζ=0.6的Maxwell滑移理论值符合较好;当壁面原子为单层石墨卷成的纳米管时,速度分布与ζ=0.1的Maxwell滑移理论值符合较好;而在孔隙方向平行于石墨晶格面的孔隙中,两侧近壁面处的甲烷速度为0,以致整体径向速度分布偏离了 Maxwell滑移理论值。在方向相同的孔隙中,滑移程度随驱动力的增大而增大,质量流量逐渐增大。随着驱动力的增加,滑移长度增加的趋势不断减小,流动摩擦阻力系数随着驱动力和雷诺数的增加而减小,减小的幅度随着驱动力的增加在降低。(2)通过构建的孔径为1～5nm的石墨多孔骨架,模拟了甲烷在温度为280K和320K下的分布形态和扩散性质。研究发现,甲烷主要吸附在石墨的六晶格壁面处,其面积越大,吸附数越多,在该壁面构成的孔隙中停留的时间越长;甲烷的吸附层是在温度与分子间势能相互耦合作用下形成的,温度对甲烷扩散的影响与固体结构原子排列关系不大。(3)通过构建的Ⅰ类和Ⅱ类干酪根有机质分子模型,模拟了甲烷在不同温度中的等温吸附和扩散过程。研究发现,干酪根对甲烷的吸附数随温度的升高而减少,且温度每升高20K,甲烷吸附量大约减少25%,吸附规律符合Langmuir等温吸附特征。Barneet Ⅱ类干酪根对甲烷的吸附量约为桦甸Ⅰ类干酪根的3.37倍。甲烷更容易从Ⅰ类干酪根中解吸出来。温度每升高40K,扩散系数大约增加2.3倍。甲烷在Ⅰ类干酪根张扩散系数大于Ⅱ类,表面其在Ⅰ类干酪根中的扩散速度更快。
[Abstract]:The nano pore fluid transport properties in shale gas is tight oil and gas reservoir development, basic scientific problems of porous nano materials development. The gas flow in nanoscale pores by size effect, the conventional theory of fluid mechanics has been unable to apply. The molecular dynamics theory and software are based on a previous study in based on the surface to simulate the transport process of methane, carbon nano pores, analyzes the methane slip in the supercritical state flow and transition flow; through the graphite porous medium model with different wall structure, simulate the distribution and diffusion of methane in the porous medium, the microscopic mechanism of methane and solid surface the wall cracks in the nano porous particle interactions revealed; finally, through the establishment of molecular type kerogen organic matter model was simulated under the different temperature of methane in cheese base The adsorption and diffusion behavior of matter in the cell, resulting in a calculation of the heat of adsorption and diffusion activation energy. The main contents and conclusions as follows: (1) by molecular dynamics model in two different diameter of the graphite in the pores of methane that changes the direction of hole graphite will lead to effects of methane and pore wall surface the potential change of flow and influence the nature of methane; in transitional flow, the radial distribution of the axial velocity fluctuation of methane is intense, graphite six lattice plane is a very smooth surface, flow rate of methane in the pore wall in fast, and the boundary slip length; the average velocity increases with the increase of methane the micro pore scale, radial distribution and axial velocity of the slow wave perpendicular to the graphitic lattice. The pore, the average speed is lower than that of methane methane flow velocity parallel to the graphite lattice plane in the pores. The slide Advection in the direction perpendicular to the porous graphitic lattice plane, the velocity distribution and the tangential momentum accommodation coefficient zeta =0.6 Maxwell slip in good agreement with the theoretical value; when the wall atoms of graphene coiled nanotubes, Maxwell theory slip velocity distribution and zeta values of =0.1 in good agreement; and in the direction parallel to the graphitic pore the pore lattice, the velocity of methane on both sides near the wall is 0, so that the overall radial velocity distribution deviates from the theoretical value. The Maxwell slip in the same direction in the pore, with the degree of slip driving force increases with the increase of mass flow increased gradually. With the increase of driving force, the slip length increased gradually decreased flow, the friction coefficient decreases with the increase of driving force and the Reynolds number, reduces with the increase in driving force decreased. (2) through the construction of the aperture is 1 ~ 5nm of graphite porous skeleton, a simulation At the temperature of alkane distribution and diffusion properties of 280K and 320K. The study found that methane mainly adsorbed on the surface of the graphite lattice wall six, the area is larger, the adsorption number in the wall of the pore to stay longer; the adsorption layer is formed on the temperature and methane molecules the potential interaction under the influence of temperature on methane diffusion and solid structure of atomic arrangement has little relationship. (3) through the construction of class I and class II kerogen organic molecular model, simulation of isothermal adsorption and diffusion process of methane at different temperatures. The study found that the number of kerogen on methane adsorption to decrease with increasing temperature, and the temperature is increased by 20K, reducing the methane adsorption capacity of about 25%, adsorption accords with Langmuir isothermal adsorption characteristics of.Barneet class II kerogen adsorption of methane is about 3.37 times that of Huadian type I kerogen methane more easily. Desorption from the first class kerogen. When the temperature is increased by 40K, the diffusion coefficient is increased by 2.3 times. The diffusion coefficient of methane in type I cheese kerogen is larger than that of type II, and its diffusion speed is faster in the first class kerogen.

【学位授予单位】：西南石油大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TQ127.11;TB383.1

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