基于分子动力学模拟的CH4溶解对原油分子间作用的影响机制研究

doi:10.11949/0438-1157.20200427

摘要/Abstract

摘要：

为了研究甲烷（CH₄）溶解对原油分子间作用的影响机制，利用分子模拟方法，分别构建正庚烷与正庚烷、蜡分子、胶质、沥青质的最低能量构型，分析CH₄氛围对原油分子间相互作用的影响。以CH₄/原油分子体系模型为研究对象进行分子动力学模拟，考察CH₄溶解对原油分子体系黏度的影响，根据原油分子间相互作用、径向分布函数、体积应变、自扩散系数以及体系内聚能密度变化规律揭示CH₄溶解对原油分子间作用的影响机制，发现CH₄的溶解增大了原油分子间的间距，削弱了体系内原油分子间的范德华作用；体积膨胀为原油分子的热运动提供了更多的空间，原油分子热运动加剧，使得油品的流动能力增强；不同原油分子体系的黏度、径向分布函数、体积应变、自扩散系数以及内聚能密度的变化趋势类似，原油分子体系中蜡、胶质、沥青质的存在并未改变CH₄对原油分子间作用的影响机制。

关键词: 甲烷, 原油, 分子模拟, 溶解, 分子间作用, 影响机制

Abstract:

To study the influence mechanism of methane dissolution on the intermolecular interaction between crude oil molecules. The lowest energy configurations of n-heptane and n-heptane, wax, colloid, asphaltene were constructed by molecular dynamics method, and the effect of CH₄ atmosphere on the interaction between crude oil molecules was analyzed. Based on the molecular dynamics simulation of CH₄ / crude oil molecular system model, the effect of CH₄ dissolution on the viscosity of crude oil molecular system was investigated. Five parameters, including intermolecular interaction, radial distribution function, volume strain, self diffusion coefficient, and cohesive energy density, were used to reveal the influence mechanism of intermolecular interaction of crude oil by CH₄ dissolution. The results showed that the dissolved CH₄ in crude oil molecular system increased the distance between the crude oil molecules, weakened the van der Waals effects between crude oil molecules. Under this condition, volume expansion provided more space for the thermal movement of crude oil molecules, intensified the thermal movement of crude oil molecules and then enhanced the flow capacity of crude oil. Moreover, there were similar rules of the effect of dissolved CH₄ on viscosity, radial distribution function, volume strain, self-diffusion coefficient and cohesion energy density in different crude oil molecular system. However, the existence of wax, colloid and asphaltene in crude oil molecular system does not change the influence mechanism of CH₄ on the intermolecular interaction of crude oil.

Key words: methane, crude oil, molecular simulation, dissolution, intermolecular interaction, influence mechanism

中图分类号:

李秉繁, 刘刚, 陈雷. 基于分子动力学模拟的CH₄溶解对原油分子间作用的影响机制研究[J]. 化工学报, 2021, 72(3): 1253-1263.

LI Bingfan, LIU Gang, CHEN Lei. Study on the influence mechanism of CH₄ dissolution on the intermolecular interaction between crude oil molecules based on molecular dynamics simulation[J]. CIESC Journal, 2021, 72(3): 1253-1263.

图/表 18

图1 原油组分分子模型

Fig.1 Molecular model of crude oil components

表1 常压条件下油样基本物性

Table1 Basic physical properties of oil samples under atmospheric pressure

油样	20℃密度/(kg/m³)	含蜡量/%	胶质含量/%	沥青质含量/%
胜利原油	878.2	24.5	18.03	0.12
南阳原油	875	38.2	—	15

表2 周期性体系中原油分子数量

Table 2 The number of molecular in periodic systems

体系	组分	摩尔比
1	正庚烷/蜡分子/胶质（胜利原油简化模型体系）	100∶13∶4
2	正庚烷/蜡分子/沥青质（南阳原油简化模型体系）	100∶25∶4

图2 原油分子体系(灰色代表正庚烷、红色代表蜡分子、蓝色代表胶质分子、绿色代表沥青质分子、黑色为氢原子)

Fig.2 System of crude oil molecular

图3 原油分子体系密度随模拟时间变化关系

Fig.3 The relationship between the density of crude oil molecular system and simulation time

表3 模拟密度与试验参考密度对比

Table 3 Comparison between simulated density and test reference density

体系	模拟时间/ps	分段密度值/(g/cm³)	标准差	平均值/(g/cm³)	试验参考值/(g/cm³)	绝对误差/%
胜利原油简化模型体系	40~60	0.86127	0.000685582	0.86150	0.878	1.879
	60~80	0.86243
	80~100	0.86080
南阳原油简化模型体系	40~60	0.84326	0.000943759	0.84199	0.875	3.773
	60~80	0.84171
	80~100	0.84101

图4 16 MPa条件下CH4在原油分子体系中溶解的构型图

Fig.4 Configuration diagram of CH4 dissolution in crude oil molecular system at 16 MPa

图5 不同压力条件下CH4在原油分子体系中溶解过程的构型图

Fig.5 Configuration diagram of CH4 dissolution in crude oil molecular system at different pressure

图6 不同压力条件下CH4分子在原油分子体系中溶解量曲线

Fig.6 Solubility curve of CH4 in crude oil under different pressure

图7 原油分子体系零剪切黏度与溶气压力关系曲线

Fig.7 Relationship between zero shear viscosity and dissolved gas pressure of crude oil molecular system

图8 20℃条件下不同分子间的最低能量构型和CH4氛围下不同分子间的最低能量构型

Fig.8 The lowest energy configuration of different molecules and that of different molecules in CH4 atmosphere at 20℃

表4 正庚烷与正庚烷、蜡分子、胶质、沥青质分子间间距及相互作用能

Table 4 Intermolecular spacing and interaction energy of n-heptane with n-heptane, wax, colloid and asphaltene

分子对	分子间间距/?			分子间相互作用能/(kJ/mol)
分子对	最低能量构型	CH₄氛围下最低能量构型	相对改变量/%	最低能量构型	CH₄氛围下最低能量构型	相对改变量/%
正庚烷/正庚烷	4.162	4.692	12.734	5.567	4.202	-24.519
正庚烷/蜡分子	9.319	10.167	9.100	1.878	1.580	-15.868
正庚烷/胶质	6.022	6.608	9.731	3.372	2.882	-14.531
正庚烷/沥青质	10.604	11.239	5.988	1.618	1.446	-10.630

图9 C-C原子间径向分布函数

Fig.9 Radial distribution function between C-C atoms

图10 不同原油分子体系的体积应变

Fig.10 Volume strain of crude oil molecular systems

图11 20℃条件下不同溶气压力条件下原油分子的均方位移

Fig.11 Mean square displacement of crude oil molecules under different dissolved gas pressures at 20℃

表5 不同溶气压力条件下原油分子的扩散系数

Table5 Diffusion coefficient of crude oil molecules under different dissolved gas pressures

溶气压力/MPa	扩散系数×10^-5/(cm²/s)
溶气压力/MPa	胜利原油简化模型体系	南阳原油简化模型体系
常压	3.315	3.105
1	3.496	3.268
4	3.795	3.564
8	3.985	3.827
16	3.784	3.697

表6 不同溶气压力条件下原油分子体系的内聚能密度及分量

Table 6 Cohesive energy density and component of crude oil molecular system under different dissolved gas pressures

溶气压力/MPa	胜利原油简化模型体系			南阳原油简化模型体系
溶气压力/MPa	CED×10^-8/(J/m³)	E_vdW×10^-8/(J/m³)	E_{Electrostatic}×10^-8/(J/m³)	CED×10^-8/(J/m³)	E_vdW×10^-8/(J/m³)	E_{Electrostatic}×10^-8/(J/m³)
常压	2.57	2.559	0.011	3.263	3.251	0.012
1	2.5	2.492	0.008	3.226	3.220	0.006
4	2.45	2.446	0.004	3.051	3.041	0.010
8	2.401	2.394	0.006	2.959	2.951	0.008
16	2.43	2.425	0.005	2.988	2.976	0.012

表7 原油模型体系参数

Table 7 Crude oil system parameters

溶气压力/MPa	胜利原油简化模型体系				南阳原油简化模型体系
溶气压力/MPa	零剪切黏度/(mPa·s)	体积应变/%	自扩散系数×10^-5/(cm²/s)	内聚能密度×10^-8/(J/m³)	零剪切黏度/(mPa·s)	体积应变/%	自扩散系数×10^-5/(cm²/s)	内聚能密度×10^-8/(J/m³)
常压	17.813	0	3.315	2.570	22.582	0	3.105	3.263
1	18.354	-1.791	3.151	2.698	23.165	-1.658	2.918	3.359
4	19.457	-3.269	2.996	2.818	23.996	-3.054	2.7415	3.409
8	19.790	-3.602	2.986	2.890	24.430	-3.401	2.723	3.462

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基于分子动力学模拟的CH₄溶解对原油分子间作用的影响机制研究

Study on the influence mechanism of CH₄ dissolution on the intermolecular interaction between crude oil molecules based on molecular dynamics simulation

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