Study on methane adsorption/desorption and flow law in the nanopores of coal based on LAMMPS
-
摘要: 基于LAMMPS(Large-scale Atomic/Molecular Massively Parallel Simulator)分子动力学方法，研究煤纳米孔隙中驱动力、孔径、温度和压力对甲烷吸附/解吸和流动的影响规律。结果表明，随着驱动力增加，甲烷分子黏度逐渐减小，流动性增强，流动速度增大，滑移长度绝对值逐渐减小，流动趋近于无滑移状态。甲烷的吸附密度与驱动力无关，主要受气−固作用影响。甲烷在流动过程中会吸附于煤孔隙壁面，当煤孔径较小时，甲烷几乎全部吸附，无游离态甲烷。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，甲烷由单峰分布转为2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。从微观角度通过建立更加真实的模型阐明了煤纳米孔隙中甲烷吸附/解吸和流动机制，研究结果可为工程应用中促进甲烷解吸、提升煤层气开采效率提供理论基础。Abstract: In this work, the influence rules of driving force, pore size, temperature and pressure on methane adsorption/desorption and flow in coal nanopores were investigated by the molecular dynamic method based on Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS). The investigation results show that the viscosity of methane gradually decreases with the increasing driving force, while its fluidity and flow velocity increase. Meanwhile, the absolute slip length decreases and the flow tends to the non-slip state. Generally, the adsorption density of methane is independent of the driving force, but greatly affected by the gas-solid action. Methane can be adsorbed on the pore wall of coal during its flowing. For a small pore diameter of coal, the methane is almost adsorbed without free status. With the increase of pore size, the influence of the wall van der Waals force on the free methane molecules is weakened, and thus the flow velocity of methane increases, leading to a large amount of free methane present in the pores. Consequently, the methane changes from the unimodal distribution to the symmetrical bi-modal distribution. For methane has lower viscosity and good fluidity in the large pores, the Hagen-Poiseuille equation is more suitable for the methane flow therein. As the temperature increases, the thermal motion of methane molecule is enhanced, the density of the adsorption layer decreases, and the methane flow rate increases. Thereby, the adsorbed methane on the pore wall of coal is desorbed into free methane, increasing the flow rate of methane. With the increase of pressure, the amount of methane in the pore increases, resulting in the strong collision among methane molecules, so that the flow resistance of methane increases to slow down its flowing. In this work, the methane adsorption/desorption and flow mechanism in coal nanopores was clarified from a microscopic perspective by establishing some more realistic models. Hence, the research results could provide a theoretical basis for promoting methane desorption and improving the efficiency of coalbed methane extraction in engineering applications.
-
Key words:
- coal /
- nanopores /
- methane flow /
- LAMMPS /
- molecular dynamics
-
图 4 不同驱动力下孔隙中甲烷的质量密度分布
Fig. 4 Mass density distribution of methane in pore under different driving forces
图 5 不同驱动力下孔隙中甲烷的速度分布
Fig. 5 Velocity distribution of methane in pore under different driving forces
图 6 不同驱动力下甲烷的滑移长度和黏度
Fig. 6 Slip length and viscosity of methane under different driving forces
图 8 3.8~7.0 nm孔径中甲烷质量密度分布
Fig. 8 Mass density distribution of methane in the pores size from 3.8 nm to 7.0 nm
图 9 不同孔径中甲烷流动速度分布
Fig. 9 Flow velocity distribution of methane under different pore sizes
图 10 不同孔径下甲烷的滑移长度和黏度
Fig. 10 Slip length and flow viscosity of methane under different pore sizes
图 11 不同温度下甲烷的质量密度和运移速度分布
Fig. 11 Mass density and flow velocity distribution of methane at different temperatures
图 14 不同压力下甲烷的质量密度与运移速度分布
Fig. 14 Mass density and flow velocity distribution of methane under different pressures
图 15 不同压力下甲烷数密度分布
Fig. 15 Number density distribution of methane under different pressures
表 1 不同压力下孔隙中甲烷数量和施加的驱动力
Table 1 Amount of methane in the pores under different pressures and the applied driving force
压力/MPa 甲烷数量 驱动力/[kcal·(mol·nm)−1] 7 100 0.012 15 200 0.006 25 300 0.004 40 400 0.003 
下载: 导出CSV
-
[1] 万玉金,曹雯. 煤层气单井产量影响因素分析[J]. 天然气工业,2005,25(1):124−126.WAN Yujin,CAO Wen. Analysis of production−affecting factors of single well for coalbed gas[J]. Natural Gas Industry,2005,25(1):124−126. [2] 吴雅琴,邵国良,徐耀辉,等. 煤层气从降压到产气过程的运移机理研究[J]. 长江大学学报(自然科学版),2016,13(20):9−13.WU Yaqin,SHAO Guoliang,XU Yaohui,et al. Research on migration mechanism of coalbed methane from its depressuring to its recovery[J]. Journal of Yangtze University (Natural Science Edition),2016,13(20):9−13. [3] ZHOU Sandong,LIU Dameng,CAI Yidong,et al. Comparative analysis of nanopore structure and its effect on methane adsorption capacity of Southern Junggar Coalfield coals by gas adsorption and FIB–SEM tomography[J]. Microporous & Mesoporous Materials,2018,272:117−128. [4] 蔺亚兵,马东民,刘钰辉,等. 温度对煤吸附甲烷的影响实验[J]. 煤田地质与勘探,2012,40(6):24−28.LIN Yabing,MA Dongmin,LIU Yuhui,et al. Experiment of the influence of temperature on coalbed methane adsorption[J]. Coal Geology & Exploration,2012,40(6):24−28. [5] 张庆玲,崔永君,曹利戈. 压力对不同变质程度煤的吸附性能影响分析[J]. 天然气工业,2004,24(1):98−100.ZHANG Qingling,CUI Yongjun,CAO Lige. Influence of pressure on adsorption ability of coal with different deterioration level[J]. Natural Gas Industry,2004,24(1):98−100. [6] 张宝鑫,邓泽,傅雪海,等. 温度对中高阶烟煤甲烷吸附–常压/带压解吸过程中煤体变形影响实验[J]. 天然气地球科学,2020,31(12):1826−1836.ZHANG Baoxin,DENG Ze,FU Xuehai,et al. Characteristics of medium–high rank bituminous coal deformation during methane adsorption−desorption with atmospheric pressure/with successively decreasing outlet pressure at different temperatures[J]. Natural Gas Geoscience,2020,31(12):1826−1836. [7] WANG Dengke,LIU Shumin,WEI Jianping,et al. A research study of the intra–nanopore methane flow law[J]. International Journal of Hydrogen Energy,2017,42(29):18607−18613.. doi: 10.1016/j.ijhydene.2017.04.195 [8] 刘永茜,张书林,舒龙勇. 吸附–解吸状态下煤层气运移机制[J]. 煤田地质与勘探,2019,47(4):12−18.. doi: 10.3969/j.issn.1001-1986.2019.04.003LIU Yongqian,ZHANG Shulin,SHU Longyong. Coalbed methane migration mechanism under adsorption–desorption condition in coal[J]. Coal Geology & Exploration,2019,47(4):12−18.. doi: 10.3969/j.issn.1001-1986.2019.04.003 [9] 肖晓春,潘一山. 滑脱效应影响的低渗煤层气运移实验研究[J]. 岩土工程学报,2009,31(10):1554−1558.XIAO Xiaochun,PAN Yishan. Experimental study of gas transfusion with slippage effects in hypotonic coal reservoir[J]. Chinese Journal of Geotechnical Engineering,2009,31(10):1554−1558. [10] 裴柏林. 煤层气储层三维渗透率变化规律实验研究[J]. 煤田地质与勘探,2013,41(4):26−30.PEI Bolin. Experimental research on variation pattern of 3D permeability in coalbed methane reservoir[J]. Coal Geology & Exploration,2013,41(4):26−30. [11] 张凯飞,刘汉涛,雷广平,等. 赵庄3#煤中甲烷吸附特性的分子模拟[J]. 中国科技论文,2020,15(1):94−99.ZHANG Kaifei,LIU Hantao,LEI Guangping,et al. Molecular simulation of adsorption properties for methane in Zhaozhuang coal 3#[J]. China Sciencepaper,2020,15(1):94−99. [12] 王宝俊,章丽娜,凌丽霞,等. 煤分子结构对煤层气吸附与扩散行为的影响[J]. 化工学报,2016,67(6):2548−2557.WANG Baojun,ZHANG Lina,LING Lixia,et al. Effects of coal molecular structure on adsorption and diffusion behaviors of coalbed methane[J]. CIESC Journal,2016,67(6):2548−2557. [13] 曲国娜,马兆鑫,贾廷贵,等. 高变质煤对CH4和CO2气体吸附试验与分子模拟研究[J]. 安全与环境学报,2022,22(1):142−147.QU Guona,MA Zhaoxin,JIA Tinggui,et al. Experiment and molecular simulation study on the adsorption of CH4 and CO2 by coal with high–metamorphic[J]. Journal of Safety and Environment,2022,22(1):142−147. [14] 唐巨鹏,邱于曼,马圆. 煤中CH4扩散影响因素的分子动力学分析[J]. 煤炭科学技术,2021,49(2):85−92.TANG Jupeng,QIU Yuman,MA Yuan. Molecular dynamics analysis of influencing factors of CH4 diffusion in coal[J]. Coal Science and Technology,2021,49(2):85−92. [15] BHOI S,BANERJEE T,MOHANTY K. Molecular dynamic simulation of spontaneous combustion and pyrolysis of brown coal using ReaxFF[J]. Fuel,2014,136:326−333.. doi: 10.1016/j.fuel.2014.07.058 [16] DANG Yong,ZHAO Lianming,LU Xiaoqing,et al. Molecular simulation of CO2/CH4 adsorption in brown coal:Effect of oxygen–,nitrogen–,and sulfur–containing functional groups[J]. Applied Surface Science,2017,423:33−42.. doi: 10.1016/j.apsusc.2017.06.143 [17] MATHEWS J P,CHAFFEE A L. The molecular representations of coal:A review[J]. Fuel,2012,96:1−14.. doi: 10.1016/j.fuel.2011.11.025 [18] RIEDER M, CRELLING J C, SUSTAI O, et al. Arsenic in iron disulfides in a brown coal from the North Bohemian Basin, Czech Republic[J]. International Journal of Coal Geology, 2007, 71(2/3): 115–121. [19] MAYO S L,OLAFSON B D,GODDARD W A. DREIDING:A generic force field for molecular simulations[J]. The Journal of Physical Chemistry,1990,94(26):8897−8909.. doi: 10.1021/j100389a010 [20] JORGENSEN W L. Optimized intermolecular potential functions for liquid alcohols[J]. The Journal of Physical Chemistry,1986,90(7):1276−1284.. doi: 10.1021/j100398a015 [21] WANG Sen,JAVADPOUR F,FENG Qihong. Molecular dynamics simulations of oil transport through inorganic nanopores in shale[J]. Fuel,2016,171:74−86.. doi: 10.1016/j.fuel.2015.12.071 [22] RICHARD R,ANTHONY S,AZIZ G. Pressure–driven molecular dynamics simulations of water transport through a hydrophilic nanochannel[J]. Molecular Physics,2016,114(18):2655−2663.. doi: 10.1080/00268976.2016.1170219 [23] HANSEN J S,TODD B D,DAIVIS P J. Prediction of fluid velocity slip at solid surfaces[J]. Physical Review E:Statistical,Nonlinear & Soft Matter Physics,2011,84(1):016313. [24] TODD B D,EVANS D J,DAIVIS P J. Pressure tensor for inhomogeneous fluids[J]. Physical Review,1995,52(2):1627−1638. [25] WHITBY M,CAGNON L,THANOU M,et al. Enhanced fluid flow through nanoscale carbon pipes[J]. Nano Letters,2008,8(9):2632−2637.. doi: 10.1021/nl080705f [26] BOTAN A,ROTENBERG B,MARRY V. Hydrodynamics in clay nanopores[J]. The Journal of Physical Chemistry C,2011,115(32):16109−16115.. doi: 10.1021/jp204772c [27] THOMAS J A,MCGAUGHEY A J H. Reassessing fast water transport through carbon nanotubes[J]. Nano Letters,2008,8(9):2788−2793.. doi: 10.1021/nl8013617 [28] HEINBUCH U,FISCHER J. Liquid flow in pores:Slip,no–slip,or multilayer sticking[J]. Physical Review A,1989,40(2):1144−1146.. doi: 10.1103/PhysRevA.40.1144 [29] MAINAK M,NITIN C,RODNEY A,et al. Nanoscale hydrodynamics:Enhanced flow in carbon nanotubes[J]. Nature,2005,438(7064):44.. doi: 10.1038/438044a [30] RHODERICK G C,CARNEY J,GUENTHER F R. NIST gravimetrically prepared atmospheric level methane in dry air standards suite[J]. Analytical Chemistry,2012,84(8):3802−3810. [31] WANG Sen,FENG Qihong,ZHA Ming,et al. Molecular dynamics simulation of liquid alkane occurrence state in pores and slits of shale organic matter[J]. Petroleum Exploration and Development,2015,42(6):844−851.. doi: 10.1016/S1876-3804(15)30081-1 [32] 冯艳艳,储伟,孙文晶. 储层温度下甲烷的吸附特征[J]. 煤炭学报,2012,37(9):1488−1492.. doi: 10.13225/j.cnki.jccs.2012.09.018FENG Yanyan,CHU Wei,SUN Wenjing. Adsorption characteristics of methane on coal under reservoir temperatures[J]. Journal of China Coal Society,2012,37(9):1488−1492.. doi: 10.13225/j.cnki.jccs.2012.09.018 [33] 马向攀,王兆丰,任浩洋. 煤吸附甲烷能力对温度压力变化的响应特性[J]. 煤矿安全,2016,47(12):8−11.. doi: 10.13347/j.cnki.mkaq.2016.12.003MA Xiangpan,WANG Zhaofeng,REN Haoyang. Response characteristics of coal adsorbing CH4 capacity under variation of temperature and pressure[J]. Safety in Coal Mines,2016,47(12):8−11.. doi: 10.13347/j.cnki.mkaq.2016.12.003 -
下载:
点击查看大图
图(15) / 表(1)
计量
- 文章访问数: 97
- HTML全文浏览量: 17
- PDF下载量: 21
- 被引次数: 0
