Methane Diffusion through Nanopore-Throat Geometry: A Molecular Dynamics Simulation Study

Sun，Runxuan1,2,3; Xu，Ke3; Huang，Tianjia3,4; Zhang，Dongxiao3,5

2023-04-01

10.2118/212289-PA

SPE Journal   影响因子和分区

1086-055X

1930-0220

Molecular diffusion dominates over pressure-driven convection as the major mass transport mechanism in nanoporous media with <10-nm pores, which is typical pore size for shale gas recovery. To study fluid behavior at this scale, molecular dynamics (MD) simulation has been widely applied. Nevertheless, classic capillary tube or slit models are of uniform geometry that miss the converging-diverging pore-throat feature, while more realistic models lose simplicity and generality. In this work, we propose a novel geometric model that can reproduce the realistic converging-diverging structure in subsurface porous media without any additional complexity compared to classic slit or capillary models. In this pore-throat model, we are able to identify how nonuniform geometry affects the methane diffusion for both pure methane and for methane mixtures with water, carbon dioxide, and helium. For a pure methane system, we demonstrate the fundamental impact of throat width on diffusion coefficient when the throat width is narrower than 20 Å and identify a critical throat width that determines whether methane can self-diffuse though the throat. This critical throat size is regulated by the energy barrier at the throat rather than by molecular size. We then introduce a semianalytical model to predict self-diffusion coefficient as a function of pressure, temperature, and throat width. For mixtures, we observe the key impact of spatially nonuniform fluid distribution in determining diffusion. Water or carbon dioxide can locally concentrate at the throat, which reduces methane diffusivity, while helium prefers to stay in the pore body, which mildly enhances methane diffusivity. Specifically, although residual water reduces methane diffusion (26% reduction for 20% water molar fraction), it completely blocks the throat and thus prohibits pressure-driven methane convection. By comparison, the dominance of molecular diffusion over convection can be extended to larger pores in presence of residual water. It provides an explanation on shale gas production when connate water is expected to block the flow path.

SCI ; EI

英语

通讯

Sinopec Petroleum Exploration and Production Research Institute[33550000- 21- ZC0613- 0314]

Engineering

Engineering, Petroleum

WOS:000981633900006

SOC PETROLEUM ENG

20231914061778

Carbon dioxide ; Diffusion in liquids ; Diffusion in solids ; Geometry ; Helium ; Molecular dynamics ; Nanopores ; Pore size ; Porous materials

Nanotechnology:761 ; Physical Chemistry:801.4 ; Chemical Products Generally:804 ; Organic Compounds:804.1 ; Inorganic Compounds:804.2 ; Mathematics:921 ; Physical Properties of Gases, Liquids and Solids:931.2 ; Solid State Physics:933 ; Materials Science:951

ENGINEERING

2-s2.0-85156256245

Scopus

1.State Key Laboratory of Shale Oil and Gas Enrichment Mechanisms and Effective Development,
2.Sinopec Key Laboratory of Shale Oil/Gas Exploration and Production Technology,
3.Department of Energy and Resource Engineering,College of Engineering,Peking University,China
4.Department of Petroleum Engineering,Texas A&M University,United States
5.Shenzhen Key Laboratory of Natural Gas Hydrates,Southern University of Science and Technology,China

Sun，Runxuan,Xu，Ke,Huang，Tianjia,et al. Methane Diffusion through Nanopore-Throat Geometry: A Molecular Dynamics Simulation Study[J]. SPE Journal,2023,28(2):819-830.

Sun，Runxuan,Xu，Ke,Huang，Tianjia,&Zhang，Dongxiao.(2023).Methane Diffusion through Nanopore-Throat Geometry: A Molecular Dynamics Simulation Study.SPE Journal,28(2),819-830.

Sun，Runxuan,et al."Methane Diffusion through Nanopore-Throat Geometry: A Molecular Dynamics Simulation Study".SPE Journal 28.2(2023):819-830.