Molecular dynamics simulations of CH4 diffusion in kaolinite: influence of water content
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Understanding the interaction of CH4 with kaolinite is significant for researchers in the fields of coalbed CH4 and shale gas. The diffusion behaviors of CH4 in kaolinite with water contents ranging from 0 to 5 wt% have been analyzed by molecular dynamics simulations. The results of the simulations indicate that CH4 molecules can jump between adjacent holes in the kaolinite matrix. CH4 diffusion coefficient was very low (3.28 × 10−9 m2/s) and increased linearly with the increasing of water content. As the water content decreased, the value of radial distribution function first peak between CH4 and oxygen was larger, meaning that with lower water content, the interaction energy between CH4 and oxygen in kaolinite is stronger. The interaction between CH4 and water is linearly positively correlated with water content, in contrast, the interaction energy between kaolinite and water as well as between kaolinite and CH4 decreased linearly with increasing water content. On the other hand, the diffusion of CH4 molecules adsorbed on the surfaces also can be accelerated by the fast diffusion of water molecules in the middle micropore of the kaolinite.
KeywordsMolecular dynamics Kaolinite Water content Diffusion Interaction energy
As the important cements and inorganic components of shale and coal seams, clay minerals interacting with CH4 have been one of the research hotspots in the energy field (Zhang et al. 2014; Zhao et al. 2016). Kaolinite is the most common clay mineral with large specific surface area and complex pore structure (from micropore to mesopore), which makes kaolinite have strong adsorption capacity (Murray 1999) and. The composition of kaolinite is Si4Al4O10(OH)8 and it consists of 1:1 dioctahedral layers which is composed of a sheet of corner-sharing SiO4 tetrahedra and a sheet of edge-sharing AlO6 octahedra linked by common oxygen atoms parallel to the (001) sheet (Warne et al. 2000). Hence, it is necessary and important to be able to understand the interaction mechanism between CH4 and kaolinite for the researchers in the fields of shale gas and coal bed methane.
Shale and coal seams are generally rich in water, making the surface of clay minerals easily occupied by water molecules (Zhang 2005; Jenkins and Charles Boyer 2008). Some molecular simulations have been implemented to study the effect of water content on CH4 adsorption on kaolinite in the past few years. Zhang et al. utilized the molecule simulations to research the effect of water content on CH4 adsorption on kaolinite. It is illustrated that the water has a side effect on the CH4 adsorption capacity and the adsorption rate of kaolinite. The oxygen atom in kaolinite is the preferential adsorption site for water molecules and CH4 molecules, while the hydrogen atom is only the preferential adsorption site for water molecules. Xiong et al. utilized GCMC simulations to research the influence of water on CH4 sorption in kaolinite. The results reflected the CH4 and water molecules competed in the kaolinite pores, and the water molecules preferentially occupied the low-energy adsorption sites, which reduced the adsorption space and adsorption sites of the CH4 molecules. However, these research outcome focusing on the effect of water content on adsorption behavior of CH4 in kaolinite paid little attention to the transport aspect of the effect of water content, for example, the influence of water content on diffusion of CH4 in kaolinite.
Diffusion is a very important phase in unconventional natural gas extraction, reflecting the speed of CH4 migration from micropores to fractures (Hu et al. 2017). This process is much slower than permeation flow, occurring in natural and artificial cracks in coal seams and shale, and is regarded as an intermediate process for CH4 production. So the molecular dynamics (MD) method was utilized to study the CH4 diffusion in kaolinite with pre-absorbed water contents of 0–5 wt% simulated at 293.15 K and 5 MPa in our paper. However, this paper mainly focuses on the water content on methane diffusion in kaolinite. Therefore, there are still many other basic parameters including pore size, pressure, temperature, etc., which also can influences the methane diffusion in kaolinite and need further research. We hope that this study can quantitatively analyze the diffusion properties of kaolinite and lay the foundation for further research on the storage and exploration of CH4 and shale gas reservoirs.
2 Molecular dynamics simulation details
2.2 Implementation of simulation
The Dreiding force field (Mayo et al. 1990; Fafard et al. 2017; Zhou et al. 2017) was selected in all simulations and can be utilized to study the physical and diffusion properties of kaolinite structures which has been confirmed in our previous works (Zhang et al. 2018a, b). The electrostatic and van der Waals interactions between CH4 and kaolinite were simulated by Ewald method with a cut-off value of 0.8 nm (Rutkai and Kristóf 2008). CH4 diffusions were computed as follows: the initial state of all models used here are the final output of Monte-Carlo simulations to obtain new kaolinite–H2O–CH4 systems. Then, these systems were run in 1 ns NPT and 1 ns NVT ensemble for minimized to relaxation. At last, 5 ns NVT MD simulations were implemented to obtain the displacement, CH4 self-diffusivity, interaction energy, and radial distribution functions (RDF). Water contents were varied to investigate its effects on CH4 diffusion. The Andersen barostat (Fernández-Pendás et al. 2014) and Berendsen thermostat (Evans and Holian 1985) were utilized to control the pressures and temperatures. And we used the Accelrys Material Studio software (X. Accelrys & Software Inc) to implement all the MD simulations.
3 Results and discussion
3.1 Diffusion trajectories
3.2 Self-diffusion coefficient
3.3 Transport diffusion coefficient
3.4 Radial distribution function
To reflect the influence of water on strength of the intermolecular interaction force between the CH4 molecules and kaolinite, the radial distribution function, g(r), was implemented that can measure the law of the change of atomic density and the distance of a specific atom (Kong and Wang 2016).
3.5 Interaction energy
3.6 Weight density distributions
The first peak values and second peak values of weight density distributions between CH4 and kaolinite with different water contents
Water content (wt%)
First peak values (kg/m3)
Second peak values (kg/m3)
Due to the pore space limitation and the strong interaction between the two walls, the kaolinite surface had a higher adsorption CH4 concentration in dry condition which determines the CH4 diffusion coefficient was very small. With the increase of water content, as the interaction of the two walls against CH4 is weakened, the number of CH4 molecules in the micro-pores was gradually increased at higher water contents (Fig. 9). In addition, because the affinity of the kaolinite wall to the CH4 molecule was significantly reduced, the CH4 molecules in the middle of the kaolinite micro-pores had faster diffusion. In addition, the rapid diffusion of CH4 molecules and water molecules in the middle of the micro-pores also accelerated the diffusion of CH4 molecules adsorbed on the surface, which leads to an overall increase in the CH4 diffusion coefficient.
Molecular dynamics (MD) simulations were used to study the CH4 diffusion in kaolinite with water contents ranging from 0 to 5 wt%. The results illustrated that CH4 molecules can jump within adjacent holes in the kaolinite matrix. The CH4 diffusion coefficient was about 3.28 × 10−9 m2/s and water had a positive effect on it. The larger the water content, the larger the value of RDF first peak between CH4 and oxygen, indicating that the interaction energy between CH4 and oxygen in kaolinite is stronger. The interaction energy between CH4 and water increased linearly with water content, in contrast, the interaction energy between kaolinite and water as well as between kaolinite and CH4 decreased linearly with the increasing water content. The rapid diffusion of CH4 molecules and water molecules in the middle of the micro-pores also accelerated the diffusion of CH4 molecules adsorbed on the surface, which leads to an overall increase in the CH4 diffusion coefficient. We hope that our research demonstrates a strategy to facilitate the further exploration of coalbed methane and shale gas.
This research was financially supported by the National Natural Science Foundation of China (Nos. U1810102, 51974194). The use of the Materials Studio software package, which is supported by the Key Laboratory of Coal Science and Technology of the Ministry of Education and Shanxi Province, is gratefully acknowledged.
- X. Accelrys (2010) A.M.S.R.N., Release 6.0, Accelrys & Software Inc, S.D.Google Scholar
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