Effect of Surface Coverage of Water Molecules on Methane Adsorption on Muscovite and Pyrophyllite: Molecular Dynamics Study

Abstract

To gain a better understanding of the effect of surface coverage of water (H2O) on methane (CH4) adsorption on shale, we performed molecular dynamics simulations. The interactions of H2O with minerals play a key role in the surface coverage of H2O; hence, two representative minerals were investigated: muscovite, which has a high layer charge, and pyrophyllite, which has a net zero layer charge. On the muscovite surface, the amount of CH4 adsorption decreases due to the formation of a structured adsorption layer of H2O, which increases the free energy barrier of CH4 adsorption. On the pyrophyllite surface, the amount of CH4 adsorption decreases due to the spread of a H2O droplet on the surface, which reduces accessible adsorption sites for the CH4 molecules. These insights from an atomic-scale viewpoint are expected to improve our interpretation of the results from the core analysis and modeling of the adsorption amount on shale reservoirs.

Correlation between Contact Angle and Water Film Energetics in Carbon Dioxide–Water–Clay Mineral Interfacial Systems: A Molecular Dynamics Study

“…This study clarified the correlation between the contact angle and film energetics in CO 2 –water–clay mineral systems through MD simulations. Our MD simulations used the force fields that were previously validated for studying the wettability of mineral surfaces − ,− as well as CO 2 –water interfacial phenomena. , The present MD simulations show that with an increasing negative layer charge, the contact angle decreases, and the amount of strongly adsorbed water and film thickness/stability increases. According to the free energy variation determined for a representative CO 2 molecule permeating the water film, the amount of work needed for this molecule to approach the surface increases with increasing negative layer charge. On the basis of these findings, we proposed a novel approach to complement direct contact angle calculations by means of computing the permeation free energy.…”

“…The system was considered to be at equilibrium once the potential energy, bulk density, and film thickness had converged [see Text S1 and snapshots in the equilibrium process (Figure S4), the time variations of the CO 2 bulk density (Figure S5), water density in the film region (Figure S6), and interfacial profile (Figure S7) during the production run]. The overall simulation times for the equilibration and production runs were comparable to or longer than those reported in previous studies on similar systems. − The reason for the long production run is that the spreading of the water film converges more slowly than the system’s temperature, the total energy, and the bulk density, and in some cases, the fluctuations seemed not negligible (see Figure S8, which shows fluctuations in the amount of water adsorption even during equilibrium). While the water film thickness does converge within approximately 150 ns, we took extra care to confirm that the amount of adsorbed water reaches a steady state after fluctuating around a constant value.…”

“…Moreover, the focus on specific factors is costly, particularly on a microscopic scale (i.e., when the effects of cation exchange or the presence of nanoparticles are considered). In the light of complementarity with experimental measurements, MD simulations have been used and shown to deepen our understanding of mineral surface wettability. − ,− Additionally, given the strong correlation between θ and Δ F Z revealed by our MD simulations, the computation of the permeation free energy is considered a promising approach to complement direct θ measurements. As θ calculated by MD simulations is sensitive to the definition of the contact plane and the criteria used for interfacial profile extraction, free energy calculations can provide a more reliable means of assessing changes in wettability and their underlying factors.…”

“…This is in good agreement with previous contact angle measurements and MD studies on the ability of water to wet talc and pyrophyllite surfaces (which also feature net-zero layer charges). 28,35,70,71 At q = −0.17 e, the surface affinities of CO 2 and H 2 O were comparable, and the interface of these two phases had little curvature. At q = −0.33 e, θ was significantly smaller than 90°, and a water film sandwiched between the CO 2 phase and the clay surface was formed.…”

“…The logarithmic mean force dynamics (LogMFD) method − was used for free energy calculations and relied on the PLUMED plug-in module in conjunction with GROMACS. The LogMFD parameters used to compute the potential of mean force (PMF) were the same as those described in our previous study. , Two types of free energy variations, F ( Z ) and F ( X ), were determined to investigate the film energetics.…”