Molecular dynamics simulation on methane hydrate formation in clay nanopores of edge surfaces

Abstract

Natural gas hydrates are predominantly buried in clay sediments in natural environments, where are some edge surfaces of clay particles directly contacting the hydrates. However, the exact nature of the interaction between these surfaces and the hydrates, as well as their influence on hydrate formation, remains elusive. Herein, microsecond molecular dynamics simulations have been performed to investigate CH4 hydrates formation in nanopores consisting of clay edge surfaces, to reveal the effects of clay edge surfaces and layer charges. The simulation results show that the clay edge surfaces affect CH4 hydrate formation by changing the distribution of water and CH4 molecules via surface adsorption, mainly ascribed to the different polarities of the groups on the edge surfaces of different clays. The greater the electronegativity of the clay, the stronger the inhibition of CH4 hydrate formation, thus, the electroneutral clays are more beneficial for CH4 hydrate formation than the electronegative clays. Moreover, in the early stage of the simulation, compared with the electronegative clays, the electroneutral clays are more favorable for the diffusion of CH4 molecules from the nanopores into the bulk solution and then promote CH4 hydrate formation. On the other hand, the ions in the solution gradually aggregate together and their distribution becomes denser and more ordered. The edge surfaces of electroneutral clay are more accessible to hydrate solids than electronegative clay. These molecular insights into the formation behavior of CH4 hydrates in clay nanopores consisting of edge surfaces help to understand the formation process of natural gas hydrates in marine sediments.