Non-equilibrium molecular dynamics study on the decomposition of natural gas hydrates under a temperature gradient: implications for hydrate exploitation via thermal stimulation

Natural gas hydrates (NGH) are a promising unconventional energy with the potential to alleviate global energy shortage. Thermal stimulation is a widely employed method for NGH exploitation, but often leads to non-uniform heating and heterogeneous hydrate decomposition. The microscopic mechanism of NGH decomposition under temperature gradients during thermal stimulation remains elusive. In this study, non-equilibrium molecular dynamics simulations are conducted to investigate the decomposition process of NGH in seawater at 15 MPa between a cooling source with 288.15 K and a heating source with 333.15 K, 343.15 K, 353.15 K and 363.15 K, i.e., under a series of temperature gradients of 45 K, 55 K, 65 K, and 75 K. The results indicate that increasing temperature gradients accelerates NGH decomposition, CH4 release and nanobubble formation. Hydrate solids on the side of heating source always decompose sustainedly, while those on the side of cooling source first decompose and then re-form under low temperature gradients of 45 K, 55 K, and 65 K. It is unraveled that adsorption of CH4 nanobubbles on the heating source slows down the decomposition kinetics of NGH by impeding heat transfer, due to the lower thermal conductivity of CH4 gas than seawater. Breakage of hydrogen-bond networks and entropy increase in NGH dominate the decomposition process. These molecular insights help to deepen our understanding of the decomposition mechanism of NGH during thermal stimulation, and could give guidance for optimizing the thermal injection strategy for NGH exploitation.