New insight into methane hydrate formation at clay-geofluid interface in natural kaolinite-stacking sediments

Natural gas hydrate , existing in clayey sediments, provide alternative solutions for clean energy resource. The edge sites and defects (e.g., nano-hill and nano-valley) are ubiquitous within naturally occurring clay stacks (i.e. the layered structure of clay minerals), which are of paramount significance in the context of hydrate nucleation and formation, yet a comprehensive molecular-scale understanding remains elusive. In present work, kaolinite stacks with defects were carefully modelled and systematic molecular dynamics simulations were conveyed to comprehend the subtle process of methane hydrate nucleation and formation at clay-geofluid interface. Simulation results highlight a consistent preference for hydrate cages to nucleate in off-surface regions rather than directly at clay surfaces. Dynamic analyses revealed disparities in ring formation between hydrophilic and hydrophobic surfaces, with hydrophilic surfaces promoting organized water ring formation and stability and facilitating hydrate cage assembly, while hydrophobic surfaces displayed limited ring and cage formation due to less structured water molecules. Besides, the unique characteristics of the edge and defect surfaces in clay-geofluid interactions and hydrate nucleation promotion can be attributed to variations in surface hydroxyl group concentration compared to the gibbsite surface. Overall, this research provides a molecular-level understanding of the complex interactions between geofluids and clay stacks, emphasizing their significant impact on natural gas hydrate nucleation and growth.