Salinity-controlled guest selectivity in CH4 + CO2 hydrates: Implications for enhanced CO2 sequestration in marine sediments

Abstract

Guest replacement-where CO2 replaces CH4 in natural gas hydrates-offers promising potential for greenhouse gas sequestration and energy recovery. However, CO2 injection into CH4 hydrate-bearing sediments creates CH4 + CO2 mixtures, leading to secondary hydrate formation, which is strongly affected by the saline conditions typical of marine environments. This study investigated the guest enclathration behavior of CH4 + CO2 hydrates, focusing on the effects of salinity compared to pure water system. Thermodynamic phase equilibria were experimentally determined at NaCl concentrations of 3.5 and 5.0 wt%, and validated against a predictive thermodynamic model. Vapor-phase composition analysis confirmed the preferential encapsulation of CO2 in both systems; however, the presence of NaCl reduced the overall water-to-hydrate conversion due to thermodynamic inhibition, with conversion of similar to 80 % in pure water and 20-40 % in NaCl solutions. This reduction was further quantified in terms of driving force variations induced by salt enrichment. Hydrate-phase composition measurements revealed enhanced CO2 incorporation in saline systems (55 % in pure water vs. 67 % in NaCl (5.0 wt%) solution). Time-resolved tracking revealed a pronounced initial preference for CO2 incorporation, indicating kinetic selectivity of CO2 over CH4. Rietveld refinement of PXRD patterns, supported by C-13 NMR spectroscopy, provided cage-specific occupancy information, revealing a decline in CH4 occupancy and a corresponding increase in CO2 occupancy with higher salinity. These findings highlight the thermodynamic influence of NaCl on hydrate formation and guest distribution, providing valuable insights for optimizing CO2 injection strategies in natural gas hydrate reservoirs, particularly in marine settings.