Self-preservation and Stability of Methane Hydrates in the Presence of NaCl

Gas hydrate, a solid transformed from an ensemble of water and gaseous molecules under suitable thermodynamic conditions, is present in marine and permafrost strata. The ability of methane hydrates to exist outside of its standard stability zone is vital in many aspects, such as its utility in gas storage and transportation, hydrate-related climate changes and gas reservoirs on the planet. A systematic study on the stability of methane hydrates divulges that the gas uptake decreased by about 10% by increasing the NaCl content to 5.0 wt%. The hydrate formation kinetic is relatively slower in a system with higher NaCl. The self-preservation temperature window for hydrate systems with NaCl 1.5, 3.0 and 5.0 wt% dramatically shifted to a lower temperature (252 K), while it remained around 270 K for NaCl 0.0 and 0.5 wt%. Based on powder x-ray diffraction and micro-Raman spectroscopic studies, the presence of hydrohalite (NaCl×2H2O) phase was identified along with the usual hydrate and ice phases. The eutectic melting of this mixture is responsible for shifting the hydrate stability to 252 K. A systematic lattice expansion of cubic phase infers the interaction between NaCl and water molecules of hydrate cages.

Raman and powder-XRD depicting specific signatures of hydrate and hydrohalite phases. Top (A), and bottom (C) segments correspond to hydrate systems with 0.0, and 15.0 wt% NaCl and the middle (B) trace is with 3.0 wt% NaCl. Computed diffraction peak positions for cubic hydrate (Pm3n), hexagonal ice (P63/mmc) and monoclinic hydrohalite (P21/c) are represented by red, blue and green coloured bars respectively.

 

The structural stability of methane hydrates in CH4 – H2O – NaCl system, inferred from the gas release (in a pressure vessel) and by the Raman spectroscopic measurements in the self-preservation window.

 

P.S.R. Prasad and B. Sai Kiran, Scientific Reports, (2019) 9:5860 | https://doi.org/10.1038/s41598-019-42336-1