Temperature‐ and pressure‐dependent structural transformation of methane hydrates in salt environments

Abstract

Understanding the stability of volatile species and their compounds under various surface and subsurface conditions is of great importance in gaining insights into the formation and evolution of planetary and satellite bodies. We report the experimental results of the temperature‐ and pressure‐dependent structural transformation of methane hydrates in salt environments using in situ synchrotron X‐ray powder diffraction, solid‐state nuclear magnetic resonance, and Raman spectroscopy. We find that under pressurized and concentrated brine solutions methane hydrate forms a mixture of type I clathrate hydrate, ice, and hydrated salts. Under a low‐pressure condition, however, the methane hydrates are decomposed through a rapid sublimation of water molecules from the surface of hydrate crystals, while NaCl · 2H2O undergoes a phase transition into a crystal growth of NaCl via the migration of salt ions. In ambient pressure conditions, the methane hydrate is fully decomposed in brine solutions at temperatures above 252 K, the eutectic point of NaCl · 2H2O.

Proposing Crystallographic Theory to Deparameterize Hydrate Phase Description: Physical Insights and Validation

“…28,29 The experiment rather attests to fixed hydrate cage occupancies when the hydrate phase is in equilibrium with the electrolytic medium. 30 These observations coherently explain the requirement of higher equilibrium pressures in the presence of salts that lead to preservation of the hydrate cage occupancies despite featuring a distorted lattice.…”

“…Despite having evidence from molecular dynamic simulations and experimental observations, the influence of salt on the hydrate lattice stability is rarely accounted for in the HLVE descriptions. Particularly, the molecular simulations report altered lattice parameters that signify a distorted hydrate lattice. , The experiment rather attests to fixed hydrate cage occupancies when the hydrate phase is in equilibrium with the electrolytic medium . These observations coherently explain the requirement of higher equilibrium pressures in the presence of salts that lead to preservation of the hydrate cage occupancies despite featuring a distorted lattice.…”

Formulating Noncovalent Interactions for Gas Hydrates with Electrolytes: A New Approach of Stability Analysis

“…This also agrees with the reported reduction in the gO− O bond lengths, indicating a shrunken hydrate phase. 53,62,63 Overall, the extent of this shrinkage is observed to vary according to the hydrate composition.…”

“…We attribute the limitations of the current approaches to the treatment of the hydrate phase, which relies on the average guest−host interactions alone. With numerous studies suggesting altered lattice dimensions in the presence of electrolytes, 53 the interaction energies between the molecules do not remain consistent. This inconsistency leads to deviation from the actual energies of the system, which are compensated for with the updates to the description of the vapor and liquid models.…”

Characterizing the variability of natural gas hydrate composition from a selected site of the Western Black Sea, off Romania

“…Furthermore, electrolyte solutions (3.5 wt% NaCl in a sediment-free solution) are known to have a negligible effect on the cage occupancy of CH 4 -hydrates (I LC /I SC ~3.8) (Moryama et al, 2015). Similarly to the case of hydrates formed from pure water (Qin and Kuhs, 2013), the ratio slightly decreases (I LC /I SC ~3.2) at higher pressure (10 MPa and 274 K), i.e., deep in the stability field, in a salt environment (4 wt% NaCl) (Shin et al, 2017). Thus, sodium ions present in the original aqueous solution do not impact cage occupancy in artificial hydrates.…”