Destabilization of intrapermafrost gas hydrates by salt migration from cryopegs: experimental results

The migration of dissolved salts from natural saline solutions (cryopeg brines or seawater) into permafrost and its effect on the dissociation of intrapermafrost gas hydrates is studied in laboratory experiments. The experiments are applied to laboratory-made frozen sand samples saturated with methane hydrate and exposed to interaction with chemically different frozen saline solutions simulating cryopeg brines. The runs were performed at atmospheric pressure and a constant subzero temperature of approximately −6 °C, i.e., under the P-T conditions that can maintain self-preservation of metastable methane pore hydrates. Variations in the NaCl/MgCl 2 concentration ratio in the solutions affect significantly the salt transport patterns and the salinity level critical for complete hydrate dissociation. Specifically, both salt migration and hydrate dissociation in frozen soils are faster at lower NaCl and higher MgCl 2 concentrations. Phase transitions in the pore moisture of frozen hydratebearing sand samples interacting with frozen saline solutions are analyzed using low-field NMR relaxometry. According to the NMR data, the front of elevated liquid water content propagates along the salt flow direction, at a rate increasing with the mobility of salt ions in the series Na 2 SO 4 – NaCl – MgCl 2 . The experimental results confirm that migration of natural saline solutions (e.g., cryopeg brines or seawater) driven by natural or production-related factors can destabilize intrapermafrost gas hydrates.