Migration of Salt Ions in Frozen Hydrate-Saturated Sand: Effect of Silt and Clay Particles

Abstract

Permafrost composed of sand with silt and clay components may store stable and metastable gas hydrates, which can lose stability under a flux of dissolved salts from natural or production-related solutions (seawater, brines from cryopegs, drilling fluids, etc.). The flux of salt ions is controlled externally by temperature and pressure as well as by the salinity of solutions and the properties of fine-grained (<0.05 mm) components in permafrost. The effect of silt and clay percentage and mineralogy (kaolinite or montmorillonite) on the patterns of salt and water transport and the related dissociation of pore gas hydrates in frozen sand is studied in laboratory experiments. The experiments are conducted in conditions that can maintain the self-preservation of gas hydrates: a constant negative temperature of −6 °C and a pressure of 0.1 MPa. Sediments with higher clay percentages show slower migration and accumulation of salt ions and the higher critical salt concentration required for the onset of gas hydrate dissociation. The flux of Na+ to sand samples containing montmorillonite is lower than to those with kaolinite. On the other hand, salt migration and hydrate dissociation are faster in sand with higher silt contents. The experimental results form the basis for a generalized model of main mechanisms that drive salt transport in frozen hydrate-bearing sediments interacting with saline solutions.