Nuclear magnetic resonance investigations of phase transformations in frozen hydrate-containing soils during interaction with salt solutions
B.A. Bukhanov, E.M. Chuvilin, A.Z. Mukhametdinova, E.O. Krivokhat
Skolkovo Institute of Science and Technology, Center for Petroleum Science and Engineering, Moscow, Russia
Keywords: permafrost, gas hydrates, NMR relaxometry, salt migration, hydrate dissociation, self-preservation, supercooled water, pore ice
Abstract
Experimental studies of phase transformations in frozen hydrate-containing fine sand samples during their interaction with frozen saline solutions have been conducted using low-field NMR relaxometry. The obtained results confirm that the amount of liquid water in the investigated samples increases over time as a result of salt transfer and dissociation of pore hydrate. In addition to the pore hydrate dissociation and supercooled water content increase, a reverse process occurs, which is associated with the freezing of the pore solution as a result of a decrease in the salt ions concentration. The performed studies enable identifying the boundary of the area with the high liquid water content in soil systems in the direction of salt migration. In this case, the velocity of this front in the target samples was determined not only by the concentration of the contacting solution but also by the initial content of the liquid water phase and the conditions of the pore hydrate existence. Thus, the movement of the liquid water front in frozen sands containing metastable hydrates occurs approximately 30-40 % faster than in similar soils containing stable hydrates, which is in good agreement with the results of previous studies. As a result, the developed NMR technique opens up a great potential for permafrost and gas hydrate studies in general and, in particular, for understanding the contribution of nonequilibrium and equilibrium liquid water to the salt ions migration mechanism in frozen and hydrate-containing soils.
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