Brine Inclusions Migration in Intact Salt Crystals under Thermal Gradient
Abstract
The behavior of water contained in rock salt under the influence of thermal gradients is critical to the performance of salt as a medium for the disposal of nuclear waste. Water contained in salt can be present as discrete inclusions within intact salt crystals, at the interface between salt crystals and aggregates, and also as hydration water and structural water present in accessory minerals present in salt. Water content in pure halite salt usually rages from 0.1 to 0.5 wt. % but is significantly higher in clay rich salt, for which water content can be up to several wt. %. Under the influence of thermal gradients brine inclusions and water associated to the accessory mineral is mobilized. Previous investigations have shown brine inclusions tend to move towards the heat source through a mechanism that involves the dissolution of salt at the hot face of the brine inclusion and its precipitation at the colder side of the inclusion. Uncertainties remain on the exact parameters that define the rate of brine migration and whether it truly migrates to towards the heat source. We performed studies under controlled thermal gradients to examine the behavior of brine inclusions in single salt crystals obtained from the underground salt mine at the Waste Isolation Power Plant (WIPP). We found that the behavior of the brine inclusions under thermal gradients is dependent on the thermal gradient magnitude and the nature of the inclusion. Full inclusions (liquid only) migrate predominantly towards the heat source, but when the inclusions are large and close to the surface they fracture the salt and release water near the surface. Inclusions that migrate towards the heat source migrate through a mechanism that involves the dissolution of salt at the hot side of the inclusion and its deposition along the migration path. SEM analysis of the migration pathways shows that brine migrates through the creation of a network of square shaped hollow channels of about 10 micron diameter. The behavior of two phase inclusions (liquid and gas) in a temperature gradient is distinctly different from that of full inclusions. The brine in the two phase inclusions still migrates towards the heat source; however, the vapor phase moves away from the heat source through narrow square shaped channels. The resulting salt crystals are much more fractured and have a distinct appearance compared to salt crystals with full inclusion. The composition of the salt deposited along the migration channels changes along migration pathway. At the start of the inclusion migration pathway the deposited salt is composed of a mixture of NaCl, MgCl2, and CaCl2, with minor other elements. However, as the brine migrates towards the heat source its composition changes and it gets enriched in NaCl. SEM images and migration channels
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFMMR33A2316C
- Keywords:
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- 3625 MINERALOGY AND PETROLOGY Petrography;
- microstructures;
- and textures;
- 3653 MINERALOGY AND PETROLOGY Fluid flow;
- 3600 MINERALOGY AND PETROLOGY