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Brine Migration Experimental Studies for Salt Repositories

Experiments were used to examine water content in Permian salt samples (Salado Formation) collected from the WIPP site.  The profile of water release and movement is recognized as a function of temperature from 30 to 275 oC using classical gravimetric methods to measure weight loss as a result of heating.

The amount of water released from heating the salt was found to be correlated with the salts accessory mineral content (clay, other secondary minerals lost up to 3 wt % while pure halite salt lost less than 0.5 wt % water).  Water released from salt at lower temperature was reversible and is attributed to clay hydration and dehydration processes.  The analysis shows a significant loss of interlayer water (13% wt) in the corrensite structure between 65 and 75 oC; no further structural changes are observed in the clay structure from 75 oC to 275 oC.  Gypsum (CaSO4·2H2O) undergoes step-wise phase transformation through dehydration to produce (abrupt phase change at ~ 75 oC with the loss of one water molecule) bassanite (CaSO4·(H2O)) followed by bassanite dehydration to anhydrite (CaSO4) beginning at a temperature of ~ 275 oC. 

Bassanite phase stability at higher temperatures is not yet determined, but experiments are ongoing.

Neutron based Filter Difference Spectrometry (FDS) was used to identify the nature of the water associated with salt.  The use of FDS in situ with thermogravimetric analysis (TGA) could be a new technique to establish the quantitative evolution of water from complex salt mixtures as a function of temperature.

The experiments examined the behavior of individual brine inclusions (one phase and two phase inclusions) in variable thermal gradients.  Results show that brine migrates toward the heat source in both types of inclusions. However, the gas phase in the two phase inclusions migrated away from the heat source. Brine migration occurs through a network of micron size channels.  As a result of the continued dissolution precipitation process the chemical composition of the brine changes as the brine migrates up the thermal gradient. Near the heat source the brine composition becomes dominated by NaCl whereas away from the heat source a magnesium rich salt is precipitated.

Data gained from this investigation highlights 1) the significance of the different water components associated with salt (brine inclusion, hydrous mineral structural water), 2) the temperature domains under which the water can be driven out of the salt and, 3) the parameters that affect the velocity of brine migration in salt.