Impact of Pore Size and Salt Precipitation on the Evaporation From Porous Media

Evaporation is a major process in the global water cycle. The literature on micro-scale evaporation mechanisms and the influence of the resulted salt precipitation within pores on evaporation processes is limited. Moreover, the role of heterogeneity on the combined evaporation - salt precipitation processes is poorly understood. This research focuses on salt deposition inside the matrix pores during the evaporation process and its correlation to gas permeability of the matrix. Another goal is to explore the combined impact of salt precipitation and heterogeneities on evaporation. Two experimental methods are used: (1) quantifying salt deposition in pores with high-resolution X-ray CT; and (2) monitoring of evaporation and salt deposition in controlled columns. In the first experiment, a Berea Sandstone, saturated with five percent NaI solution was allowed to evaporated naturally for five days while the water, air, and salt distributions were monitored periodically using X-ray CT.Klinkenberg permeabilities were determined prior to and following the evaporation process to explore the impact of salt deposition within the matrix on gas permeability. In the second experiment, eight columns were packed with fine and coarse sand as well as fine and coarse sand combined, with a textural interface between the two grain sizes. Four columns were saturated with DI water and the rest with salty solution. The columns were placed in a temperature controlled room with continuous monitoring of evaporation rates. Fifty percent of the salt precipitatedat the top seven mm of the sandstone sample, near the evaporating interface. Large pores (>200 micron) remained open with very small amount of precipitated salt crystals, while the smaller pores were mostly clogged with salt. These observations indicate that the solution migrated to the evaporation surface by capillarity. The gas permeability was reduced by ten percent supporting the observation that most of the large-pores remained open. Preliminary results from the column experiments indicate that both grain size and pore-water salinity play a major role in the evaporation rate.