Effect of wettability changes on evaporation rate and the permeability impairment due to salt deposition

Abstract

Pore-scale visualization was employed to investigate evaporative drying of brine and associated salt deposition at different wetting conditions, using a 2.5D etched-silicon/glass micromodel based on a thin section image of a carbonate rock. We also compared air drying with CO2 drying, with the latter having important applications in CO2 sequestration processes. The resulting permeability impairment was also measured. For deionized water in a water-wet model, we observed the three classical periods of evaporation: the constant rate period (CRP), the falling rate period (FRP), and the receding front period (RFP). The length of the deionized water CRP was much shorter for a uniformly oil-wet model, but mixed wettability made little difference to the drying process. For brine systems at all wetting conditions, the dry area became linear with the square root of time after a short CRP. This is due to the deposited salt acting as a physical barrier to hydraulic connectivity, unlike the case of deionized water, which is due to capillary disconnection from the fracture channel. For the water-wet model, we observed two regions of a linear downward trend in the matrix and fracture permeability measurements. A similar trend was observed for the mixed-wet systems. However, for the oil-wet systems, fracture permeability only changes slightly even for 360 g/L brine, a result of the absence of salt deposits in the fracture caused by the early rupture of the liquid-wetting films needed to aid hydraulic connectivity. Overall, matrix permeability for all wetting conditions decreased with increasing brine concentration and was almost total for the 360 g/L brine. Finally, using CO2 rather than air as carrier gas makes the brine phase more wetting, especially in the deionized water case, with the result that hydraulic connectivity was maintained for longer in the CO2 case compared to dry out with air.