Effective Darcy-scale contact angles in porous media imbibing solutions of various surface tensions

Surface tensions of high-salinity solutions are significantly different from those of pure water. Our objective was to develop and test a methodology to determine whether these surface tension effects predictably alter imbibition into dry and moist porous media. Static and dynamic experiments were performed using four grades of quartz sand to determine the effects of solution salinity on imbibition. Results were quantified as apparent contact angles between the sand and three solutions (pure water, 5 molal NaNO₃, and n-hexane). Contact angles determined using a static method in initially air dried sand ranged from 23° to 31°, with the same values found for both water and the NaNO₃ solution. Effective contact angles determined for the air-dried sand using a dynamic method based on a modified version of the Green and Ampt model were about twice those found using the static method, averaging 45° and 62° for water and the NaNO₃ solution, respectively. In prewetted sands, the dynamic imbibition data yielded apparent contact angles of 2° and 21° for water and the NaNO₃ solution, respectively, with the latter value comparing well to a predicted value of 25° for the NaNO₃ solution solely on the basis of surface tension contrast. The results of this study indicate that on the Darcy scale, saline solutions appear to follow the relationship of nonzero contact angles with other miscible fluids of different surface tensions used to prewet the sand grains, in agreement with the macroscale infiltration results of Weisbrod et al. (2004).