TRANSIENT ANALYSIS OF ADVANCING CONTACT ANGLE MEASUREMENTS ON POLISHED ROCK SURFACES

Determining the wettability of rocks is necessary for understanding how multiphase fluids move within the subsurface. Traditional quantifications of wettability are done by measuring the contact angle of a sessile liquid droplet on a solid surface. Previous work on rock surfaces has ignored the dynamic behavior of such droplets over time which can affect contact angle measurements. We investigated the dynamic behavior of the contact angle for water on rock surfaces in the presence of air. Sessile droplet diameter and contact angle measurements were made over time (t) on flat polished disks of Burlington limestone, Crossville sandstone, Mancos shale, Sierra White granite, Vermilion Bay granite A and B, and Westerly granite. The measurements were normalized against their initial values at t = 0 and fitted to two-parameter models using non-linear regression, to yield a measurement of the mean equilibrium advancing contact angle for each rock type. The regressions of normalized diameter over time and contact angle over time had median coefficients of determination of 0.88 and 0.98, respectively. Mean equilibrium advancing contact angles ranged from 37.24^o for Mancos shale to 75.61^o for Burlington limestone. Analysis of variance indicated statistically significant differences in mean contact angles between rock types at the 95% confidence level. Observation of sessile droplets was also conducted using dynamic neutron radiography and showed sessile droplet imbibition into the polished rock disks. The approach used to model sessile droplet behavior over time permits a more meaningful estimate of the equilibrium advancing contact angle for rocks than the simple averaging of a time series of measurements as is frequently done.