Estimating Wettability of Rock Fracture Surfaces from Contact Angle and Roughness Measurements
Date of Award
Master of Science
Larry D. McKay, John S. Tyner
Estimates of wettability are important for modeling multi-phase flow and transport in the subsurface. This study investigates the wettability of rock fracture surfaces through contact angle measurements and surface roughness characterization. Droplet diameter and advancing contact angle were determined at 0.5s intervals for ~90s on flat polished disks of Burlington Limestone, Crossville Sandstone, Mancos Shale, Sierra White Granite, Vermilion Bay Granite (2 types), and Westerly Granite using a Krüss DSA 30 Drop Shape Analyzer. The droplet diameter and advancing contact angle data sets were nonlinearly regressed against time using two different two-parameter models. The resulting parameter estimates were used to compute the apparent equilibrium contact angle, θe, for each disk following droplet diameter stabilization. Estimates of θe ranged from 37.2° for Mancos Shale to 75.6° for Burlington Limestone. Analysis of variance indicated statistically significant differences in θe between the rock types at the 95% confidence level. Height maps for fractured surfaces of these rocks were made using a Phenom Pro X SEM and analyzed for Wenzel roughness factor and fractal dimension using multi-image variography. The Mancos Shale and Crossville Sandstone were found to be fractal, both having a mean fractal dimension of 2.16. The Wenzel roughness factor was found to increase with increasing length scale. Mean Wenzel roughness factors for the 200 to 600 μm fields were found to have significant differences between rock types. Using the Wenzel roughness factor and contact angle results in the Wenzel model, four of the rock types were found to have a rough surface contact angle of 0°, indicating complete wettability; the Burlington Limestone, Mancos Shale, and Westerly Granite had contact angles greater than zero but less than their contact angles for flat polished surfaces. This is, to the best of our knowledge, the first time the Wenzel model has been used to quantify rough surface contact angles for rocks. The transient analysis employed in this study also permits a more meaningful estimate of the equilibrium contact angle for polished rock surfaces, rather than taking the initial value or averaging over time as is frequently done.
Gates, Christopher H., "Estimating Wettability of Rock Fracture Surfaces from Contact Angle and Roughness Measurements. " Master's Thesis, University of Tennessee, 2018.