Micro-scale wettability of carbonate rocks via high-resolution ESEM imaging

The wettability of several materials has been traditionally quantified using macro-scale contact angles. However, precise identification of the three-phase contact (TPC) line is often difficult due to the resolution limit of macro-scale setups. Moreover, micro-level surface chemical heterogeneities can have a notable impact on the predicted wetting behavior which limits macro-scale contact angles. Thus, here, we investigate the micro-scale water wettability of condensed micro-droplets on carbonate rock surfaces via a high-resolution Environmental Scan-ning Electron Microscopy (ESEM). Macro-and micro-scale contact angles were evaluated under three conditions:1) natural carbonate surfaces, 2) surfaces aged in crude oil, and 3) surfaces aged in cationic surfactant to allow for a broader insight of the impact of rock composition and surface morphology on wettability. At the macro-scale, carbonate rocks were preferentially oil-wet to intermediate-wet. However, a profound variability was observed in wetting behavior at the micro-scale where a weakly water-wet state (50 degrees >= theta >= 80 degrees) was prevalent with evidence of minor oil-wet patches too. At the micro-scale, for the 100% dolomite sample, the contact angle (theta) varied from similar to 66 degrees to 76 degrees under natural conditions, while the same sample aged in crude oil depicted a remarkable variability, i.e., theta ranged from 58 degrees to 132 degrees with the majority of micro-droplets having theta of similar to 85 degrees - thus suggesting a mixed-wet behavior. For the same sample aged in surfactant, theta was <5 degrees at micro and macro-scales, with few micro-droplets having theta of similar to 89 degrees. However, the macro-scale theta values were 105 degrees (natural) and 90 degrees (oil-aged) - suggesting notable variability at macro-and micro-scales. These findings reflect: a) significant differences among macro-and micro-scale contact angles, and b) surface wetting at the micro-scale captures physical and chemical properties of the rock, i.e., surface roughness, pore size and distribution, and chemical composition. The study herein presents qualitative and semi-quantitative analyses of the non-uniform wetting behavior of carbonate rocks and the associated rock/fluid interactions through a multi-scale perspective and thus have broader implications for flow in porous medium.