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 Scanning 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° ≥ θ ≥ 80°) was prevalent with evidence of minor oil-wet patches too. At the micro-scale, for the 100% dolomite sample, the contact angle (θ) varied from ∼66° to 76° under natural conditions, while the same sample aged in crude oil depicted a remarkable variability, i.e., θ ranged from 58° to 132° with the majority of micro-droplets having θ of ∼85° – thus suggesting a mixed-wet behavior. For the same sample aged in surfactant, θ was <5° at micro and macro-scales, with few micro-droplets having θ of ∼89°. However, the macro-scale θ values were 105° (natural) and 90° (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.