Impact of surface roughness on wettability of oil-brine-calcite system at sub-pore scale

Wettability alternation appears to be an important physicochemical process in carbonate reservoirs during low salinity water flooding. Contact angle measurement is widely used as a simple and direct method to demonstrate wettability alteration by low salinity water. The effect of various parameters, e.g., brine salinity, oil composition, and rock mineralogy on contact angle have been well documented. However, uncertainty over effect of rock surface roughness on contact angle of oil-brine-calcite is a major impediment to upscaling laboratory results and predicting wettability at field scale, knowing oil-brine-rock interaction is governed by electrostatic forces. We thus measured contact angle of oil on calcite substrates with different surface roughness (17 nm, 366 nm, and 943 nm), in high and low salinity brines. Moreover, we compared our experimental results with contact angles predicted by Wenzel's equation. Contact angle results show that in high salinity brine, contact angles decreased from 170 degrees to 134 degrees (36 degrees decrease) with increasing surface roughness from 17 to 943 nm, suggesting a less hydrophobic system. Similar correlation between contact angles and surface roughness was observed in low salinity brine. Nevertheless, contact angles only slightly decreased from 117 degrees to 101 degrees (16 degrees decrease) in low salinity brine, suggesting the effect of surface roughness on contact angle is more subtle in low salinity condition. We also found that for oil-brine-calcite system, the correlation between contact angle and surface roughness contradicts the trend predicted by Wenzel's equation. This is largely because the surface forces that govern oil-brine-calcite interactions are not captured by Wenzel's equation. Therefore, we hypothesize that at pore-scale level, wettability alteration by low salinity brine will likely be more subtle than that shown by contact angles when performed on smooth substrates (at sub-pore scale). To predict contact angle at pore-scale, surface roughness and surface forces governing oil-brine-calcite interactions need to be considered. The findings of this research will provide further insight into water-assisted EOR in carbonate reservoirs. (C) 2019 Elsevier B.V. All rights reserved.