Current understanding of shale wettability: A review on contact angle measurements

Despite the success of hydrocarbon production from unconventional shale reservoirs, the understanding of shale wettability under in-situ conditions remains a challenge. Shale wettability reported in the literature has a wide spectrum from water-wet to mixed-wet and oil-wet. This review however shows that the wettabilities of shales are quite consistent when converted to reservoir conditions for hydrocarbon/brine/shale systems. The review reveals that while shale is oil-wet in presence of air when compared to water (oil contact angle is much lower than water contact angle in air) based on contact angle measurements, it is preferentially water-wet when converted to hydrocarbon/brine/shale system using Young equation no matter what composition shale has or what type of oil or brine is used i.e. it is with the caveat that the water is free from surfactants. This is a significant finding that can explain many unexpected wettability observations of shale rocks. What makes the shale wettability more interesting and in some way adds to its complexity is the effect of state variables such as ionic strength, pressure, temperature, pH, clay content, and organic matter content on contact angle. This review further discusses various studies that have up to now attempted to characterize the effect of these state variables on wettability based on contact angle measurements for different shales around the world.