Experimental Investigation of the Effect of Surface Cleavage and Exposure Time on Rock Wettability

The wettability behavior of reservoir rocks is recognized as a key factor that influences oil recovery. Several recent studies have investigated factors affecting wettability. However, a thorough understanding of all the elements influencing rock wettability remains elusive. Surface wettability is known to be influenced by a range of operating conditions and the surface-related parameters e.g., surface roughness, surface chemistry etc. While these factors have been rigorously reported before, one key factor that received little or no attention is the surface cleavage. In particular, pure calcite mineral demonstrates multiple cleavage plane, and thus wettability of freshly cleaved calcite can vary depending on the cleavage plane, which can also be very different to the wettability of calcite surface exposed to the atmosphere. Moreover, the traditionally reported macro-scale contact angles lack a bit in terms of the identification of three-phase contact line, and thus the use of high-resolution micro-scale approaches for contact angle measurement becomes essential. Thus, in this study, we investigate the wettability of freshly cleaved and exposed calcite and quartz surfaces and examine the difference in the observed wetting behavior. The contact angles were measured at both micro- and macro-scale using a conventional sessile drop method and environmental scanning electron microscopy (ESEM), respectively to also examine any additional impact of the scale of observation. Advanced image analysis algorithm is then used to process the droplet image and examine the contact angles. Additionally, FTIR spectroscopy is also used to determine the surface chemistry changes in the exposed surfaces that would lead to any wettability alteration. The results of this study indicate that the contact angles observed for both freshly cleaved calcite and freshly fractured quartz are much less than the contact angles of the exposed calcite and quartz – suggesting that freshly cleaved surfaces are more hydrophilic than their exposed versions. Thus, much caution should be considered during dealing with calcite and quartz; as exposure time could be a crucial factor in wettability determination.