Understanding the Wettability of Calcite (CaCO3) Using Higher Spatial Resolution

Wettability alteration of carbonate rocks from an oil-wet state to a water wet or mixed-wet state during water flooding is known to enhance the recovery of oil from the reservoirs. Previously, the wettability of porous mediums has been extensively investigated using numerous macroscopic methods. Among those, the contact angle is a highly employed method for wettability measurements; however, this technique, because of its lower spatial resolution, fails to provide a complete chemical understanding of all the factors affecting the reservoir wettability. In an attempt to overcome this, we employed a multiscale approach involving macro-, micro-, and nanoscopic analytical techniques to investigate the wettability of calcite. Studies were performed by aging two different planes of freshly cleaved calcite in ambient atmosphere and with deionized (DI) water. Contact angle measurements and AFM force profiles were recorded at the macroscale and nanoscale, respectively. Wettability transition was observed from super hydrophilic to hydrophobic nature in ambient atmosphere and super hydrophilic to hydrophilic nature in DI water. When AFM studies were performed on samples aged in DI water there were always patches of water present, which were observed only at the nanoscale. These water patches affect the contact angle measurements and make the macroscopic wettability results inherently ambiguous. This work has shown that the contact angle measurements should not be taken as the absolute measurement of wettability.