464536 Wettability at the Pore Level: A New Set of Spectacles for Higher Spatial Resolution
In this study we have investigated the wettability on an idealized flat surface of a mineral which is representative of a carbonate reservoir, i.e., atomically flat idealized calcite crystal. We first observed the wettability of calcite at the macroscopic level using Static Contact Angle (SCA) and then observed the wettability at Nanoscopic level. This study was performed to observe if the macroscopic results are a true measure of sample wettability.
Freshly cleaved calcite undergoes a wettability transition from being super hydrophilic to hydrophobic nature on being exposed to the atmosphere for 120 hours. The SCAs changed from being 0° when freshly cleaved on both the planes of calcite, before stabilizing at ~82.8° for Plane 1 and ~ 90.3° for Plane 2 after 120 hours. When observed with a higher spatial resolution using an Atomic Force Microscope (AFM), it was observed when calcite is freshly cleaved in atmosphere the dangling bonds of calcite are balanced by hydrolyzed water. The transition in wettability is due to the growth of these hydrate layers which were confirmed by AFM imaging and FTIR spectrum. The quantitative analyses at the nanoscale i.e., the force of adhesions were in good agreement with the macroscopic results.
When calcite was aged in Deionized water the SCAs changed from 0° when freshly cleaved to ~ 15.4° for Plane 1 and ~18.4° for Plane 2 after aging for 240 hours. The calcite crystal undergoes dissolution with time which was observed with AFM, FTIR and ICP mass spectroscopy. The macroscopic studies were carried out by drying the sample with Nitrogen and then measuring the SCA. However, when probed under an AFM it was observed that there is always a layer of water present on top of the sample surface. When the droplet from contact angle goniometer is put on top of the sample surface, it will interact with water layers present at the nanoscale and therefore alter the contact angle results; thereby putting the macroscopic wettability results into question. These observations also hint that macroscopic measurements alone are not sufficient to understand the cause for the observed time dependent wettability transition of calcite and call for the use of higher spatial resolution techniques.
We believe that our work gives a nanoscopic insight into the wettability of calcite when aged in different mediums. Our work has shown that SCA results of calcite when aged in liquid environments like DI water cannot be taken to be the true measurement of the sample wettability unlike in the case when calcite was aged in ambient atmosphere.
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