Sample preparation for rock wettability studies via atomic force microscopy

The wettability of a reservoir rock is one of the most essential parameters in oil and gas recovery applications and gas storage schemes. However, bulk techniques, which are commonly used to analyse rock wettability, for example the United States Bureau of Mines test, are not sensitive enough to probe mixed-wettability scenarios. Furthermore, these measurements are conducted at millimetre–centimetre scale, while wettability is determined at the atomic scale, and some rocks (e.g. shale) have a very fine structure even at nanoscale. Additionally, in the case of shale rocks, standard wettability measurements cannot be applied due to their extremely low permeability. To overcome these limitations, wettability can be directly measured at the nanoscale with advanced analytical methods, such as scanning electron microscopy (SEM) and atomic force microscopy (AFM). While such techniques are well-established in various disciplines, there exists no standard procedure for rock wettability analysis at nanoscale. Thus, this study elaborates on the optimal methods that can be used for the preparation of an AFM-cantilever-rock grain sample, with which the rock wettability can be measured at atomic scale. Therefore, this work aids in the wider-scale implementation of AFM as a rock wettability measurement tool.