Conventional methods for wettability assessment are based on volumetric measurements such as displacement tests, and have to be modified for shales because of their low porosity, ultra low permeability with natural fractures, and reactive components. These methods are time consuming and the results are often questionable; these facts that have motivated the search for new approaches to estimate this property. A methodology for measuring shales wettability that uses X-Ray Photoelectron Spectroscopy (XPS) is presented in this paper. Results are compared with conventional imbibition tests. XPS is being used as a faster way to infer the wetting condition on sandstones rocks.
XPS allows knowing the chemical composition of the outer layers of a solid surface. Using XPS the organic carbon content at the surface is evaluated, and then it is correlated with the wetting condition from imbibition tests. Water wet and oil wet shales from different countries were investigated. Samples were prepared rigorously for mineralogical and XPS measurement.
The relationship between wettability and surface composition as determined from XPS is reported. Wettability index measured by imbibition tests is used in this study as an indicator of the wettability of shales. The XPS spectra of oil-wet surfaces of shales reveal the existence of organic carbon and also of an organic mineral species such as Si–CH. These species have a well-defined binding energy which differs from the inorganic species of mineral grains. In this study we present quantifiable evidence that chemisorbed organic material on the surfaces defines the oil-wetting character of oil shales. In the case of gas shales gas is adsorbed onto clay surfaces, as well in natural fractures and porous giving them oil wet condition. This view is supported by a strong correlation between the organic matter content on shale surfaces, the mineral composition and the wettability. In this paper we demonstrate a different approach for measuring shale wettability using XPS analysis. Instead of waiting for many days for the results from imbibition test, XPS takes less than one day. As a result, the time needed for the wettability measurement is reduced significantly.