Use of XPS for reservoir sandstone wettability evaluation. Application to kaolinite and illite

X-Ray Photoelectron Spectroscopy appears to be an attractive tool for wettability evaluation. Due, however, to carbon contamination, it is not possible to determine a direct relationship between surface carbon content and oil-wettability. Reference clays were aged with crude oil or with crude oil+brine; clay-size fractions from actual core samples were extracted, cleaned and submitted to the same procedure. XPS was used to determine the surface composition of the samples after each treatment. Some carbon was fixed on all the clay fractions after contact with crude oil. After contact with oil+brine, two behaviors were found: either more carbon was fixed than with crude oil, or the quantity of carbon fixed was close to the value before contact. Expected values for elemental surface composition versus bulk composition were calculated for kaolinite and illite, assuming variable thicknesses of carbon coating on the clay surface. The ratios Si/Al, K/Al and O/Al were not modified by a homogeneous carbon deposit: so XPS does not provide an answer about the presence of a homogeneous film or a distribution of patches on the surface, but possible changes in these ratios could be interpreted as a particular location of the carbon. Comparison of these simulation results with experimental results allowed an evaluation of the carbon coating thickness, or of the coverage ratio. Cleaning of the samples, which were more charged in carbon, showed that the major part was weakly retained. The final carbon content was still somewhat higher than before contact, which was consistent with a part of carbon being more strongly adsorbed. The major difference in the behavior of kaolinite and illite was found after aging in the presence of water, and might be related to the surface cations. Comparison of the amounts of surface carbon versus bulk carbon shows two mechanisms: adsorption and retention. Wettability changes are obviously the consequence of both phenomena.