Theoretical and experimental evidence of different wettability classes

In this paper we analyze data from thirteen North Sea sandstone reservoirs that fall into the large group of intermediate wettability. By dividing this group into three sub-classes, fractionally-wet (FW), mixed-wet large (MWL) and mixed-wet small (MWS), we find an improved understanding of the intermediate-wet state. In the fractionally-wet state oil- and water-wet sites are random with respect to size, while in the mixed-wet state water- and oil-wet pores are sorted by size. We assume two classes of mixed wettability; MWL where the large pores are oil-wet and MWS where the small pores are oil-wet. The paper includes both theoretical considerations, pore–scale observations by ESEM, SEM and thin sections, as well as core scale wettability indices that indicate that these wettability states may exist in reservoirs.