Interpretation of mixed wettability states in reservoir rocks

It has recently been noted that the diffuse electrical double layers which exist at the oil/ brine and the mineral/brine interfaces in sandstone reservoirs will in many cases be quite similar with respect to electric charge and potential. Extremely thin aqueous wetting films separating such interfaces are thus stabilized by the electrostatic repulsive force acting between the double layers. In the present paper the limits under which stable thin films can exist are examined in more detail. It is shown that there exists a lower limit to the pore size in which thin wetting films will occur and that this limiting size depends on the salinity of the brine. It is also shown that whether this limiting pore size becomes a factor in determining the wettability of the rock/brine/oil system depends on the pore size distribution curve and on the initial brine saturation of the rock. Geological factors which may come into play in establishing the initial water saturation in a given case are discussed.