Spontaneous imbibition for mixed-wettability states in sandstones induced by adsorption from crude oil

Abstract

Wettability control by methods that are of practical relevance to oil recovery is an ongoing problem in wettability research. The preparation of mixed-wettability cores by adsorption from an asphaltic crude oil at elevated temperature and pressure has been investigated. After aging, crude oil was displaced by flow of decalin, an intermediate solvent that was compatible with the crude oil with respect to asphaltene precipitation but not likely to cause undue desorption of polar components of crude oil from pore walls. The decalin was in turn removed by flow of refined oil. In tests of the stability of the induced wetting states obtained by this approach, imbibition measurements were repeated for up to six cycles of imbibition and drainage. In most cases, wettability changed slightly towards water wetness after the first cycle, but there was comparatively little or no change for further cycles of spontaneous imbibition and forced drainage. Factors in preparation of mixed-wet cores such as the volume of decalin used to flush crude oil from the core and the temperature of flushing were investigated during the course of development of the experimental procedures. Once established, the induced wetting states were stable with respect to further flushing with decalin and also to flushing with toluene, a stronger solvent for asphaltenes. In experiments on displacement of brine by high viscosity refined oils, the initial water saturation of mixed wet cores could be readily reduced to much lower values than the water saturation that was present at the time of aging. However, in contrast to the drastic reduction in imbibition rate for cores aged at low water saturation, low initial water saturation obtained by this means had very little effect on subsequent spontaneous imbibition. This observation is consistent with the concept of mixed wettability postulated by Salathiel (1973) [JPT (1973) 12616] that the distribution of water at the time of aging controls the distribution of adsorbed components of crude oil.