Wettability in chalk, effect of initial water saturation on the adsorption of polar oil components

Abstract

Experimental study of wettability alteration was aimed at three laboratory designated chalk material, collected from the same reservoir block, at different initial water saturations (0%, 10%, and 40%). Crude oil sample with fixed Acid Number (AN=0.35 mgKOH/g) and Base Number (BN=0.20 mgKOH/g) was used to saturate and age the chalk cores at 50°C. Extensive acid and base number measurements of output oil was made during saturation to create a fingerprint of the wettability alteration processes. It is understood that content and interactions of oil components to rock sur- faces affect nature of fluid wetting and has a direct impact on ultimate oil recovery [1] . The observations confirm that initial wetting of Carbonates are mixed-wet at relevant initial water saturation and that wetting condition is strongly related to the acidic oil components. The following observations were made; 1) Adsorption of the polar oil compo- nents take place in two different mechanisms; the adsorption of base groups is related to physical attraction, while acids are adsorbed by chemical covalent bonds. 2) Oil wetting state is related to strength of chemical bonding between the oil and rock surface, this was proved by cleaning the core with oil solvents, to detach loosely bonded oil components, such that only strongly competitive carboxyls can adsorb to the chalk surface during a secondary oil saturation. 3) Presence of water will decrease the rate of adsorption processes and the fractional wetting by water is increased at higher water content. 4) Oil recovery by spontaneous imbibition increased with increasing water wetting.