Adsorption of polar crude oil components exert a major impact on chalk wetting, and adsorption effects have been reported on water-wet pure Stevns Klint outcrop chalk (>99% CaCO3). Aalborg chalk is a silica-rich material also used as analogue rock material for chalk reservoir studies. This work aims to characterize the impact silica minerals will have on polar crude oil component adsorption, and core wettability.
Silica-rich (6-8 At %) Aalborg chalk cores with initial water saturation (Swi) of 10% were flooded with a crude oil with acid and base numbers (AN and BN) equal to 0.35 mg KOH/g of oil. Effluent crude oil samples were analyzed, detecting changes in the AN and BN concentrations at the core outlet. Wettability was determined by spontaneous imbibition (SI), and further confirmed by chromatographic wettability tests.
Adsorption of polar crude oil components was observed to be an instantaneous process. The adsorption of polar acidic components onto the Aalborg chalk surfaces was less profound as previously observed in Stevns Klint chalk. However, adsorption of polar basic components was significantly higher in Aalborg chalk, which could be explained by the presence of negatively charged silicate surfaces. The water wetness of the Aalborg Chalk core was significantly reduced after the crude oil exposure.
This comparative study on both Aalborg and Stevns Klint chalk highlights the impact of silica content on adsorption of polar components and initial wetting. The findings contribute with essential information for the development of theoretical and chemical models to describe initial reservoir wetting and explain oil production profiles in chalk reservoirs during normal water injections, or during wettability alteration with Smart Water.