Wettability alteration in chalk: 2. Mechanism for wettability alteration from oil-wet to water-wet using surfactants

The results from a study regarding wettability alteration (oil-wet to water-wet) in low permeability chalk using water-soluble surfactants are presented. Fourteen different surfactants were tested for spontaneous imbibition into oil-wet chalk cores. Cationic surfactants of the type R–N+(CH3)3 were able to desorb organic carboxylates from the chalk surface in an irreversible way, and 70% of the oil in place was recovered within 30 days by a spontaneous imbibition of the aqueous surfactant solution at 70°C. The fluid distribution inside the core was studied by cleaving the core and taking pictures. The mechanism for the wettability alteration is supposed to take place by an ion-pair formation between the cationic surfactant and adsorbed negatively charged carboxylates from the oil. The efficiency of the surfactant is related to properties including CMC-value, hydrophobic property, IFT-value, and steric effects close to the N-atom. Anionic surfactants were not able to desorb anionic organic carboxylates from the crude oil in an irreversible way. Ethoxylated sulfonates with high EO-numbers did, however, displace oil spontaneously in a slow process. The brine imbibed non-uniformly, and the mechanism is suggested to involve the formation of a water-wet bi-layer between the oil and the hydrophobic chalk surface. At a very low IFT-value, 0.08 mN/m, oil is displaced spontaneously from the core by gravity forces.