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 degreesC. 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. (C) 2000 Elsevier Science B.V. All rights reserved.