Calcium Ion Bridging of Aqueous Carboxylates onto Silica: Implications for Low-Salinity Waterflooding

Release of crude oil from reservoir rock by low-salinity waterflooding is thought to occur by decreasing the advancing crude oil/brine/rock water contact angle to depin three-phase contact lines (i.e., by increasing reservoir water wettability). Crude oil likely adheres locally to reservoir rock asperities by deposition of asphaltene agglomerates formed at the crude oil/brine interface, following collapse of protective water films. One mechanism proposed for asphaltene adhesion and subsequent release is ion bridging of asphaltenic carboxylate groups protruding through molecularly thin water layers to calcium-occupied rock surface exchange sites, the so-called multicomponent ion exchange mechanism. To our knowledge, however, no experimental evidence directly establishes divalent cation bridging of aqueous carboxylates to anionic mineral surfaces. Using a quartz crystal microbalance with dissipation, we measure adsorption of aqueous carboxylates (benzoate, pentanoate, and hexanoate) onto silica, with and without calcium ion present at near-neutral pH. We find little to no adsorption on a silica surface to within the detection limit of our measurement (±1 Hz, 0.18 mg/m). Modeling of the silica surface chemistry, using classical ion-complexing triple-layer formalism, reveals that less than 10% of surface-hydroxylated sites actually ionize at pH 7. Of these, not all bind calcium ions. Accordingly, the calcium ion surface density is insufficient to reverse the negative surface charge of silica or to promote significant ion bridging of aqueous carboxylates. In so far as aqueous-soluble carboxylic acids mimic those incorporated in crude oil asphaltenes, we conclude that calcium ion bridging alone does not adhere asphaltenes to silica surfaces.