Mechanism of calcite wettability alteration by alkyl polyglucoside

Abstract

Surfactant injection into oil-wet carbonate reservoirs offers a large potential for increased oil recovery due to reservoir wettability alterations. Here, the mechanism of wettability alteration by non-ionic surfactant alkyl polyglucoside (APG) is investigated and compared to that of the cationic surfactant dodecyltrimethylammonium bromide (DTAB). Techniques used include 1H and 13C magic angle spinning NMR, zeta potential, adsorption and contact angle (CA) measurements. APG monomers effectively desorb stearic acid (SA) from an SA treated calcite surface by replacing it at cationic sites on the surface, both above and below the APG critical micelle concentration (CMC). Increased polarization of the APG hydroxyl groups upon interaction with Ca+ sites is a driving force for APG adsorption. CAs show that APG treatment changes wettability to water-wet both below (CA = 66.9°) and above (CA = 77.2°) CMC. An observed sphere-to-rod transition occurring in APG solution above 1 mM is accompanied by a structural transition from half-micelles to patches of partial bilayers at the calcite surface, which further increases APG adsorption and CA (CA = 88.1°). In the presence of SA, adsorption of APG onto calcite is similar to that of DTAB below CMC, while above CMC APG adsorption is 2–3 times higher than DTAB. In absence of SA, DTAB adsorption is negligible. CAs are comparable after APG and DTAB treatment, making APG an interesting candidate for such purposes due to its effectiveness at very low concentrations.