Wettability Alteration by Combinations of Chelating Agents and Surfactants

Wettability alteration significantly impacts the adsorption of oil on rock surfaces and capillary pressure, which in turn can enhance oil recovery, particularly from unswept regions such as dead-end pores, narrow pore throats, and low permeability rock matrices. Various materials have been shown to modify rock wettability effectively. Chelating agents have recently demonstrated excellent wettability alteration performance in carbonate rocks. The combined effects of chelating agents and surfactants on wettability require further exploration. This study investigates the enhanced oil recovery implications of mixtures of different surfactants with EDTA, a widely used chelating agent, at varying concentrations and pH levels in carbonate rocks. Indiana limestone with low permeability (<20 mD) was chosen to represent tight carbonate rocks, as confirmed by X-ray diffraction (XRD) analysis, which showed 100% calcite composition with trace impurities. Surfactants with different ionic properties-anionic (SDS), cationic (CS, locally synthesized), and a zwitterionic surfactant (ZS, locally synthesized)-were tested. pH and temperature were modified to test EDTA solutions of different concentrations and their mixtures with surfactants. Interfacial tension reduction by chelating agents alone, surfactants alone, and their mixtures were measured. Wettability alteration brought by chelating agents and surfactants was evaluated using the static contact angle method. At low pH, high-concentration EDTA solutions face stability issues. At high pH, excess OH- resulted in the precipitation of metal ions in the brine. The application of chelating agents requires considerations in a proper pH range. 1 wt% EDTA at a neutral pH has a limited impact on interfacial tension (IFT). When pH was high, however, the IFT was significantly reduced. Adding surfactants in this case can further enhance the IFT reduction, indicating a synergic effect between each surfactant and EDTA. EDTA alone is found to be effective in the wettability alteration of carbonate rock. At a high pH (13), a higher EDTA concentration led to a stronger water-wetness on the rock surface. At a low pH (3), however, this trend was not observed. In both CS and ZS cases, combining the surfactant with EDTA further enhanced the wettability alteration, modifying the strongly oil-wet carbonate sample to water-wet or strongly water-wet conditions. SDS, however, had a negative impact. When SDS was mixed with EDTA, the wettability alteration performance was negligible, much weaker than EDTA alone. Based on the obtained results, chelating agents can be applied as low-cost additives for surfactant stabilization in high-salinity conditions. When chelating agents are to be applied in brine, an optimal applicable pH range of 5~9 is recommended. The combination of a cationic/zwitterionic surfactant with a chelating agent forms a wettability modifier that can change strongly oil-wet rock to water-wet conditions, thus significantly increasing the residual oil recovery from fractured oil-wet carbonate formations.