Synergistic effects of chelating agents and surfactants for chemical EOR in carbonates

Surfactants play a critical role in enhanced oil recovery (EOR) applications; however, their performance is often compromised in harsh reservoir conditions, such as high temperature and high salinity, due to precipitation caused by interactions with multivalent metal ions. Chelating agents were introduced into oilfields for various purposes due to their ability to sequester metal ions. In this work, we conducted a comprehensive investigation about chelating agent-surfactant (CS) flooding for carbonate reservoirs, as an alternative to the well-established alkaline surfactant (AS) flooding used in sandstone. The tested surfactants include sodium dodecyl sulfate (anionic) (SDS), dodecyltrimethylammonium bromide (cationic) (DTAB), Triton X100 (nonionic), and a locally synthesized zwitterionic surfactant. The tested chelating agents include diethylenetriaminepentaacetic acid (DTPA), ethylenediaminetetraacetic acid, and glutamic acid N , N -diacetic acid. pH and temperature, as dominant factors in chelating agent solubility and brine stability, were modified to test chelating agent 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 on carbonate rock surfaces was evaluated using the static contact angle method. Based on the obtained results, chelating agents can be applied as low-cost additives for surfactant stabilization in high salinity conditions. The addition of chelating agents significantly improved the stability of SDS and DTAB in salt solutions and seawater. At a relatively low concentration (0.25 wt%), DTPA was able to stabilize DTAB of 1.00 wt% in seawater at high temperature (90 °C). DTPA, among the tested three chelating agents, exhibited a stronger stabilization effect on surfactants of different ion types. When chelating agents are to be applied in brine, an optimal applicable pH range of 5–9 is recommended so not to induce solubility issue of chelating agents or stability issues of metal ions. In this range, IFT reduction is more significant at high pH, while wettability alteration is more significant at low pH. The combination of a cationic surfactant with a chelating agent forms a low adsorption wettability modifier which can change strongly oil-wet rock to water-wet conditions, thus significantly increasing the residual oil recovery from oil-wet carbonate formations. Zwitterionic and nonionic surfactants are also applicable to combine with a chelating agent for EOR purposes. Anionic surfactant SDS, however, showed a growing inhibition on the wettability alteration effect induced by EDTA as the concentration of SDS increased.