Surface charge study of EDTA interaction with carbonate rock during chelating agent flooding

Altering the carbonate wettability is considered as an effective approach for enhancing the oil recovery from carbonate reservoirs. Wettability alteration from oil-wet to more water-wet condition can be assessed by studying the changes in rock surface charges. In this study, the surface charges of carbonate rock samples during chemical flooding is investigated. The impact of flooding carbonate rocks with Ethylenediaminetetraacetic acid (EDTA) on changing the surface charges and increasing the oil recovery is studied. The effect of brine salinity, EDTA concentrations and solution pH on the oil recovery is discussed. The performance of EDTA solution was compared with high-salinity seawater (SW), diluted seawater (50%SW) and deionized water. Results of zeta potential measurements, ions concentrations analysis, nuclear magnetic resonance, spontaneous imbibition and coreflooding experiments are reported in this paper. The rock samples were characterized using scanning electron microscope (SEM) and X-ray diffraction (XRD) analyses. SARA analysis, gas-chromatography (GC) and rheological measurements were performed to characterize the crude oil. This study showed that injecting EDTA solutions into carbonate rocks can change the carbonate surface charges from positive to negative values indicating that the rock wettability was altered to more water-wet conditions, and consequently more oil was recovered during the coreflooding and the spontaneous imbibition tests. Incremental oil recovery of 20% was achieved by flooding the carbonate sample with EDTA solutions. During spontaneous imbibition tests, 80% of the oil was recovered using 5 wt% EDTA chelating agent, while 50% and 20% were recovered using deionized water and seawater brine, respectively. The ions concentrations analysis showed that the amount of calcium ions was significantly increased during the EDTA flooding, indicating that the injected chemicals increase the Ca2+ leaching from carbonate surface and improve the wettability conditions. Finally, NMR analysis confirmed that the injected chemicals did not induce severe bulk damage and only surface dissolution was taking place.