Effect of Ferrous Ion Fe2+ on Wettability: Oxidation and Cation Bridging

Abstract Reservoir rock and crude oil samples are usually exposed to oxygen during storage and in laboratory experiments. Ferrous ions (Fe2+) on mineral surfaces and in brines can be oxidized to ferric ions (Fe3+). Rock samples can also be contaminated with Fe3+, e.g. from muds. The objective for the study has been to investigate the effects of Fe2+ oxidation and cation bridging by Fe2+ and Fe3+ on wettability. Oxidation of Fe2+ to Fe3+ was first investigated by aging formation water (FW) solutions at reservoir temperature. The effect of Fe3+ on wettability was then studied for the clay mineral glauconite. The wettability was characterized using a flotation method that relies on the affinity of the minerals to either the brine or oil phase. Flooding experiments were carried out in native and restored reservoir cores using FW, sea water (SW) and low salinity water (LSW) as injection waters. The element composition of effluent samples was determined. Geochemical simulations were carried out to further investigate the interactions of brine with the mineral and rock in the flotation tests and core flooding experiments. During aging of FW samples for 2 days, 46% of Fe2+ (50ppm) was oxidized to Fe3+. FW containing 50ppm Fe3+ gave less water-wet glauconite clay than FW without Fe3+. The flotation method showed that Fe3+ increased the concentration of oil-wet glauconite by 366% and 67% for two different FW/STO systems. The effluent Fe concentration was very low during injection of FW and SW to native reservoir cores, but a Fe peak was observed in LSW floods. No Fe was detected in the effluents during injection of FW, SW and LSW to the restored reservoir cores. Geochemical simulations showed that 50ppm Fe3+ was not affecting the concentration of multivalent cations onto clay surfaces. The alteration of wettability of glauconite to less water-wet was most likely due to precipitation of surface active iron-minerals on glauconite surfaces. According to the simulations, the solubility of Fe3+ was very low in all brines (FW, SW and LSW) injected to the reservoir cores. The observed Fe-peak during injection of LSW to the native reservoir core plugs can therefore not be explained by the simulation results. The solubility of Fe3+ may have been higher than simulated because of complexes between Fe3+ and carboxylic acids. The study has shown that geochemical simulations can be helpful in interpretation of different types of experiments. Similar method can also be used to evaluate the potential for oxidation of Fe2+ and thereby the risk for precipitation of surface active Fe-minerals. These can give wettability conditions not representative for the actual oil reservoir.