Interplay between emulsion stability and wettability alteration: An application for water-based enhanced oil recovery methods

Low salinity waterflooding (LSW) is a popular enhanced oil recovery method, which is mainly grounded on rock wettability improvement. Nevertheless, according to recent studies, the application of LSW is accompanied by some practical challenges, such as the formation of unwanted water-in-oil (w/o) emulsions, which emphasizes the necessity for optimization of the injected brine composition. The present study aims to assess the simultaneous role of brine chemistry and its corresponding potential determining ions (PDIs) on rendering rock hydrophobicity and the formation of tight w/o emulsions. To this end, seawater (SW) and its dilutions combined with a number of ion-engineered solutions consisting of PDIs, namely Mg2+, Ca2+, and SO42- were used to perform a set of contact angle and emulsion stability experiments. The design of experiment (DoE) methodology was also used to optimize the brine chemistry for low salinity water injection based on the outcomes of the experiments. We found that lowering ionic strength of brine from 0.8 mol.L-1 down to 0.1 mol.L-1 provides favorable conditions to generate stable w/ o emulsions, with a separated water volume (SWV) as low as 5% of the total initial water after one day of aging at 80 degrees C. Also, as to the dynamic contact angle results, reduction of brine's ionic strength down to 0.1 mol.L-1 without ion-tunning improved the wettability alteration index (WAI) merely about 0.2. However, for the ion-tuned solutions, the presence of Mg2+ cations was found not to be in favor of the low salinity effect. Since not only does it cause a small change in the wetting state (WAI < 0.5), but it also boosts the emulsion's longevity, markedly (SWV < 15%). Contrarily, the presence of SO42- in the brine solution appeared to be more promising because of its strong potential for rendering surface wettability and also reducing emulsion longevity (SWV greater than 50%). As to the presence of Ca2+ cations, results indicated more water-wet surfaces with the WAI of 0.8. Nonetheless, based on the SWV results, Ca2+ cations are prone to form stable emulsions, with SWV ranging from 30% to 5% of the total initial water. Finally, based on the optimization results from DoE, a brine composition in which the concentration of divalent cations (i.e. Mg2+ and Ca2+) is kept minimal and SO42- is the dominant divalent ions is highly likely to induce a unit desirability condition. Also, it was found that the presence of Mg2+ and SO42- in the aqueous phase significantly affects wettability alteration and emulsion generation processes. According to the findings of this study, both brine salinity and its ionic composition should be carefully formulated as they can direct the low salinity effect either in positive or negative ways by improving wettability or generating stable emulsions. The outcomes of the present study can serve as a method for optimizing the water-based enhanced oil recovery approaches. (C) 2022 Elsevier B.V. All rights reserved.

Automated summary

Lowering the ionic strength of brine can generate stable w/o emulsions with a separated water volume as low as 5% of the total initial water. Ion-tuned solutions, especially the presence of Mg2+ cations, can boost emulsion longevity. The presence of SO42- in the brine solution shows strong potential for rendering surface wettability and reducing emulsion longevity.