Investigation of the effects of an excess of Ca2+ and Mg2+ ions in low salinity water in the process of oil removal, dissolution, and solvation in calcites

The transition from an oil-wet to a water-wet calcite surface is crucial for enhanced oil recovery and can be influenced by low salinity (LS) water. While previous studies have explored the effects of LS water or single-ion solutions on rock-oil interfaces, the impact of excess active ions in LS on wettability remains uncertain. This work aims to determine whether Ca²⁺ or Mg²⁺ ions are more effective in altering wettability and oil recovery and to identify the optimal salinity for the Nujol-calcite system. Oil-wet calcite surfaces were characterized before and after treatment with LS water rich in Ca²⁺ or Mg²⁺, diluted 25, 50, 75, and 100 times (LS25, LS50, LS75, and LS100) compared to formation water. LS75, with a salinity of 3173 ppm, was the most effective for oil removal, achieving 81 % oil recovery. LS75Mg removed 75 % of the oil, and LS75Ca removed 65 %, as confirmed by ATR-FTIR and contact angle measurements. LS75Mg also showed a larger contact angle (∼160°) than LS75Ca (145°), indicating better oil detachment. Surface changes, including dissolution and oil fragmentation (especially with LS75Mg and LS100Mg), were observed via topographical analysis. XPS results revealed higher Na⁺, Cl⁻, and Mg²⁺ levels on treated surfaces, with LS50Mg showing the most significant reduction of surface carbon content. While a combination of Ca²⁺ and Mg²⁺ at optimal salinity enhances oil removal, excess Mg²⁺ alone is less effective. The competitive mechanisms behind oil removal and their dependence on brine composition are discussed.