Oil Removal in Prewet Calcite: Active Versus Inactive Ions Investigated by a Fourier Transform Infrared and X-ray Photoelectron Spectroscopy Study

In presalt reservoirs, natural rocks interact simultaneously with formation water (FW) and mineral oil. The prehydrated calcites with FW and demineralized water (DW) are suitable model systems to investigate oil adsorption (removal) on (from) their surfaces by Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. Preliminary characterization of the chemical composition of fresh cleaved calcite conditioned directly with low salinity (LS) waters, FW, and DW indicates that calcite undergoes (i) surface dissolution once in contact with DW and diluted LS water (LS100) as testified by the split of ν₃ vibration bands of the CO₃^2- group and (ii) partial chemical modification of calcite e through Mg incorporation, water anchoring at the surface, and salt deposit formation at the calcite/FW interface. Pristine and (FW, DW) prewet calcites were conditioned with Nujol, and an overall larger CH₂ and CH₃ vibration band intensity of Nujol was observed on the FW prehydrated surface than calcite/DW/oil and calcite/oil interfaces. The final conditioning of calcite/oil, calcite/FW/oil, and calcite/DW/oil with LS water ended up with greater oil removal for a saline solution of 0.2 mol/L ion strength, independent of the prehydration. Our results indicate that altered calcite chemical composition or dissolution affects subsequent oil adsorption and removal, and, therefore, there is a competitive role between brine ion strength and oil and surface compositions in the enhanced oil recovery process using LS water.