Publications

Why oil sticks to limestone

Brady, Patrick V.

A coordination chemistry analysis of oil-calcite adhesion allows waterflood chemistry controls over enhanced oil recovery from limestones to be understood. The model relies on temperature-dependent surface complexation models of calcite and oil. The primary electrostatic bridges holding oil to calcite are calculated to be [-COO^-][>CaOH₂⁺], [-COO^-][>COOCa⁺], [>CaSO₄^-][-COOCa⁺] and [-COOCa⁺][>COO^-] (“>” denotes calcite surface groups; “-” denotes polar oil surface groups; Mg²⁺ can substitute for Ca⁺²). The [-COO^-][>CaOH₂⁺] bridge between oil carboxylate and protonated calcite calcium sites is most sensitive to changes in waterflood chemistry. Model calculations predict that increased levels of Ca⁺², Mg⁺², and SO₄^-2, alone or in combination, will increase oil recovery from limestones by decreasing the number of [-COO^-][>CaOH₂⁺] bridges. Divalent cations decrease the local interfacial potential by decreasing the net negative charge on oil carboxylate groups; SO₄^-2 coordinates to protonated calcite calcium sites to decrease charge and electrostatic attraction. Increases in ionic strength should increase adhesion by increasing the net charge on each surface, though the effect will be less on calcite. The model presented here requires no fitting parameters yet accurately reproduces observed oil mobilization trends suggesting the model to be a potentially valuable tool for designing chemistries of waterfloods employed in limestones.