Systematic observations of enhanced oil recovery and associated changes at carbonate-brine and carbonate-petroleum...

Enhanced oil recovery (EOR) from carbonates is obtained by injection of controlled ionic strength brines containing "active ions" (e.g., SO₄^2-, Mg²⁺, Ca²⁺). It is generally believed that this occurs through the interaction of the active ions at the carbonate-brine interface (e.g., within a thin brine layer separating the petroleum and the carbonate phases). Here, in-situ observations show how one active ion, SO₄^2-, alters behavior at the carbonate-petroleum interface. Displacement of petroleum from initially oil-wet carbonate rocks using brines with variable SO₄ concentrations systematically changes oil recovery, in situ contact angles, and connectivity of the oil phase, confirming that the active ion alters interactions at the oil/brine/carbonate interface, as expected. Measurements of model calcite-fluid interfaces show that there is no measurable sorption of SO₄ to carbonate-brine interfaces but reveals that the carbonate-petroleum interface is altered by previous exposure to SO₄-containing brines. These results suggest that EOR in carbonates is controlled indirectly by active ions. We propose that this may be due to a reduced oleophilicity of the carbonate caused by chemical complexation between the active ion and petroleum's acidic and basic functional groups. This mechanism explains how both anions and cations act as active ions for EOR in carbonates.