Ion Modification of Hybrid Low-Salinity Enhanced Oil Recovery Methods for Improved Spontaneous Imbibition in Fractured Carbonate Reservoirs

Spontaneous imbibition (SI) is the key mechanism governing oil production in fractured carbonates, where capillary forces drive oil from the low-permeability matrix into the fractures. Its application is limited by the strongly oil-wet nature of carbonate rocks and by high interfacial tension (IFT), which restrict the capillary forces needed to mobilize oil from the matrix. This work examines whether a combined low-salinity water (LSW)-surfactant enhanced oil recovery (EOR) process can be optimized for SI by tuning the potential determining ions (PDIs) of the hybrid-stage brine. Experiments were performed with a fixed LSW preconditioning stage, while the hybrid-stage composition and surfactant concentration were varied. Contact angle screening of three ion-modified LSW formulations showed that LSW3Ca3Mg-SO₄, with tripled Ca²⁺ and Mg²⁺, reduced the contact angle from 178.5° to 47.3° within 6 days and lowered the IFT from 33.2 to 17.5 mN/m without surfactant. In SI tests on limestone cores, ion-modified hybrid systems showed substantially higher early-time imbibition rates than conventional LSW-surfactant systems, together with equal or higher ultimate recovery. Notably, with 0.5 wt.% surfactant the ion-modified system reached an early-time rate of 0.75%/h and 36.36% of the original oil in place (OOIP), compared with 0.36%/h and 35.94% OOIP for conventional LSW with 1.0 wt.% surfactant, i.e., at half the surfactant dosage. Wettability-controlled systems, such as ion-modified LSW without surfactant, accelerate early imbibition but recover only about 6-7% OOIP regardless of ionic tuning. IFT reduction, in contrast, is considered to lower the capillary resistance in tight pore throats, allowing much higher recovery even while the rock surface remains oil-wet (40.15% OOIP with the ion-modified hybrid system at 1.0 wt.% surfactant). These findings suggest that tuning the ionic composition of the hybrid-stage brine can accelerate SI and allow a lower surfactant dosage, pointing to a potentially cost-effective ionic design strategy for EOR in fractured carbonate reservoirs.