Significant work is ongoing within the industry to determine a best practice for maximizing oil recovery from fractured oil-wet shale reservoirs. Rapid decline curves are often observed and water flooding can be largely ineffective because of negative capillary pressure. The goal of this work is to identify a chemical solution that can maximize oil and gas recovery in unconventional reservoirs by reduction of hydrocarbon adhesion to shale rocks. In order to evaluate an optimal solution, numerous formulations were developed and tested for their impact on adsorption and adhesion on rock and/or sand, changes in interfacial tension, and how the formulation affects the wettability of the formation. Shale rocks were characterized for their surface energy, as this governs the adsorption and adhesion tension of crude oil, water, and chemicals to the solid surface. Formulations were selected that minimized the adsorption on rock and sand surfaces since such adsorption may lead to an increase in the surface tension of fluid pumped into the well and the interfacial tension between the crude oil and fluid. Contact angle measurements were used to determine the Van der Waals and Lewis acid-base components of surface energies for Barnett, Eagle Ford, Niobrara, and Bakken shales. In addition, contact angle measurements and interfacial tension were used to determine the adhesion of crude oil to the rock. Numerous chemical formulations were evaluated to identify products that can decrease the work of adhesion, making oil recovery more efficient (i.e. less work is required to remove oil drops from the rock surface and mobilize them). Competitive adsorption of formulations at the oil-water and rock-water interface was evaluated. The amount of natural surfactants in the oil and their adsorption on the rocks (reversible vs. irreversible) affect whether the rock is oil-wet or water-wet. If the adsorption is reversible, the rock would be more water-wet, resulting in higher oil recovery. Formulations which altered the wettability to water wet rapidly, but reduced the interfacial tension slightly, exhibited the highest oil recoveries.
Based on wettability alteration, interfacial tension, and work of adhesion, a novel product was developed that is salt tolerant (in 30% TDS), thermally stable (115°C), and produces high oil recovery (i.e., 60% OOIP). Kinetics were also improved compared to conventional treatments and brine alone. In addition, this product showed a low static adsorption on the Bakken shale (0.20 mg product active/gram rock) and no emulsion tendency.