Adhesion and surface energy of shale rocks

Abstract

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. 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. Higher values of work of adhesion between surfactant solution and rock promote water-wet conditions but also exhibit higher surfactant adsorption. An optimal solution was developed that decreased the work of adhesion, minimized the adsorption, and altered the rock wettability, thus making oil recovery more efficient (i.e. less work is required to remove oil drops from the rock surface and mobilize them). This novel product is salt tolerant (in 30% TDS), thermally stable (115°C), and produces high oil recovery (i.e., 60% OOIP). In addition, it showed a low static adsorption on the Bakken shale (0.20mg product active/gram rock) and no emulsion tendency. Formulations which altered the wettability to water wet rapidly, but reduced the interfacial tension slightly exhibited the highest oil recoveries.