Interfacial Visco-Elasticity of Crude Oil - Brine: An Alternative EOR Mechanism in Smart Waterflooding

Abstract

Abstract Injection of water with designed chemistry has been proposed as a novel enhanced-oil recovery (EOR) method, commonly referred to as low-salinity or smart waterflooding, among other names. The plethora of names encompasses a family of EOR methods that rely on modifying injection water chemistry to increase oil recovery. Despite successful laboratory experiments and field trials, underlying EOR mechanisms remain controversial and poorly understood. The vast majority of the proposed mechanisms rely on rock-fluid interactions. In this work, we propose an alternative fluid-fluid interaction mechanism, i.e. an increase in crude oil-water interfacial visco-elasticity upon injection of designed brine as a suppressor of oil trapping by snap-off. A crude oil from Wyoming was selected for its known interfacial response to water chemistry variation. Brines were prepared using analytic grade salts to test the effect of specific anions and cations. The ionic strength of brines was modified by dilution with deionized water to the desired salinity. A battery of experiments were performed to demonstrate the impact of dynamic interfacial viscoelasticity on recovery including double-wall ring interfacial rheometry, dilational rheology, direct visualization in microfluidic devices and coreflooding experiments in Berea sandstone cores. Interfacial rheological characterization shows that interfacial viscoelasticity generally increases as brine salinity is decreased, regardless of what cations and anions are present in brine. However, the rate of elasticity buildup and the plateau value depend upon specific ions available in solution. Snap-off analysis in a microfluidic device, consisting of a flow-focusing geometry, demonstrates that increased viscoelasticity suppresses interfacial pinch-off and sustains a more continuous oil phase. This effect was examined in coreflooding experiments using sodium sulfate brines. Corefloods were designed to prevent wettability alteration by maintaining a low temperature (25 °C) and short aging times. Geochemical analysis provided information on in situ water chemistry needed to establish a direct link between brine composition and oil displacement. Oil recovery and pressure responses were shown to directly correlate with interfacial elasticity, i.e. recovery factor is greater the larger the induced interfacial viscoelasticity. Our results demonstrate that a largely overlooked interfacial effect of engineered waterflooding can provide as an alternative and more complete explanation of low-salinity or engineered waterflooding recovery. This new mechanism offers a direction to design water chemistry for optimized waterflooding recovery in engineered water chemistry processes and opens a new route to design EOR methods.