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.

No abstract

Summary Injection of water with a designed chemistry has been proposed as a novel enhanced-oil-recovery (EOR) method, commonly referred to as low-salinity (LS) or smart waterflooding, among other labels. The multiple names encompass 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. At present, 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 viscoelasticity 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 responsiveness to water chemistry. Brines were prepared with analytic-grade salts to test the effect of specific anions and cations. The brines’ ionic strengths were modified by dilution with deionized water to the desired salinity. A battery of experiments was performed to show a link between dynamic interfacial viscoelasticity and recovery. Experiments include double-wall ring interfacial rheometry, direct visualization on microfluidic devices, and coreflooding experiments in Berea sandstone cores. Interfacial rheological results show that interfacial viscoelasticity generally increases as brine salinity is decreased, regardless of which cations and anions are present in brine. However, the rate of elasticity buildup and the plateau value depend on 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 with sodium sulfate brines. Corefloods were designed to limit wettability alteration by maintaining a low temperature (25°C) and short aging times. Geochemical analysis provided information on in-situ water chemistry. Oil-recovery and pressure responses were shown to directly correlate with interfacial elasticity [i.e., recovery factor (RF) is consistently greater the larger the induced interfacial viscoelasticity for the system examined in this paper]. Our results demonstrate that a largely overlooked interfacial effect of engineered waterflooding can serve as an alternative and more complete explanation of LS 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.

Low salinity waterflooding is a promising Improved Oil Recovery (IOR) process showing growing activity since discovery. However, incremental recovery over traditional waterflooding varies significantly. Numerous investigations have attempted to prove or disprove recovery mechanisms associated to this process. In our earlier research, we proposed that buildup of the crude oil-brine film viscoelasticity leads to suppression of trapping mechanisms during low-salinity waterflooding. We also advanced the idea that film response depends upon combined characteristics of both crude oil and water. In this paper, interfacial viscoelasticity measurements were conducted on several Wyoming crude oils as well as processed versions of the same oils with selected asphaltene content. Dual-wall ring shear rheology and pendant-drop dilational rheology were run to investigate the connection between polar content in the oil and interfacial viscoelastic response. To further investigate the connection between the interface viscoelasticity and low-salinity waterflooding mechanisms, coreflooding experiments intended to minimize geochemical events during flooding were completed using Berea sandstone. Oil recovery and pressure responses were monitored as well. Film viscoelasticity results turned out consistent with our hypothesis, namely that high content of polar components leads to high viscoelasticity of the crude oil-water interfacial film. Carefully selected coreflooding experiments were run and these results were combined with our earlier ones to unveil recovery trends. Our observations show that a good relationship exists between polar component content and interfacial viscoelasticity, and consequently with oil recovery factor, but outlying results, though favoring low-salinity waterflooding, indicate that a more complex set of interactions need to be further investigated. The conclusions of our work support an additional mechanism for low salinity waterflooding that should improve industry's ability to select candidates for this process by directing fluid-fluid characterization efforts not frequently executed at present.