Waterflooding via injection of chemistry-optimized low-salinity – also, low ionic strength/concentration – waters, such as SmartWater, is becoming increasingly attractive for improved oil recovery, especially in carbonate reservoirs. In this manuscript, we describe the results from a series of experiments and theoretical modeling to determine the mechanisms that govern the ‘SmartWater Effect', whereby reducing the ionic strength (concentration) of the injection fluids (compared to high ionic strength formation water), also known as SmartWater flooding, has been found to improve oil recovery.
We measured various interrelated crude-oil(CO)/brine(W)/calcite(R) interfaces, focusing on their physical and chemical – both static and dynamic – changes, such as contact angles, macro- to nano-scale surface topography (e.g., roughening, restructuring), and surface chemical composition (e.g., due to dissolution, precipitation). The experimental aqueous brine solutions varied in ionic strengths ranging from 350,000 ppm (high ionic strength, ~7 mol/L) to pure water (ultra-low ionic strength). Our results indicate that the SmartWater Effect on decreasing the CO/W/R adhesion energy – which results in increased water-wettability and, in turn, increased oil recovery – in carbonates is due to three different but interrelated mechanisms. We propose a semi-quantitative model to explain these effects, and demonstrate numerical solutions using realistic values for the relevant system parameters. From our experimental results and theoretical modeling, we conclude that the SmartWater Effect is due to the combination of: (1) changes to the well-known colloidal interaction forces (electric double-layer, van der Waals, and hydration), which has been the conventional explanation for the SmartWater Effect in carbonates; (2) increased roughness due to (electro)chemical reactions involving dissolution, pitting, and adsorption-(re)precipitation, resulting in physico-chemical changes (roughening, restructuring) of the calcite surfaces, especially at low ionic strengths. Both of these effects act together synergistically to reduce the adhesion energy between the oil and rock (calcite) surfaces across the aqueous brine (‘water') film, which increases the water-wettability; and (3) detachment of organic-ionic layers that adsorb onto the rock surfaces during aging as thin and suspended flakes. The detachment of these flakes into the solution removes organics from the rock surfaces, thereby directly increasing oil recovery. All three of these interrelated contributions – reduced colloidal forces, increased surface roughness, and detachment of pre-adsorbed organic-ionic layers – appear to be essential for the SmartWater Effect to be fully effective at all solution concentrations. We also discuss the very different time-scales or ‘dynamics’ of these three processes, and their relationships to flooding rates and core pore geometry and topography.
The results presented in this manuscript are of practical significance to provide a better understanding of SmartWater flooding mechanisms in carbonates at multiple length scales, including subnano-, nano-, micro-, and macroscopic scales. The new fundamental understandings presented in this study will also guide the optimization of SmartWater flooding processes in other reservoir systems.