Modulation of the Viscosity of Guar-Based Fracking Fluids Using Salts

Fracking is an enhanced oil recovery technology, which uses viscoelastic fluids (fracking fluids) to fracture oil reservoirs and to transport sand within the fractures, to prop them open. This technology enables oil recovery from scarcely permeable formations. Fractured formations release saline water over time. This saline water (called produced water) is discarded rather than used to produce fracking fluids because it can decrease fracking fluid viscosity. Nonetheless, it would be advantageous to use produced water to reduce freshwater consumption and wastewater production. Our study analyzes the effect of chloride salts (CaCl2, MgCl2, and Fe(III)Cl) and of sulfate salts (MgSO4 and FeSO4) at different concentrations (0.05-1 M) on the viscosity of aqueous guar solutions. All chloride salts tested increase the viscosity of guar solutions in the concentration range analyzed and promote the formation of small guar aggregates. At 0.05 M concentrations, MgSO4 has effects similar to chloride salts. In contrast, 1 M MgSO4 decreases the viscosity of guar solutions. FeSO4 also decreases the viscosity of aqueous guar solutions, at either 0.05 or 1 M concentrations. The decrease in viscosity of guar solutions is attributed to large guar aggregate formation (as opposed to a cohesive network). Sodium cocoyl glutamate (SCG) increases the viscosity of non-cross-linked guar solutions and the shear viscoelastic moduli of guar solutions cross-linked with sodium tetraborate. Specifically, SCG restores the viscosity of guar solutions with MgSO4 and increases it above values measured in deionized (DI) water in the presence of MgCl2. Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy shows that hydrogen bonding was more significant in guar + SCG + 0.7 M MgCl2 samples than in guar + SCG + 0.7 M MgSO4, indicating that the formation of a hydrogen-bonded network was correlated to high viscosity. ATR-FTIR also indicates that MgSO4 weakened hydrogen bonding of water clusters, whereas SCG restored it, enabling guar hydration even in the presence of MgSO4. Our study highlights which salts are most problematic (e.g., FeSO4) and proposes a potential additive (SCG) to enhance the viscosity of guar in the presence of selected salts (e.g., magnesium salts) by promoting hydrogen bonding.

Automated summary

Fracking is an enhanced oil recovery technology that uses viscoelastic fluids to fracture oil reservoirs. The study shows that chloride salts and sulfate salts at different concentrations can affect the viscosity of aqueous guar solutions, which is important for hydraulic fracturing applications.