Compatibility and Rheology of High-pH Borate Gels Prepared With Produced Water for Hydraulic-Fracturing Applications

Abstract

Summary Fracturing fluids are commonly formulated with pond water to ensure reliable rheology. However, pond water is becoming more costly, and in some areas, it is difficult to obtain. The use of produced water in hydraulic fracturing has gained increased attention in the last few years, because it could solve freshwater-acquisition difficulties and reduce disposal costs. A major challenge, however, is its high content of total dissolved solids (TDSs), which could cause formation damage and negatively affect fracturing-fluid rheology. The objective of this study is to investigate the feasibility of using produced water to formulate crosslinked-gel-based fracturing fluid. This paper focuses on the compatibility of produced water with the fracturing-fluid system and the effect of salts on the fluid rheology. Produced-water samples were analyzed to determine concentrations of key ions. The fracturing-fluid system consisted of natural guar polymer, borate-based crosslinker, biocide, surfactant, clay stabilizer, scale inhibitor, and pH buffer. Compatibility tests of the fluid system and its components were conducted at different ion concentrations. Apparent viscosity of the fracturing fluid was measured with a high-pressure/high-temperature (HP/HT) rotational rheometer. All rheology tests were conducted at 300 psia and 180°F with a 3-hour test duration. Further investigations to study the effect of adding chelating agents to the fluid system were also carried out. Results indicate the potential of untreated produced water to cause precipitates and, hence, formation damage. Precipitates were successfully prevented by diluting the produced water with fresh water. Divalent cations were found to be the main source of precipitation, and reduced amounts of each ion were determined to prevent precipitations. The separate and combined effects of Na, K, Ca, and Mg ions on the viscosity of the fracturing fluid were also studied. Regardless of the concentration of monovalent cations, divalent cations reduced fluid viscosity by up to 100 cp. Monovalent cations reduced the viscosity of fracturing fluid only in the absence of divalent cations, and showed no effect in the presence of Ca and Mg ions. The use of chelating agents has reduced the precipitation of divalent cations and enabled the formulation of fracturing fluid at higher Ca and Mg concentrations. Some chelating agents showed the ability to complex with the boron ion and/or reduce the system's pH value; consequently, viscosity measurements indicated the breaking of the fluid viscosity after the addition of the chelating agent. This paper contributes to the understanding of the main factors that enable the use of produced water for hydraulic-fracturing operations. Maximizing the use of produced water could reduce water-disposal costs, mitigate environmental impacts, and solve freshwater-acquisition challenges.