Study on Abnormal Viscosity Development in High-TDS Produced Water

With limited availability of fresh water, more and more stimulation operations turn to produced water. In such operations, polymers must sufficiently hydrate first to enable effective crosslinking and ensure proppant transportation and placement. On the other hand, produced water from unconventional formations (such as Marcellus and Bakken shales) is famous for its high total dissolved solids (TDS) and high divalent cation content. Produced water contents change from time to time, and from location to location, posing a tremendous challenge in stabilizing the performance of the fracturing fluids prepared in such water. Predictable performance of fracturing fluids made with unpredictable produced water is highly desirable in order to utilize produced water.

This paper will present studies of hydration behavior of non-charged polysaccharides, such as, guar, HPG (hydroxypropyl guar) and HEC (hydroxyethyl cellulose), in water with controlled salinity and multivalent cation concentration, including Ca²⁺, Mg²⁺, Fe³⁺, etc., under various temperatures. The results are surprisingly more complicated than most may assume. For example, higher salinity may suppress linear gel viscosity, while multivalent cations do exactly the opposite. Contrary to the common knowledge that viscosity generally decreases with elevated temperatures, in some temperature ranges, an opposite trend is observed instead; and over a wider temperature range, a U-shaped trend is observed. Such counter-intuitional results will be presented and the mechanism will be explored to explain such observations. Semi-empirical equations will be proposed to predict the viscosity response of polymers in real-world produced water. Results generated in laboratory synthetic high-TDS water are then compared to those obtained in field produced water to validate the study. With systematic studies, optimized formulas of fracturing fluids based on various produced water samples have been found for stimulation operations for unconventional formations and conventional formations as well.

This paper will provide in-depth understanding of effects of a number of produced water ingredients on the performance of the fracturing fluids prepared with produced water, with emphases in areas such as polymer hydration, crosslinking, and high-temperature stability.