Influence of inorganic ions in recycled produced water on gel-based hydraulic fracturing fluid viscosity

• Produced water reuse is a good water management practice in hydraulic fracturing.
• A water quality standard was developed for guar and CMC based frack fluids.
• Higher valence ions require lower concentrations to reach critical concentrations.

In an effort to determine impacts of the increased use of treated produced water in new fracs, the rheology of two fracturing fluids was observed for varying water qualities. Specific ions of interest were spiked at varying concentrations into tap water that was used as a base for the fluids. Apparent viscosities were measured using a Chandler 5500 viscometer once fluids were formulated. Empirically, it was determined that at the chosen concentrations for this study, aluminum, iron, phosphorous, potassium, and sodium all have negative impacts on fracturing fluid stability. Calcium and magnesium improved fluid stability until a critical concentration was reached, resulting in lowered viscosities and a less stable fluid. The carboxymethyl cellulose (CMC) fluid was more resilient to aluminum, potassium, and sodium, and other ions that negatively impacted stability than the guar based fluid. The guar based fluid also benefited from divalent cations more than the CMC fluid. The effect of using concentrated gel hydrated with fresh water and then diluted with concentrated ion solutions was also evaluated and did not show any improvement in fluid stability.It appears that specific cations compete for crosslink sites on the gel polymer either through shielding or complexing with active sites that the crosslinker metal would normally complex with. This results in less crosslink sites available for the quatrovalent metal and a less stable fluid. In addition to crosslinked sites that an added crosslinker would complex with, hydrogen bonds can also make a weak crosslink. In the case of calcium and magnesium, the added cations displace hydrogen bonds and form a slightly stronger crosslink. However, this crosslink is not as strong as the ones made by the added crosslinker, and when a critical concentration of calcium or magnesium is reached, the cation competes with the added crosslinker as well, reducing fluid stability.

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