# Removal of Divalent Cations from Produced Water and Its Impact on Rheological Properties and Proppant Settling Velocity

## TL;DR

This study examines how removing divalent cations from produced water affects its flow properties and ability to carry proppant in hydraulic fracturing.

## Contribution

The paper reveals that while removing divalent cations improves viscoelastic properties, it does not significantly enhance proppant transport.

## Key findings

- Removing divalent cations increases relaxation times, improving viscoelastic gel characteristics.
- Higher friction reducer dosages increase relaxation times but still fall short of deionized water standards.
- Shear viscosity increases only modestly, with no significant impact on proppant settling velocity.

## Abstract

The petroleum industry seeks to optimize the reuse of flowback and produced water (FPW) in hydraulic fracturing to reduce environmental impacts and costs. This study investigates how controlling divalent cations in FPW influences its rheological properties and proppant carrying capacity, both of which are crucial for efficient fracturing. Synthetic FPW, modified to simulate treated and untreated conditions, was analyzed to determine the impact of gel-based additives such as anionic polyacrylamide-based friction reducers (FRs). Results indicate that removing divalent cations increases relaxation times from 0.12 s in untreated FPW to 1.00 s in a 1 gallon per thousand gallons (gpt) FR solution, demonstrating improved viscoelastic gel characteristics. However, these changes do not significantly increase proppant carrying capacity. Even with relaxation times increasing to 4.5 s at higher FR dosages (3 gpt), the treated FPW still does not achieve the relaxation time observed in FR solutions using deionized (DI) water, which remain above 10 s. The removal of divalent cations from FPW resulted in only minor changes to its shear viscosity, with a modest 15% increase that was not enough to significantly affect the settling velocity of the proppant. Thus, removal of divalent cations can positively influence rheological behavior; it does not necessarily improve proppant transport efficiency in hydraulic fracturing operations.