Salt-Tolerant Non-Crosslinked Polymer Networks for High-Temperature Weighted Fracturing Fluids in Deep Hydrocarbon Reservoirs

Abstract

High-density potassium formate fracturing fluids require polymers that hydrate in concentrated brine while providing low friction and proppant suspension at high temperature. We hypothesized that the 2-acrylamido-2-methylpropanesulfonic acid (AMPS) content balances salt-tolerant hydration against molar-mass-dependent chain entanglement, enabling a reversible non-crosslinked network. Five hydrophobically associating copolymers based on acrylamide (AM) and AMPS, with AMPS feed contents of 5.5–23.8 wt%, were synthesized by low-temperature micellar copolymerization followed by post-hydrolysis. Polymer structure, molar mass, and solution conformation were determined by proton nuclear magnetic resonance spectroscopy and size-exclusion chromatography coupled with multi-angle light scattering. Hydration and thickening were evaluated in 1.40 g/cm3 potassium formate brine, followed by rheological testing at 160 °C, pipe-flow drag-reduction testing, and static proppant-suspension testing. Increasing the AMPS feed content decreased the weight-average molar mass from 25.83 × 106 to 9.26 × 106 g/mol, increased the chain-conformation exponent from 0.570 to 0.610, and shortened the time required to reach 90% hydration (t90) from approximately 118 to 48 min. SN-4, containing 17.9 wt% AMPS, provided the best overall balance. A 0.60 wt% SN-4 fluid retained approximately 63 mPa·s after shearing at 160 °C and 170 s−1 and recovered 92.1% of its viscosity after 300 s. Fresh and thermally sheared–recovered fluids maintained drag-reduction rates above 60%, and the recovered fluid reduced proppant settling velocity by 97.4%. These results support the proposed balance and demonstrate the potential of this reversible network for high-temperature weighted fracturing fluids in deep hydrocarbon reservoirs.