Triple-Cross-Linked Seawater-Sourced Hydrogels with Extreme Thermo-Shear Stability for Ultradeep Marine Reservoir Exploitation

Abstract Hydrogels play an important role as materials in the oil and gas industry. Direct preparation of hydrogels using seawater is attractive for offshore energy production but remains challenging because conventional partially hydrolyzed polyacrylamide with a single-cross-linked network degrades rapidly via hydrolysis under high-temperature, high-salinity conditions. Here, ultrahigh-molecular-weight acrylamide (AM)/acrylic acid (AA)/2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) copolymers are combined with a Zr4+/polyethyleneimine (PEI) composite cross-linker to fabricate seawater-sourced hydrogels featuring hierarchical triple cross-linking. Upon heating, Zr4+ initially coordinates with carboxyl groups to form an ionic network. PEI subsequently undergoes chemical cross-linking with amide groups at high temperature, whereas Ca2+/Mg2+ in seawater provide additional ionic reinforcement during continuous heating. The optimized hydrogel exhibits good viscoelasticity under thermo-shear conditions from 30 to 200 °C, retaining a high viscosity of 169.5 mPa·s after the full test cycle. The long-term thermal stability experiments, elemental analysis, and density functional theory calculations reveal that AMPS-rich segments kinetically suppress amide hydrolysis and mitigate chain collapse, whereas excessive AMPS compromises network construction. This study establishes a molecular and network design strategy for water-soluble polymer-based hydrogels in extreme saline and thermal environments, enabling direct seawater fracturing in ultradeep marine reservoirs.