Fabrication of Cardanol Side-Chain Hydrophobically Associating Thermosensitive Gel System with Thermally Induced Network Evolution for Deep Fluid Diversion

Conventional polymer gels for deep fluid diversion in moderate-temperature (40–60 °C) reservoirs are constrained by the chromatographic separation of crosslinkers and poor gelation controllability. In this work, a side-chain pendant decoupling molecular design strategy is proposed. Using cardanol glycidyl ether, diethylenetriamine (DETA), and polyethylene glycol diglycidyl ether (PEGO), a stepwise route exploits the differential reactivity of mono- versus di-epoxide groups to graft cardanol C15 alkyl chains as freely pendant side groups onto a PEGO/DETA covalent network. The covalent network provides a mechanical skeleton, while the pendant C15 chains act as independent thermoresponsive switches, liberating the C15 chains from topological constraints. This endows the hydrogel with a clear thermoresponsive transition around 40 °C, with G′ increasing to 980 Pa and the transmittance dropping from 28.9% to below 4%. Kinetic analysis indicated a transition from diffusion-controlled to chemically controlled network growth. The Avrami exponent n ranged from 0.85 to 1.28 across the tested temperatures. The poor linearity of the Arrhenius fits (R2 = 0.62 and 0.35) indicates non-Arrhenius behavior, consistent with the proposed synergistic dual mechanism. Given the unavoidable salinity of formation water in actual reservoirs, the effect of salinity on gelation kinetics was further examined. At 45 °C, varying the NaCl concentration from 0 to 5000 mg/L affected gelation kinetics, while the measured plateau storage moduli remained similar under the investigated conditions. Physical simulation experiments showed a plugging rate of 96.1% at 60 °C in deionized water, and in situ gelation in the core under simulated reservoir brine led to a 99.98% permeability reduction within 60 min. This cardanol-derived hydrogel with engineered side chains establishes a molecular engineering principle in which pendant grafting liberates hydrophobic chains from topological constraints, offering a new paradigm for cardanol-derived intelligent oilfield chemicals in deep reservoir conformance control.