A thermoresponsive temporary plugging agent for drilling fluids in marine gas hydrate reservoirs: Synthesis, performance, and reversible plugging mechanism

Abstract

During drilling in marine gas hydrate reservoirs, drilling-fluid invasion can induce hydrate dissociation, wellbore instability, and reservoir damage, thereby compromising drilling safety and reservoir protection. To address the low-temperature agglomeration and limited self-unplugging capability of conventional temporary plugging agents, a water-soluble thermoresponsive temporary plugging agent (TRP) was synthesized via free-radical copolymerization. Its structural characteristics, phase-transition behavior, plugging performance, reversible unplugging capacity, and underlying mechanism were systematically investigated through laboratory experiments and low-field nuclear magnetic resonance (NMR) imaging. The results show that TRP exhibits a lower critical solution temperature (LCST) of approximately 15°C, matching the thermal conditions of marine hydrate reservoirs. Above the LCST, TRP rapidly aggregates to form a dense plugging layer, effectively suppressing drilling-fluid invasion; below the LCST, it redissolves and restores formation permeability, with a maximum permeability recovery of 96.8%. Sand-disk filtration tests further demonstrate that TRP provides more effective fluid-loss control than ultrafine CaCO₃ and nano-emulsion under the tested conditions. Low-field NMR imaging directly visualizes the invasion-plugging-unplugging process. Mechanism analysis indicates that plugging originates from hydrophobic association of polymer chains above the LCST, which enhances filter cake compactness and increases the rock-surface contact angle from 41.5° to 77.4°. This reversible thermoresponsive strategy provides a practical basis for intelligent reservoir protection during deep-sea hydrate drilling.

Keywords

Natural gas hydrates / Combustible ice / Clean energy / Unconventional energy / Gas hydrate reservoir / Water-based drilling fluids / Thermoresponsive polymers / Temporary plugging agent / Mechanism analysis / Low-field nuclear magnetic resonance imaging / Lower critical solution temperature

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