Physical–Chemical Composite Plug-Removal Technology for Clayey-Silt Gas-Hydrate Reservoirs near the Wellbore

Clayey-silt gas hydrate reservoirs in the northern South China Sea are characterized by weak consolidation, fine-grained sediments, and high clay contents, making sand-control screens (SCSs) susceptible to progressive plugging during long-term production. This study investigates the plugging mechanisms of SCSs and evaluates a physical–chemical composite plug-removal method for clayey-silt gas hydrate reservoirs. X-ray diffraction (XRD), hydrate dissociation, long-term plugging, mineral dissolution, chemical plug-removal, and high-pressure water-jet experiments were conducted, together with computational fluid dynamics–discrete element method (CFD–DEM) simulations revealing the progressive transition from particle migration and deposition to pore-throat blockage under coupled fluid–particle interactions. The results show that the clay-mineral content increased from approximately 36.4% in the original sediment to 41.5% in the plugging layer, accompanied by enrichment of montmorillonite and ordered illite/smectite mixed-layer minerals. Screen plugging developed progressively from initial particle bridging to fine-particle invasion and pore-throat locking, followed by progressive compaction and structural stabilization. A mud-acid system containing 10% HCl and 2% HF was selected as the core chemical treatment for weakening the carbonate–silicate composite plugging structure, while dispersing, chelating, and hydrate-inhibiting agents were used as auxiliary components. High-pressure water jetting achieved plug-removal efficiencies of 12.4–46.2% depending on the plugging stage and operating conditions. After chemical pretreatment followed by water-jet stripping, the composite method achieved pressure-drop reduction ratios of 74.1–78.0%, 64.6–69.6%, and 85.1–85.6% at the initial, middle, and late plugging stages, respectively. These results demonstrate that chemical weakening followed by mechanical stripping provides an effective approach for restoring the flow capacity of plugged SCSs in clayey-silt gas hydrate reservoirs.