Sand transport characteristics and critical sand-carrying velocity of gas-water mixtures in wellbores during hydrate production

• Sand transport mechanisms identified under multiphase flow. • Inclination angle strongly affects critical transport velocity. • Gas-liquid ratio alters sand suspension regimes. Sand production in weakly cemented silty-fine sediments represents a critical bottleneck for the sustained efficiency of horizontal hydrate production wells. This study transcends site-specific observations to elucidate the fundamental gas-water-sand multiphase transport mechanisms through systematic laboratory simulations. By isolating the synergistic coupling of inclination angles and gas-liquid ratios (GLR), the evolution of particle-fluid interactions was characterized. Results reveal three distinct transport regimes: wall-concentrated, critical, and wall-dispersed flow. A mechanical “turning point” was identified at a deviation angle of 55°, where the sand-carrying capacity reaches its minimum. This phenomenon is mechanistically attributed to the extremum in the transverse gravitational component and interfacial friction, which maximizes particle slippage. Quantitatively, the transport efficiency exhibits a non-linear sensitivity to GLR, with a critical threshold at 50; beyond this value, the marginal enhancement of gas-phase drag diminishes. Furthermore, the critical sand-carrying velocity is found to be physically coupled with the churn-to-annular flow transition, where the motive force shifts from liquid-phase buoyancy to gas-phase shear. A mechanistic-empirical model was established with high fidelity (R 2 = 0.99577), demonstrating a liquid-phase saturation effect where additional liquid volume provides negligible gains in carrying capacity. These findings provide a scalable theoretical framework and precise operational envelopes for optimizing sand management strategies in marine hydrate recovery.