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

Abstract

Sand production in weakly cemented silty-fine sediments poses a significant challenge to the stable and efficient operation of horizontal hydrate production wells. This study moves beyond site-specific observations to elucidate the fundamental gas-water-sand multiphase transport mechanisms through systematic laboratory simulations. By examining the coupled influence 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 critical deviation angle of 55° was identified, at which 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 nonlinear dependence on GLR, with a critical threshold around 50, beyond which further increases provide limited enhancement in gas-phase drag. 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 (R² = 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.