Study on Coal Particle Properties and Critical Velocity Model in Coalbed Methane Horizontal Wells

During the drainage process of coalbed methane (CBM) horizontal wells, wellbore fluctuations exert a significant influence on gas–liquid–solid three-phase flow behavior and coal particle migration. This study investigates the effects of wellbore inclination, gas–liquid flow rates, and coal particle sizes on migration characteristics through laboratory-scale experiments, based on an initial analysis of coal particle physical properties. A critical velocity model accounting for wellbore fluctuations is developed and refined. The migration states of coal particles under various operational conditions are examined, and the corresponding critical velocities and movement patterns are analyzed. The results show that coal particle migration is predominantly governed by the liquid phase, while the presence of particles has limited impact on the overall gas–liquid flow regime. Under different wellbore inclinations, the critical velocity increases with particle size; however, the influence of inclination is more pronounced than that of particle size. Coal particle entrainment follows three distinct stages: hopping, rolling, and suspension. The velocity during the rolling stage is identified as the critical velocity. At steeper inclination angles, particles are more easily entrained by the flow, and the associated critical velocity is higher. Based on the fitted experimental data, the model is revised to improve its predictive capability for coal particle transport in CBM wells. Finally, the model is validated using field data from a CBM well in the Ordos Basin. The results confirm the model’s ability to predict coal particle accumulation trends within the wellbore. This study provides new insights into coal particle migration mechanisms under fluctuating wellbore conditions, offering both experimental and theoretical support for understanding gas–liquid–solid flow behavior. It also presents technical guidance for optimizing drainage performance, controlling particle deposition, and formulating wellbore cleaning strategies.