Transport mechanism of coal particles in multiphase flow during early coalbed methane production

During coalbed methane dewatering and production, variations in the dip angle of coal seam create uneven V-shaped grooves in horizontal wellbores. To investigate the critical velocity and transportation of coal particles under different pipeline structures and operating conditions, this study combines experiments to analyze pressure fluctuations and flow pattern changes in a gas–liquid–solid three-phase system during the early dewatering stage. The intrinsic transport mechanism and critical discharge velocity of coal particles are identified. Additionally, simulations are used to calculate the coal particle deposition thickness and transportation efficiency under various conditions, providing direct guidance for field operations. Results show that as the gas mass flow rate increases, the periodic pressure differential in the V-shaped pipe increases, and the flow pattern transitions from the slug flow to the intermittent flow. Changes in the inclination angle increase the critical velocity for coal particles and accelerate flow pattern transitions, with a direct shift to plug flow at 35°. A reduced fluid velocity and increased coal particle concentration significantly increase deposition thickness, particularly in pipes at 45°. Moreover, the transport efficiency at a velocity ratio of 0.4 is only 86%, increasing with velocity ratio in a power function while decreasing exponentially with inclination angle under certain conditions. Consequently, matching appropriate fluid velocity with the pipeline design and discharged coal particle concentration can effectively mitigate horizontal well blockage.