Start-migration law of coal powder with different particle sizes under multi-phase flow conditions in coalbed methane wells

In order to systematically study the pulverized coal start-transport law under multi-phase flow conditions in coalbed methane wells, this paper took sand pulverized coal as the research object and controlled pulverized coal output by controlling different phase flows (liquid-solid two-phase, gas-liquid-solid three-phase). It carried out a pulverized coal start-transport test simulation of different particle sizes (less than 0.075 mm, 0.075～0.25 mm, 0.25～0.85 mm, over than 0.85 mm) in multi-phase flow conditions, and then analyzed the effects of flow rate, differential pressure, pipe inclination and grain size on coal start-transport. The results showed that under the liquid-solid two-phase flow, with the gradual increase of liquid flow, the pulverized coal gradually transitions from static state to sliding state to intermittent rolling state to rolling state to layer moving state to layer moving and suspending state and then to suspending state. The smaller the particle size of pulverized coal was, the lower the inclination angle of pipeline would be and the lower the starting flow rate would be; the flow rate showed a good linear relationship with the pressure difference, and they were both affected by the inclination of the pipeline.The particle size of pulverized coal and the inclination of the pipeline hadan important influence on the difficulty of starting-transporting of the pulverized coal, but the influence of both was complicated. The start-transport flow of different particle sizes of pulverized coal is not a simple linear relationship with the inclination of the pipe. The relationship between the particle size of pulverized coal particles (except for large-sized pulverized coal larger than 0.85 mm) and the starting flow rate can be characterized by a linear relationship at different angles, and the equation can also be used to predict the starting flow rate of coal powder with different particle sizes under specific pipe inclination angle.Under the gas-liquid-solid three-phase flow, the main controlling factor of pulverized coal start-transport was flow pattern, which was affected by gas-liquid flow ratio and pipe inclination, it which mainly included bubble flow, plug flow, stratified flow, wavy flow and slug flow. The transport capacity of each flow pattern is gas flooding laminar flow > slug flow > plug flow > bubble flow > stratified flow.With the increase of gas-liquid ratio, the pressure difference decreased rapidly first and then tended to be flat, and eventually rebounded slightly. The smaller the inclination angle of pipeline was, the faster the pressure difference decreased.