Experimental investigation on particle transport of coal fines in unsteady terrain slug flow

Abstract The coal fine output from coalbed methane (CBM) formation during the production of CBM wells seems to be a common occurrence. At the lower section of wellbore, the aggregation of coal fine particles may occur, that lead to the increase of local coal fine concentration and the negative effect on the work of downhole equipment. To investigate the migration of coal fine particles in gas-liquid flow, the gas-liquid-solid three-phase flow experiments were carried out. The experiments were conducted in a downbent V-shaped pipe with different incline angles (5°, 15° and 25°), and recorded the real-time liquid holdup at certain positions of the lower elbow with the conductivity method. The terrain-induced slugs can be identified on the time series curves of liquid holdup. And the characteristic parameters of each slug, such as slug length and translational velocity, were calculated. By injecting marked coal fine particles into the multiphase flow loop and observing whether they can be discharged from the V-shaped pipe, the lower limits of gas and liquid velocities to void the particle retention at the lower elbow were collected. According to the experimental observation of the coal fine particle movement in one slug unit, we defined dimensionless particle velocity function F D to analyze the particle transport mechanism in the terrain slug flow. The F D value distribution of terrain slugs, calculated by the use of slug parameters, changed with the slug dissipation and coalescence in the experimental incline pipeline. Detail analyses of F D statistical parameters show that the mean values of F D increase with the rise of mixture flow velocity, and the decrease of inclination angle. The mean values of F D at the critical particle discharge conditions are positively correlated to particle concentration. And a linear relationship between the critical means of F D and the particle concentrations is presented.