Study On Coal Fine Particle Deposition And Accumulation In Undulating Wellbore

Pulverized coal from formation is inevitable in the production process of CBM wells.Due to the insufficient fluid velocity and the accumulation of pulverized coal in the low part of the flow passage,increasing the concentration of pulverized coal may affect the normal operation of underground equipment.As an organic-inorganic mixture,pulverized coal has different characteristics of gravel.In the fluctuant pipeline,the force of particles in the flow field is changed due to the change of flow direction.In order to study the properties of pulverized coal particles,a series of experiments are given.The density of pulverized coal,settling speed and critical starting speed in liquid phase are measured.In order to study the movement rule of pulverized coal particles in gas-liquid flow,experiments of gas-liquid-solid three-phase flow in elbow section were carried out.The real-time data of liquid holdup at representative positions of pipe segments were recorded by conductivity method.The fluctuating slug at the measuring unit can be identified based on the liquid holdup data,and the characteristic parameters of each slug,such as slug length and velocity,can be obtained.The dimensionless particle velocity function F_D is defined by simplifying and modeling the movement of pulverized coal particles with liquid phase in a slug cell.By injecting labeled pulverized coal into the gas-liquid-solid three-phase flow loop and observing whether it can be discharged from the pipeline,the critical conditions of gas-liquid flow velocity that pulverized coal will not be detained at the bottom of elbows with different inclinations are obtained.The dimensionless particle velocity function F_D corresponding to each slug element is calculated according to the measured fluctuating slug parameters.The influencing factors of F_D distribution characteristics are analyzed,and the F_D average value under the critical condition of pulverized coal discharge is given.According to the measured critical F_D average value at the outlet of the inclined tube,the linear relationship between the critical F_D and the particle concentration was fitted.Based on the gas-liquid two-phase drift model(DFM model),the dynamic model of gas-liquid-solid three-phase flow in undulating pipeline can be obtained by adding the motion equation and mass equation of solid phase.By using a discrete strategy,the calculation of the kinetic parameters of solid particles can be added without changing the gas-liquid calculation process,which reduces the workload of solving the model.According to the experimental data of three-phase flow,the empirical parameters of the formula used in the model are adjusted,so that the calculated results of the model are in better agreement with the experimental results.The gas-liquid-solid three-phase flow dynamic model was used to simulate a series of cases.Gas-liquid flow velocity conditions,pulverized coal concentration,particle size,injection concentration,pipeline dip will affect the distribution of pulverized coal in the fluctuating section and the concentration change at the feed point in the well section.According to the dynamic model,the dynamic change of particle concentration can give the instantaneous peak value of concentration under the condition of unstable gas-liquid flow,which can provide a basis for the design of solid-phase treatment capacity of downhole equipment.The results show that the concentration of pulverized coal particles in water phase increases gradually with the production process because of the agglomeration of pulverized coal particles at the bottom of the undulateing wellbore section.