Study on the construction of a model for particle size distribution of coal fines in the full wellbore of coalbed methane wells.

During the drainage process of coalbed methane wells in the Panhe Block of the Qinshui Basin, severe coal fines production restricts productivity release. This study aims to systematically elucidate the vertical distribution patterns of coal fines particle sizes within the wellbore, providing a basis for quantitative optimization of drainage systems. The research adopts a combination of field sampling and laser particle size testing, physical simulation experiments, and experimentally calibrated Fluent DPM numerical simulation. A database of critical carrying velocities for six particle size intervals was constructed through physical simulations and used to calibrate particle drag parameters in the numerical model. The calibrated numerical model simulated the transport and sedimentation of mixed-size coal fines throughout a 1000 m, 2-7/8-inch tubing wellbore. The results show that the median particle size (D50) at the wellhead is 30.08 μm, with a maximum particle size of 1910 μm at the bottom hole, where fine particles (< 93.56 μm) account for 60%. Physical simulation experiments indicate that the critical carrying velocity of coal fines increases nonlinearly with particle size, from 0.005 m/s for 200-400 mesh to 0.075 m/s for 20-40 mesh. Based on the particle size distribution model from numerical simulation, the D50 value differs by 5.18 times from the bottom hole to the wellhead, with particles larger than 80 μm mainly settling within a 10 m interval from the bottom hole. The study developed a two-stage model for the particle-size distribution of coal fines, combining a high-precision prediction model for the near-bottom section with a full-wellbore extrapolation model. These findings provide a theoretical basis for preventing and controlling coal fines in coalbed methane wells.