Experimental characterization and flow laws of graded coal fines under rheological modulation in coalbed methane horizontal wellbores

Initial drainage in coalbed methane (CBM) horizontal wells is critical, as adhesive micron-sized coal fines mix with water to form slurry, altering flow and bed formation. These effects impose long-term constraints on drainage strategy regulation during subsequent production. Existing studies overlook rheology changes in the mixed slurry caused by micron-sized coal fines. This limits guidance on bed evolution during the initial drainage phase. To this end, we experimentally determined the viscosity of the mixed slurry with 0%–6% micron-sized coal fines by mass. We then established a fitted relationship between viscosity and micron-sized coal fines mass fraction. On this basis, we used the Hertz Mindlin–Johnson Kendall Roberts contact model to describe the coal particles adhesion behavior. A Computational Fluid Dynamics-Discrete Element Method coupled model was developed, integrating slurry rheological characteristics with particle adhesion. This framework links experimental characterization to the flow patterns of graded coal fines in horizontal CBM wellbores. The results show that the viscosity of the mixed slurry is one of the key factors controlling the evolution of bed thickness and morphology. Its influence becomes more pronounced at higher drainage velocities. At a drainage velocity of 0.3 m/s, increasing the viscosity from 1.03 to 2.00 mPa s reduces the bed thickness by approximately 21.6%. The proposed model provides theoretical support for regulating drainage strategies during the initial drainage stage of CBM horizontal wells.