Coal Fines Migration, Deposition, and Output Simulation during Drainage Stage in Coalbed Methane Production

Fracture and cleats development controls the permeability of coal reservoirs, while the migration of coal fines can block fractures and affect coalbed methane (CBM) production performance. To reveal the migration, agglomeration, and settlement of coal fines within fractures, and also to investigate permeability variations, an experimental simulation was conducted. Factors like coal fine particle sizes (<48, 48–75, 75–96, 96–120, and 120–180 μm), fracture width (0.05, 0.10, 0.15, and 0.20 mm), coal fines suspension concentration (0.5‰, 1.0‰, 1.5‰, 2.0‰, and 2.5‰), and injection flow rates (2.5, 5.0, 7.5, and 10.0 mL/min) were considered, and the deposited and output coal fines qualities were recorded. The results showed that by controlling stable injection flow rates, the injection pressures increased with the extension of time, indicating fractures were blocked gradually by coal fines. The pressure variation curves could be divided into three types: (1) steady slow rise type, (2) stable to sharp rise type, and (3) fluctuating type. The quality of deposited coal fines in fractures varied between 0.0068 and 0.917 g, and the quality of output coal fines was between 0 and 0.1028 g. With fracture width incrementation, the deposited and output values of coal fines of different sizes all increased. As coal fines concentration increased, the output values first increased and then decreased, and vice versa for deposited values. When the injection flow rate increased, the deposited coal fines values initially increased and then diminished. In the largest coal fines deposition, an injection flow rate of 5.0 mL/min was observed, while larger and stable coal fines migration and production were observed at 2.5 and 7.5 mL/min. Under most of the circumstances (except for 120–180 μm coal fines), permeability losses were concentrated between 70% and 95%, and the permeability loss with time was concentrated between 0.15 and 0.178 mD/h. Thus, the induced damages of coal fines to fractures were similar and could damage most of the flow channels. The quality of deposited coal fines was positively correlated with permeability reduction. For better coal fines output during CBM production, the flow rate should be controlled at certain rates for stable coal fines migration (e.g., 7.5 mL/min in this study). Also, further research should be focused on the dispersion of coal fines and their precise control.