Experimental Study on the Influence of Hydrochemical Properties on the Migration of Coal Fines Containing Kaolinite in Propped Fractures

Coal fines generation poses significant challenges to the efficient extraction of coalbed methane. This study aims to investigate the influence of various hydrochemical properties on the migration of coal fines containing kaolinite within propped fractures. Experimental tests are conducted using a coal sample from the No.3 coal seam in the Shanxi Formation of the Hancheng mining area. The fracture permeability, variation of mass concentration, particle size distribution, and morphology of the produced fines are analyzed. The results show that under the same hydrochemical conditions of both Na2SO4 and MgCl2 solutions, the increase of salinity leads to the increased mass concentration of the produced fines, the increased permeability of the core sample, and the decreased particle size of the produced fines. Under the same salinity (10,000 mg/L) conditions, the mass concentration of coal fines produced using the Na2SO4 solution is higher than that of the MgCl2 solution, the permeability of the sample is larger, and the particle size of the produced fines is smaller. Kaolinite added to coal fines increases the hydrophilicity and promotes coal fines migration. The presence of metal cations enhances the aggregation of particles. The average sizes of the produced fines exhibit an increase ranging from 14.81 to 49.41%. Similarly, the D90 values exhibit an increase within the range of 0.42–34.90%. The aggregation effect of Mg2+ on coal fines is greater than that of Na+. Coal fines migration dredges the propped fractures, leading to an increase in permeability. Coal fines aggregate and block fractures, resulting in pressure rise, which yields a decrease in permeability. In cases where the fines aggregation effect dominates, the aggregated fines tend to remain in their initial position within the sample throughout the experiment. Conversely, if the fines migration effect prevails, the coal fines tend to migrate toward the end of the sample, causing end blockage. This research sheds light on potential strategies for effectively managing and controlling coal fines.