The migration and deposition patterns of coal fines and their initiation mechanisms during the gas–water two-phase flow stage in coalbed methane wells

Abstract The deposition of coal fines in fractures reduces fracture conductivity and hampers efficient coalbed methane production. The results of the multiphase flow visualization experiment indicate that, at low flow rates, fines primarily deposit at the bottom of fracture. At medium flow rates, they exhibit finger-like migration patterns. At high flow rates, some fines initially transported out, but later they tend to form agglomerates. Fine coal fines migrate through suspension and rolling, while medium coal fines form filter cakes both inside and outside the fractures, and coarse coal fines block gas–water flow by forming filter cakes at the fracture fronts. Smaller particle sizes result in higher fines production. The output of coal fines peaks at concentration 4% before declining. Higher concentrations promote the formation of filter cake; however, gas–water injection can effectively clear these blockages. Multiple forces drive fines migration, with capillary forces playing a significant role in this process. The initiation velocity decreases with increasing particle size, but it rises again at a particle size threshold of 22 μm. In the early stages, fines are more influenced by adhesion forces, but gravity prevails as particle size grows. At flow rates between 0.2 and 0.29 cm/s, fines ranging from 0.4 to 52.44 μm are discharged from the coal seam. The results provide insights into coalbed methane production during gas–water flow.