The migration and placement patterns of proppant in the natural-artificial fracture networks within coalbed methane reservoirs

In coalbed methane reservoirs, intricate cleat fractures are well-developed and intertwine with artificial fractures. The uniform placement of fracturing proppants within these complex, interlaced cleat fractures is crucial for fracture conductivity and coalbed methane production. To investigate the impact of cleat characteristics and filling conditions in coal seams on proppant migration and placement, two fracture networks were established: one with full connectivity and another with a 50% fracture development degree. Utilizing computational fluid dynamics methods, we simulated proppant migration and settlement behavior in these two fracture network models under varying conditions, including fracturing fluid injection speed, sand-carrying fluid viscosity, and proppant particle size. We analyzed the mechanisms of these factors from three dimensions: the proportion of sand dome area, the balance height of the sand dome, and the horizontal distance from the entrance. The results indicate that: (1) Increasing the injection rate enhances the proppant's ability to migrate through multibranch fractures, and a higher injection rate can effectively fill the unconnected cleat fractures within a fracture network where the face cleats are 50% developed. When the sand filling ratio is consistent across different fracture network types, the network with 50% face cleat development requires a higher fracturing fluid injection rate. (2) Fracturing fluids with higher viscosity have stronger carrying capacity, effectively transporting proppant deeper into various fracture networks. (3) Smaller proppant particles exhibit better migration ability within the complex fracture network. When the proppant particle size is 0.25 mm, the overall sand filling ratio is similar in both development modes. Larger proppant particles can cause fracture plugging, particularly in coal seams with poor fracture development, where this plugging phenomenon is more pronounced. (4) Under the same fracturing fluid injection parameters, the overall sand filling ratio of the fully connected joint network is higher than that of the joint network with 50% developed face cleats. The migration of proppant in the fracture network with different development modes shows complexity and diversity, so it is necessary to formulate a suitable fracturing plan according to the degree of cleat development and construction conditions in the coal-rock reservoir.