Effects of fracture network connectivity in coal seams on proppant transport

The horizontal well staged multi-cluster fracturing used in the development of coalbed methane could jointly form a complex fracture network with cleats in the coal seam. Due to the different density and aperture of cleats, there are differences in the connectivity between various clusters of fracture networks, which directly affects the proppants transport within the fracture network. In order to explore the influence of the connectivity of various clusters of fracture networks in coal seams on the placement of proppants, two types of fracture networks, connected and unconnected, were established. The fracture network consists of two clusters of hydraulic fractures connected in series by a horizontal well section and combined with orthogonal cleats. The connectivity between the two clusters of fracture networks depends on the connectivity of secondary fractures. Considering the proppants sphericity, the Mixture model in the multiphase flow model of Eulerian Eulerian is used to simulate the distribution characteristics of proppants in different levels of fractures in connected and unconnected fracture networks under different slurry viscosity, slurry displacements, and proppant particle sizes. The sand filling ratio of proppants in these two types of fracture networks is compared to clarify the influence of network connectivity on the proppants distribution in fractures. The simulation results indicate that due to the interference between the sand carrying fluids entering from two clusters of fractures in the connecting part of the fracture network, there is almost no proppant in this connecting part. However, compared to unconnected fracture networks, the proppant is transported further within the main fracture of the connected fracture network. In terms of the total sand filling ratio of the fracture network, the sand filling ratio of the unconnected fracture network is much higher than that of the connected fracture network, with an average increase of 53.3% under the simulation parameter conditions of this study. In both connected and unconnected fracture networks, areas with sand concentration less than 5% in each level of fracture account for a large proportion, followed by areas with sand concentration greater than 10%, and areas with sand concentration between 5% and 10% have the smallest proportion. Further, reducing the slurry viscosity, increasing the slurry displacement, and using smaller particle size proppants can effectively improve the proppants distribution in both types of fracture networks. Especially, the slurry displacement in the interconnected fracture network should not be less than 12 m3/min, and the particle size of the proppant should not larger than 100 mesh.