Numerical simulation study on the placement behavior of composite particle size proppants in hydraulic fractures of coal rock reservoirs

Slippery water fracturing in coal reservoirs commonly employs composite proppants to improve filling efficiency; however, most studies simplify the proppant system to a single particle size, which may not accurately represent actual conditions. To address this, a CFD-DEM coupled approach with particle settings closer to field conditions was used to investigate the transport and placement of composite proppants in fractures under different engineering scenarios. Based on the simulation results, the flow capacity of different particle size combinations was evaluated. The results indicate the following: (1) Stepwise proppant injection outperforms mixed injection. For millimeter-scale coal cleavage fractures, the “70/140 + 40/70” mesh proppant size combination is recommended. (2) Adjusting the proppant size ratio can optimize sand bank distribution in branch fractures. In complex networks, increasing the 70/140 mesh proppant ratio from 1/2 to 2/3 increases the placement area in secondary cleavage fractures Frac21 and Frac22 by 2.04% and 0.8%, respectively. (3) Mixed-size proppants tend to segregate and form layered distributions, causing longitudinal variations in stacking flow capacity. Therefore, in numerical simulations, sand bank flow models assuming a single particle size may not accurately reflect fracture productivity. (4) Fracture flow capacity is positively correlated with proppant particle size. The 20/40 mesh proppant pack has higher porosity and looser packing, yielding higher post-propping fracture flow capacity, while the 70/140 mesh pack is more orderly and dense, with relatively low flow capacity. These findings provide a numerical basis for optimizing proppant placement sequences to enhance coalbed methane production.