Proppant transport at the intersection of coal cleats and hydraulic fractures

This work is the first computational study of proppant leak-off through coal cleats that accounts for proppant retention in cleats, occlusion formation at cleat entrances, the resulting control of fluid leak-off, and the influence of realistic cleat roughness on these factors. The fluid-particle suspensions are directly simulated with a coupled lattice Boltzmann method-discrete element method which explicitly models all pertinent physics, including shear-thinning fluid rheology. Firstly, using a simplified computational geometry, it is demonstrated that leak-off and mounding are both minimised when proppant invades and is retained in the cleat. This occurs most effectively with wide proppant size distributions, such as 100/635 mesh. When the proppant is larger than the cleat aperture, however, occlusions form at the cleat entrance which can lead to significant mounding, which is observed for 100 mesh and 40/70 mesh. These findings are commensurate with an existing benchmark experiment. Furthermore, mounding is significantly reduced for shear-thinning fluids compared to Newtonian fluids. Simulations are then conducted with rough cleats in a computational geometry that simulates a small fraction of the total hydraulic fracture. Leak-off is smallest for high cleat roughness and when cleats are narrower than a critical width, while proppant retention is largest at the same critical width but only above a certain roughness. Mounding is primarily dependent on the width, as opposed to the roughness. These results are presented as high-fidelity maps which can be directly incorporated into hydraulic fracturing simulations for improved predictions of fluid leak-off and propped reservoir volumes, with relevance to microproppant placement in coal seam gas reservoirs. • Proppant transport through rough cleats is computationally studied for the first time. • The experimental results of the STIM-LAB inc. 1995 benchmark study are reproduced, for relationships between proppant size, cleat size, proppant mounding, cleat retention and fluid leak-off. • Shear-thinning fluids reduce proppant mounding compared to Newtonian fluids. • High-fidelity maps for proppant mounding, cleat retention and fluid leak-off in rough cleats are produced, for incorporation into hydraulic fracturing simulators.