Proppant transport at the intersection of coal cleats and hydraulic fractures

Abstract

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. Suspensions are simulated with a coupled lattice Boltzmann method-discrete element method, which explicitly models all 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. However, when the proppant is larger than the cleat aperture, occlusions form at the cleat entrance, which can lead to significant mounding; this is observed for 100 mesh and 40/70 mesh. These findings are commensurate with an existing benchmark experiment. The present study additionally demonstrates that mounding is significantly reduced for shear-thinning fluids compared to Newtonian fluids. Simulations are then conducted with rough cleats in a realistic fracture channel. 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, and which can be tailored for different treatment and reservoir conditions.