Investigation of the conductivity of a proppant mixture using an experiment/simulation-integrated approach

Abstract

Proppant selection in hydraulic fracturing operations is a crucial decision that influences the productivity and performance of stimulated wells. In hybrid completion designs, proppants of various sizes and materials are often mixed and incorporated into the pumping schedule. Optimization of mixing proppants of various sizes and materials has the potential to maximize proppant pack conductivity and to enhance the reservoir production performance. In this work, an experiment/simulation-integrated workflow, which combines the discrete element method (DEM) and lattice Boltzmann (LB) modeling with laboratory penetrometer experiments, was adopted to investigate the effects of proppant mixtures of different sizes and materials on the fracture conductivity. The DEM was used to simulate proppant compaction and rearrangement. Proppant embedment was determined by a load-embedment correlation obtained from the penetrometer experiments. The pore structure of the proppant pack was extracted from the DEM model and then imported into the LB simulator as internal boundary conditions of flow modeling to determine the time-dependent conductivity of the proppant-supported fracture. The integrated workflow demonstrates that the conductivity of a mixed-sized proppant pack is not simply the arithmetic average of the conductivities of the pure proppant packs. The selection of proppants with close sizes has a minor impact on the fracture conductivity when proppants develop to multilayers; whereas mixing with a much finer proppant size will downgrade the fracture conductivity significantly. It is demonstrated that proppant size combination has a lesser effect on the fracture conductivity compared to proppant material combinations. This study also suggests that the application of combinations of sand and premium proppants and modification of proppant injection schedule in the fracturing treatment design can mitigate fracture closure and improve fracture conductivity. This study investigated fundamental mechanisms of proppant mixture at the pore scale, which have significant implications to the optimization of hydraulic fracturing and proppant placement designs.