A growing number of hydraulic fracturing stimulation treatments rely solely on the deployment of 100-mesh (i.e. 150μm) proppant. Further, these may be preceded by post-pad injection of microparticles (i.e. 5-75μm) with the intent of activating natural fractures. The objective of this work is to describe fundamental new insights on the behaviour of polydisperse microparticle suspensions in hydraulic fractures in terms of screen-out and leak-off.

This study was undertaken using a high-fidelity computational model of suspension transport based on the lattice Boltzmann method (for fluid mechanics) and discrete element method (for particle mechanics). The approach has been previously validated against analytical solutions and experimental data for suspension flows in channels, and applied to study fundamental aspects of size segregation and clogging (i.e. screen-out) of microproppant. This fully-resolved modelling approach captures two-way hydrodynamic coupling, electrostatic interactions, fracture roughness and tortuosity, and non-Newtonian fluid rheology. The sole assumption of significance is that particles are assumed to be perfectly spherical.

One of the challenges of deploying micron-scale proppant in hydraulic fracturing stimulation treatments is poor control of the particle size distribution. This inevitably results in a proportion of the injected proppant being small enough to be susceptible to electrostatic (rather than just mechanical and hydrodynamic) forces. This study clearly demonstrates the increased probability of screen-out that results as a consequence of electrostatic particle-particle and particle-wall interactions, and shows how this effect reduces as the minimum particle size increases. Analysis of particle leak-off off into transverse cleats also demonstrated how the combination of cleat width and particle size result in the formation of occlusions at cleat intersections, the reduction of leak-off rates in cleats, and the retardation of proppant transport in the primary fracture. These findings are significant because tight size control is not economically feasible for microproppant, and so hydraulic fracture engineering must accommodate their characteristic behaviours. This will result in more effective stimulation of coalbeds, but is applicable to all jobs where small proppant is proposed for injection into natural fracture systems.

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