Characterising the behaviour of hydraulic fracturing fluids via direct numerical simulation

Current design tools used for predicting the placement of proppant in fractures are based on the solution of a simplified conservation equation that is heavily dependent on empirical relationships for particle settling and suspension viscosity. In light of these shortcomings, this paper presents the development of a computational fluid dynamics (CFD) model capable of micromechanical simulation of hydraulic fracturing fluids. The model developed in this research employs the discrete element method (DEM) to represent the proppant for a range of sizes and densities. For the fluid phase, the lattice Boltzmann method (LBM) is utilised in a generalised-Newtonian form. Full hydrodynamic coupling of the LBM and DEM is achieved via an immersed moving boundary condition. The developed model has the ability to simulate Navier-Stokes hydrodynamics, a range of rheological models (e.g. Bingham, power law), thermal effects as well as electromagnetic and electrostatic forces between particles and walls. The model captures the detailed interactions of proppant particles as well as the non-Newtonian rheology of the fracturing fluid in both experimental and fracture geometries. Simulations of small-scale experiments are used to describe suspension rheology as a function of proppant concentration while small-scale fracture models explore the settling and injection of a number of candidate formulations. These results show that the direct numerical simulation (DNS) approach presented in this paper represents a potentially valuable complement to contemporary models which can provide insight on the rheology of new or novel fracturing fluid formulations as well as explore the influence of complex in-situ features on the efficacy of a hydraulic fracture. More detailed knowledge of how proppant is transported from the wellbore to the fracture tip will provide insights that could be used in the optimisation of the hydraulic fracturing process. This is particularly relevant in coal seam gas reservoirs which can include bi-directional fracture networks, non-planar fracture paths, interburden terminations and other leak-off points.