  We numerically study mechanisms and conditions of fracture initiation in
granular material induced by non-Newtonian polymer solutions. A coupling
approach of computational fluid dynamics and discrete element method is
utilized to model the fluid flow in a porous medium. The flow behavior of
polymer solutions and drag force acting on particles are calculated based on a
power-law model. The adequacy of the numerical model is confirmed by comparing
the results with a laboratory experiment. The numerical results are consistent
with the experimental data presenting similar tendencies in dimensionless
parameters that incorporate fluid flow rate, rheology, peak pressure, and
confining stress. Results show that fluid flow rate, rheology, and solid
material characteristics strongly influence fracture initiation behavior.
Injecting a more viscous guar-based solution results in wider fractures induced
by a grain displacement. A less viscous XG-based solution creates more linear
fractures dominated by an infiltration. The peak pressure ratio between two
fluids is higher in rigid material compared to softer material. Finally, the
dimensionless parameters $1/\Pi_1$ and $\tau_2$, which consider fluid and solid
material properties accordingly, are good indicators in determining fracture
initiation induced by shear-thinning fluids. Our numerical results show that
fracture initiation occurs above $1/\Pi_1 = 0.06$ and $\tau_2 = 2\cdot
10^{-7}$.
