Exploring the influence of rock inherent heterogeneity and grain size on hydraulic fracturing using discrete element modeling.

The effects of rock inherent heterogeneity and grain size on hydraulic fracture initiation and propagation for different propagation regimes are investigated through two dimensional discrete element modeling. Random particle assembly is used to mimic rock inherent heterogeneity in the numerical model while regular particle assembly is used as the reference. The rock inherent heterogeneity mainly affects the hydraulic fracture net pressure in the viscosity dominated regime and the effect is more profound in the toughness dominated regime. In the toughness dominated regime, in addition to the increase of net pressure relative to the regular particle sample, the hydraulic fracture profiles in the random particle sample also show larger tortuosity and asymmetry caused by the local heterogeneity, and the fracture growth of one of the wings can be temporarily arrested. Numerical simulations show that the effective toughness of the random particle sample is larger than that of the regular particle sample. This is caused by tortuosity, in which case the net pressure in the random particle sample is also affected by the local geometrical arrangements of the particles. Also, the apparent toughness is influenced by the magnitude of initial stress, which comes in addition to the tensile strength of the contact bond and the particle radius. The effect of stress anisotropy has limited effect on the hydraulic fracture propagation for both the viscosity and toughness dominated regimes. [ABSTRACT FROM AUTHOR]