Most hydraulic fracturing models assume that the rock is homogeneous at a pore scale. However, reservoirs are highly heterogeneous at all length scales. Pore scale heterogeneity is evident from thin sections and scanning electron microscopic images (SEM). Heterogeneity at larger length scales is evident from logs and cores. In this paper, the effect of micro-scale (pore and core scale) heterogeneities caused by varying mineral composition and the presence of pores and microfractures, on fracture propagation has been investigated.

A model that solves the solid displacements and fluid pressures both inside and outside the fracture and allows for the creation and propagation of multiple fractures is presented. This peridynamics-based hydraulic fracturing model is used to model the growth of multiple, complex fractures in a heterogeneous rock. Thin section and SEM images of rocks are used to represent the geometry of the rock grains and the pores in several rock samples. Far-field stresses are then applied and a fluid induced fracture is propagated in the rock matrix.

The results of the model reveal that the stress distribution and the fracture geometry can be quite complex at the micro-scale. Fracture branching and turning is induced by variations in elastic moduli and stress concentration at the grain scale. The microstructure of the fracture is, therefore, determined by the geometry and distribution of mineral grains, their mechanical properties, and the initial stress anisotropy due to the co-existence of different mineral grains. A similar effect is observed at the core scale where differences in the microstructure of the rock can result in stress concentration at layer boundaries. For example, we show that the presence of a brittle mineral like calcite in the rock matrix causes fractures to branch at the mineral interface. Multiple fractures are shown to open, some that may not be in hydraulic contact with each other. As the fracture propagation continues, only the least tortuous path remains open. All other branches are bypassed hydraulically and are eventually closed. This fracture complexity occurs despite macroscopic stress anisotropy. Several examples of fracture propagation in rocks that are heterogeneous at a pore scale are provided to show that such fracture complexity should be expected in most lithologies.

These results clearly show that while we have traditionally represented fractures as planes perpendicular to the minimum horizontal stress, the fracture surfaces may indeed be much more complex due to existence of different minerals grains with widely different mechanical properties. Cracks can form away from the crack tip along planes of weakness. These damage zones resulting from strains induced by fracture propagation may explain the creation of the stimulated reservoir volume (regions of enhanced permeability) around fractures in shale reservoirs.

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