Mechanism of complex fracture creation in hydraulic fracturing

Abstract

Hydraulic fracturing is an essential technique for the development of unconventional oil reservoirs. If fracture network formation is evaluated before a real practice of hydraulic fracturing, the permeability of the rock could be optimized after the fracturing. Since a lot of factors are involved in fracture complexity, the mechanism of fracture network formation is not fully revealed. Although the strength heterogeneities of rock mass is known to be one of the factors, strength heterogeneities is rarely taken into consideration because of less understanding of the influence on complex fracture creation. We perform a series of numerical simulation using the discrete element method and investigate the mechanism of fracture network formation, focusing on the strength heterogeneities and brittleness, which is often used as an index of fracture network formation. In heterogeneous models, complex fracture is formed by micro cracks generated around the tip of main fracture and pores with specific shape. On the other hand, in a model with high brittleness, a lot of branches are created by shear failure with main fracture propagation. These results indicate that the mechanism of complex fracture formation due to strength heterogeneities is completely different from that due to brittleness, and that the effect of the strength heterogeneities of rock should be considered as a key factor of the complication of fracture networks.