The role of rock strength heterogeneities in complex hydraulic fracture formation – Numerical simulation approach for the comparison to the effects of brittleness –

It is imperative to develop deep understanding of the behavior of hydro fracture propagation, in which numerous rock-physics factors could be involved, to manage or control fracturing to form a network of interconnected fluid pathways. Since it is known that the strength heterogeneities of rock mass are one of the factors, we conduct numerical experiments using the discrete element method to estimate how the formation of fractures are quantitatively influenced by the strength heterogeneities in the process of fracture network formation. We first justify the validity to introduce the heterogeneities in the numerical models under an assumption that the microscopic strengths of our numerical models conform to the Weibull distribution, and then the simulation of hydraulic fracturing is conducted. In our heterogeneous models, a complex fracture network growth is induced by branching of micro cracks around the tip of main fracture and pores depending on the uniformness of the surrounding stress field. It is revealed that the complication caused by the strength heterogeneities is quite different from that introduced by the brittleness, although the complication factor is thought directly related to the brittleness in practice in the field. Our results indicate that the effect of the strength heterogeneities of rock should be considered as a key factor of the complication of fracture networks, and the evaluation accuracy could be improved by taking into account the strength heterogeneities of rock.