The Mechanism of Complex Hydraulic Fracture Creation Due to Strength Heterogeneity

Abstract

AbstractUnderstanding the mechanism of complex hydraulic fracture creation could develop a design and management of subsurface fluid paths in hydraulic fracturing. Although many studies about fracture network formation caused by various geophysical factors has been conducted, the influence of strength heterogeneities of rock is not fully revealed. We perform a series of numerical simulations of hydraulic fracturing with a particle method to reveal the generation mechanisms of hydraulic fractures whose geometrical complexity could depend on strength heterogeneities of subsurface materials. We use microscopic strength distribution, which is set randomly to follow the Weibull distribution, in our models as a substitute for the strength heterogeneities of real subsurface material after confirming the validity of this substitution by numerical analysis of uniaxial tensile tests. As a result of numerical simulations of hydraulic fracturing with our models with a range of strength heterogeneities, we found geometrically complex fractures are comparatively generated in the models as the increase in the strength heterogeneities. Because of weak interparticle connections included in the model due to heterogeneities, micro cracks are generated to cause the furcation of advancing hydraulic fractures around their tips. The further propagation of hydraulic fracture is strongly affected by these preceding micro cracks, and hydraulic fracture does not propagate straightly and complex network of hydraulic fractures is created. In addition, pores with specific shape and microscopically weak portions cause fracture to be complex. The direction of tensile stress induced by fluid infiltration depends on the shape of pores and fractures. The tensile stress would act in the maximum principle stress direction at the pore with specific shape and newly fracture perpendicularly to main hydraulic fracture is formed from the portion of weak strength at the pore wall. On the other hand, the models with weak strength heterogeneities show a simple linear extention of hydraulic fractures because fewer branches are formed around the tip of the advancing hydraulic fractures. In conclusion, micro cracks generated around the tip of hydraulic fractures and pores with specific shape cause the hydraulic fracture to become complex geometrically in the model after the furication with strong microscopic strength heterogeneities.