Study of natural and hydraulic fracture interaction using semi-circular bending experiments

Hydraulic fracturing is an indispensable technique for developing unconventional resources such as shale gas and tight oil. When hydraulic fractures interact with pre-existing natural fractures, it can result in a complex fracture network. The interaction depends on in-situ stresses, rock and natural fracture mechanical properties, approach angle and hydraulic fracture treatment parameters. Most simulation studies treat natural fractures as frictional interfaces with cohesive properties. However, from core observation, partially cemented and fully cemented natural fractures are widely present and it is not clear whether they would fit the common description. In this study, semi-circular bending test is utilized to examine the propagation paths and strength of samples with pre-existing cemented fractures. Synthetic hydrostone samples are used to represent the rock and different inclusion slices with different mechanical properties are used to mimic cemented natural fractures. In a series of experiments, we assess the influence of the fracture approach angle, inclusion strength, and inclusion thickness on fracture propagation. Current results show that fractures tend to cross the inclusion when the approach angle is high and divert into the inclusion when the approach angle is low. The crossing surface is not a clean cut, but often has a jog distance. The thickness of the inclusion does not change the crossing/diverting behavior for orthogonal approaching samples, however it does change the jog distance along the interface. Preliminary simulation results using finite element software, ABAQUS, are presented better to analyze the experimental observations. The assessments of fracture interaction in this study are in good agreement with previous work and theories.