Experimental study on the characterization of hydraulic fracture tip using digital image correlation method

Hydraulic fracture is of crucial importance in the enhancement of unconventional oil and gas production. Due to the high requirements for the experimental technics, the characterization of hydraulic fracture tip has always been a difficult problem to the researchers. Using digital image correlation method, this study provides an experimental technic to identify the full field displacement of the 2-dimensional hydraulic fracture tip. A small-scale hydraulic fracture device based on double cantilever beam theory is used to induce Mode I hydraulic fracture. Optical microscope is used to observe the morphology and the development of the process zone of hydraulic fracture tip. Digital image correlation method is used to calculate the displacement of the hydraulic crack tip during its propagation. Results show that the process zone develops before the initiation of hydraulic fracture. The sizes and the morphologies of the process zone near the hydraulic fracture tip are different in various materials. The process zone in granite consists of small fissures and damaged rock pieces. The digital image correlation method reveals the details of the process zone, which is not identifiable via naked eyes. The direct observational results also show that the hydraulic fracture is prone to propagates alone the boundary of the granules. The use of digital image correlation method is proved to be powerful as an experimental technic in the characterization of laboratory hydraulic fracture. The study provides a better understanding for the initiation of the hydraulic fracture.