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.
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 laboratory hydraulic crack tip has always been a difficult problem to the researchers. Hydraulic fracturing is an inherently unstable process. Even the controlled and overall stable hydraulic fracture propagation is unstable at the small scale. Most of the physical process in oil and gas well engineering is a combined dynamic (Dong et al., 2019a; Chen et al., 2018; Meng et al., 2019a; Meng et al., 2019b; Tan et al., 2019) and quasi-static process (Shan et al., 2018; Wu et al., 2017; Li and Gao, 2019; Zhao et al., 2018; Xiong et al., 2019), but researchers usually treated them as a quasi-static process, including the propagation of hydraulic fracture. The rock property has also been one of the main issues that the researchers considered (Chen et al., 2017; Geng et al., 2017; Geng et al., 2018; Li et al., 2018).