Abstract: Hydraulic fracturing is widely used for the stimulation of oil and gas reservoirs in addition to the reconstruction of enhanced geothermal systems that target reservoirs. However, hydraulic fracturing behaviour is not comprehensively understood and requires further investigations, such as the damage process of crack initiation, propagation, and coalescence. Therefore, a series of hydraulic fracturing tests on granite specimens was conducted. Through a specially designed hydraulic seal device, the fracturing processes were captured by a high-speed camera. Direct observations of fracturing processes at different scales were achieved through lenses with different magnifications, and displacement and strain fields were obtained during the whole fracturing process based on DIC method. Some experimental laws on hydraulic fracturing were obtained: The geometry of pre-existing cracks and stress conditions affect the damage process and the failure mode. Before hydraulic fracturing, the obvious strain localization appears near the pre-existing flaws. Further mesoscopic observations show that the strain localization, full of microcracks, is the damage process zone of the specimen. The degree of damage increases further with the increase of pressure until crack propagation. Therefore, damage variables can be proposed to describe the hydraulic fracturing process according to the strain field. The accumulation of damage before hydraulic fracturing may take tens of seconds, while the crack propagation process is very short, only serval micro seconds.

Biography: Dr. Cheng Zhao earned his PhD degree at Nagasaki University, Japan in 2009 and then worked at Tongji University, China. At present, he is a Professor at Tongji University, China. Currently, he is focusing on brittle materials, especially on rock mechanics and geological disasters. He has published more than 80 journal paper, and he is also a peer-reviewer of more than 10 international periodicals.