Direct observations of hydraulic fracturing in rock bridge of granite specimens in grain-scale

Abstract

Hydraulic fracturing is a crucial and widely-used technology in shale gas exploitation and enhanced geothermal systems; the rock bridge will significantly affect the crack type and coalescence pattern of the hydraulically induced cracks. This paper aims to directly observe the fracturing process in rock bridge of granite specimens in grain-scale. Speckles are sprayed directly on the granite surface, which avoids covering hair-line cracks, and image quality assessment shows that the speckle pattern performs well in digital image correlation (DIC) calculations. Hydraulic fracturing was performed on two double-flawed granite specimens; a high-speed camera captured the entire specimens, and another recorded the rock bridge in grain-scale at a high frame speed simultaneously. The strain localization, crack propagation and morphology were analyzed based on the DIC method. Results showed that strain localization occurs for the first time approximately 20 seconds before failure, which is earlier than previously observed. When the displacement jump reaches a specific value, hair-line cracks appear, and then the crack propagation is the process of crack width increasing. The competition between hydraulically induced cracks often occurs in 120° rock bridge, which will cause one of the hair-line cracks near the inner tip of the flaw to close before failure.