Effect of Cleavage Anisotropy on Hydraulic Fracturing Behavior of Pocheon Granite

The main bedrock of Pohang Enhanced Geothermal Systems (EGS) in Korea consists of granite and granodiorite. Anisotropy is often among the important feature of granites, which is represented by three orthogonal cleavage planes frequently called rift, grain and hardway, and they have different physical and mechanical properties. In this study, we investigate the influence of cleavage anisotropy on hydraulic fracturing behavior of granites. A series of hydraulic fracturing tests were performed on granite specimens collected from Pocheon, and cored along the three different directions of cleavages. X-ray CT technique was used to observe the fractures inside the specimens after testing. It is found that under the same conditions, the mean breakdown pressures of fractures occurring along the rift cleavage is around 2 MPa lower compared to the mean value of those happening along the grain plane. This is because the rift plane has the lowest tensile strength. Moreover, fracture roughness analysis shows a small difference between the fractures occurring along the rift and grain cleavages.