Effect of Water Infiltration, Injection Rate and Anisotropy on Hydraulic Fracturing Behavior of Granite

Hydraulic fracturing tests on Pocheon granite cylinders at seven different injection rates varying from 1 to 100 mm3/s were conducted. They were compared with sleeve fracturing tests in which borehole was sleeved, and therefore, water infiltration influence was excluded. Hydraulic fracturing behavior of granite is significantly influenced by water infiltration, which is closely related to the preexisting microcracks in granite as well as the cleavage anisotropy. There was a threshold injection rate to fracture the granite specimen under given stress conditions. When the injection rate is below the threshold, water infiltrated granite matrix with slow increment of injection pressure, and the specimen finally reached a full saturation without fracturing. Injection pressure developed nonlinearly with time during water infiltration, while approximately linearly when infiltration was excluded. For both hydraulic and sleeve fracturing tests, breakdown pressure increases with increasing injection rate. The breakdown pressures by sleeve fracturing were more than two times higher than those in hydraulic fracturing. X-ray computed tomography (CT) observations show that induced fractures are along the weaker cleavage parallel to the direction of the vertical stress. The higher breakdown pressure results in a larger aperture of fractures in hydraulic fracturing tests.