Hydraulic fracturing experiments were conducted on 15 cm-cubic Inada granite and Ogino tuff using the fracturing fluids of water and super critical carbon dioxide. Horizontal stresses of 5 and 3 MPa, and vertical stress of 1 MPa were applied to the specimen. Vertical fractures along borehole were created in the specimen by the injection of water, while horizontal fractures along the rift or bedding planes of the specimen were tended to be created by the injection of the super critical carbon dioxide. The number of branches of fractures obtained by the super critical carbon dioxide was greater than that obtained by the water. These results suggest that both viscosity of the hydraulic fluid and weak planes of rocks influence the formation of hydraulic fractures.

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