In order to clarify fracture initiation and propagation in unconsolidated sand, we carried out hydraulic fracturing tests in laboratory focusing on moderate-permeability sand. Silica sand with particle size of about 100 μm has been generally used as the simulated formation materials in laboratory fracturing experiments. Permeability of such sand is high in the order of few darcy. Thus we mixed finer particles of kaolinite with sand, and finally succeeded to prepare the mixture of sand and kaolinite with permeability of about 5 mD. We layered the mixture in a mold to form a cubical specimen of 200 x 200 x 200 mm3. At fracturing tests, the specimen was set in a loading frame of a newly-developed test system. Tri-axial compressive stresses of 2∼3 MPa were applied to the specimen. Fracturing fluid of a machine oil with viscosity of 300 cP was pumped by a constant rate of 10∼100 mL / min into a simulated borehole of a steel pipe which was buried vertically in the specimen, and then the fluid flew out inside the specimen passing through the slit of the steel pipe. After the tests, we excavated the specimen bit by bit and observed how the fracturing fluid invaded into the specimen.
The laboratory tests demonstrated that fracture-like parting / fracture formation at macro scale was induced in the specimen by fluid injection. During fracture propagation, the pore pressure in the vicinity ahead of fracture tip seemed to increase due to fluid invasion. Fracture patters were obviously changed with injection flow rate. Lower injection flow rate led to a single and straight fracture and higher injection rate led to branching fractures, while in all cases, overall alignment of fractures was in the direction of the maximum horizontal compressive stress. Such phenomena are different from those observed in competent rocks.