Laboratory study for pore water effect on hydraulically-induced fracture behavior in unconsolidated sands

We developed recently a new apparatus which allows laboratory fracturing experiments under tn-axial compression up to 15 MPa with pore water pressure up to 15 MPa. Silica sands with particle size of about 125 im are used as the simulated formation matenals. In addition to the sand, some amount of kaolinite flour is mixed for adjusting permeability. The mixture is layered in a mold to form a cubical specimen of 200 x 200 x 200 mm3 with aid of a specially-designed press machine. A fracturing fluid with viscosity of 300 mPa s is injected into a specimen through a slit of a steel pipe buried in the specimen. After the tests, we excavate the specimen bit by bit and observe how the fracturing fluid has invaded into the specimen. In the present study, to examine the effect of pore water on the fracture formation, we carried out the tests for the specimens under various conditions of water saturation, pore pressure and confining stresses. Then we found that the fracturing pressure changes in proportion to the confining stress, and it is not influenced by water saturation and the initial value of pore pressure.