Hydraulic fracture in borehole is used to determine the geo-stresses. The complexity of rock property and stress state makes the great difference between the real curve and the ideal curve of water pressure time. The water pressure to crack the boreholewall may be lower than that to spread the fissure. The pore pressure in rock results in the decrease of the tensile strength. Cylinder specimens were directly tensile under confining, hollow cylinders were fractured by inner pressure under different axial load, and rings were Brazilian split. The experimental results show that two indexes of deformation and stress are related to the failure of rock specimen when it is suffered non-uniform stresses.
Hydraulic fracturing (HF) is a borehole field-test to estimate the state of in-situ stresses. Figure 1 shows the borehole status and water pressure in the process of hydraulic fracturing (Cai 2000). A section of a borehole is sealed off by use of two inflatable rubber packers sufficiently pressurized so that they adhere to the borehole wall. Water is pumped under constant flow rate into the section, gradually raising pressure on the borehole wall until fracture is initiated in the rock. Pumping is stopped, allowing the interval pressure to decay. Several minutes into the shut-off phase, the pressure is released and allowed to return to ambient conditions. Key pressure values used in the computation of the in-situ stresses are picked from the pressure–time record. The attitude of the induced HF is obtained using an oriented impression packer or borehole camera (Haimson & Cornet 2003).
The fracture reopening pressurePr is not the peak point in subsequent cycles as explained in (Lee & Haimson, 1989), but the breakdown pressure Pb usually is the peak pressure attained in the first pressure cycle.