In permeable rock the hydraulic fracturing stress measurement is carried out using such impenetrable shell as a packer and a sleeve that prevents from fluid injection into the fracture and rock. The stress states are determined from several fracture reopening pressures on condition that cracks have different orientation in reference to a maximum stress. Numerical modeling of this problem shows the gradual character of the crack opening in the borehole and the influence of an indefinable initial crack length on the reopening pressure value Pr of other fractures. As a solution the using of a singular radial fracture and the measuring of fracture opening value on the outline dependence of the pressure in the impenetrable shell are proposed. The fractures are induced by hydraulic fracturing in advance. The test of the solution is carried out using singular integral equations of linear fracture mechanics. The principal stress ratio estimation algorithm is developed. The ratio of principal stresses is an additional parameter and it is used to find out the stress with the fracture reopening pressure on the borehole wall. Technical and methodic issues of practical single and repeated investigations realization are considered.
Rock masses are initially stressed in their natural state and state of stress (magnitudes and directions) is an essential parameter required for the stability evaluation of underground openings. Hydraulic fracturing has been widely used so far for stress measurements. Its general concept is as follows (see e.g., Haimson 1989); the state of the remote stresses in a plane perpendicular to a borehole axis is evaluated from the reopening pressure, the shut-in pressure and orientation of a pair of longitudinal cracks which is induced by hydraulic fracturing, where the longitudinal is the crack parallel to the borehole axis.
