The interpretation of horizontal and mixed-mode fractures in hydraulic fracturing tests in rocks

For the design of underground structures in rocks, the states of initial stresses in the rock mass are required. For structures located at great depths, the hydraulic fracturing test for stress measurements is the only practical method. For vertical fractures, existing solutions for calculation of stresses from test results are satisfactory. For horizontal or mixed-mode fractures, appropriate solutions are required. Closed-form solutions for horizontal and mixed-mode fractures including strength anisotrophy are presented. The method enables the determination of which fracture (horizontal or vertical) occurs first at the first breakdown pressure during the test, so appropriate stress calculation may be carried out. Results of hydraulic fracturing tests in three case histories have been reanalyzed using the method developed. It is shown that for horizontal fractures the ranges of stress values computed are considerably reduced compared with existing solutions. The reinterpreted horizontal stresses in a case record are consistent with results of field observations in underground excavations. Experimental requirements for the measurements of rock parameters relevant to the specific stress paths in hydraulic fracturing tests are discussed. Key words : hydraulic fracturing, stress measurements, mixed-mode fractures, underground structures, rock strength.