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| NIRE Annual Report | 
| 1998 |
Estimation of the Fracture Aperture by Hydraulic Fracturing and Injection TestsGeomechanics DivisionGeotechnology Department |
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Objectives
The main object of the experiment is to estimate the fracture aperture, it was found that the stress concentration around a mine tunnel, which is induced by primary stress, played an important role. An artificial fracture was made by hydraulic fracturing. After hydraulic fracturing, water was injected into the fracture and movements of wall surface were measured. An analysis was made by a combined Gandi-Lomize model. There was a fair agreement between measured and estimated value by the model. This analysis could be used for not only fracture aperture but also the in-situ stress state.
Results
The experiment was conduced in Mozumi tunnel in Kamioka mine. A borehole for the test had been drilled horizontal direction to a gallery and the hydraulic fracturing test was carried out in the borehole at the depth of 1.7 m. After hydraulic fracturing test, the injection rate of water into the artificial fracture was changed step by step and injection pressure was measured (Fig.1). The wall surface displacements during the injection test were also measured by twenty-four LDVTs. The magnitude of a stress which is perpendicular to the fracture includes the influence of stress concentration caused by the gallery. Also, a distribution of the magnitude of a vertical stress to the fracture was not uniform, because the fracture was not parallel to a wall of the gallery. In this borehole, the magnitude and the direction of a stress state within a perpendicular plane to the borehole axis were already known. Therefor, assuming the magnitude of primary stress which is parallel to the borehole axis, the distribution of the magnitude of vertical stress to the artificial fracture which was caused by stress concentration of the gallery were calculated by FEM. Furthermore, a surface displacement of the gallery wall was calculated from the vertical stress to this artificial fracture and hydraulic pressure distribution within the fracture during injection test. As a result, calculated injection pressure and the surface displacements (Fig.2) were agreed very well with measured values.