An Estimation of Fracture Aperture and Pressure Distribution in a Fracture Made by In Situ Hydraulic Fracturing Test

摘要

An in-situ hydraulic fracturing test was conducted in the Kamioka mine to estimate a fracture aperture and a pressure distribution within a fracture. The test was carried out in a borehole at a depth of 1.7 m. The borehole had been drilled vertical to the wall of the opening. After making an artificial fracture which was parallel to the wall, water was injected into the fracture. Another borehole which had an offset of 1.5 m away from the injection borehole was drilled to measure the pressure within the fracture at that location. The injection rate of water was increased step by step and a distribution of displacements of wall surface was measured by twenty-four LVDTs during the injection tests. Assuming that the water flow was straight, the distribution of water pressure within the fracture was calculated by the Lomize's equation. By using this pressure distribution as a boundary condition, an two-dimensional FEM model together with the Gangi bed-of-nails model was used to estimate the distribution of the fracture aperture and pressure distribution within the fracture to match the measured result of wall surface displacements, injection pressure and a pressure within the fracture. There is a good agreement between measured results and calculated values. Thus, the proposed analysis is thought to be effective. From this estimation, the distribution of the fracture aperture has two peaks. One peak is located near the injection borehole and another peak is located about 2 m away from the borehole. The maximum aperture of the fracture was estimated to be 230 micro-m.