We developed a new reusable gage designed upon a concept completely different to that of the conventional ones. The gage allows us to measure the tangential deformation of borehole wall. We tested the gage by a laboratory experiment and examined how it could measure accurately borehole deformation caused by packer inflation. The rock specimen had two drilled borehole of 76 mm in diameter with hydraulically-induced axial fractures. The results show that the deformation measured by the gage located near by the fracture mouth increased linearly with the packer pressure initially and it deviated from the initial trend afterwards. The deviation from linearity of the deformation-packer pressure curve was associated with the fracture opening. Then we confirmed that the packer pressure at the beginning of deviation was close to the reopening pressure Pr estimated by the Kirsch equation, i.e. Pr = 3Sh - SH, where SH and Sh (SH = Sh) are the biaxial compressive loads externally applied to the specimen as the direction of Sh being normal to the fractures. This indicates that we can estimate the magnitude of SH from the detected reopening pressure and the Kirsch equation on condition that the magnitude of Sh is estimated using other approaches.
Change in stress of rock mass should appear as borehole deformation. Based on this principle, many researchers attempted to estimate the change in rock stress from borehole deformation. To do that, various ways have been proposed so far for measuring borehole deformation such as the USBM gage, the cell of Bonnechere, the CSIR HI cell etc . The former two are reusable, and they can be installed quickly into boreholes. For the last one, gluing is required so that relatively long time is needed for installation and it can not be reused. On the other hand, the change in rock stress affects tangential deformation of borehole wall more sensitively than its diametrical deformation as can be seen easily. However, the conventionally-developed reusable gage such as the USBM gage and the cell of Bonnechere can measure only diametrical borehole deformation but not tangential one, while the fixed cell such as the CSIR HI cell can measure tangential borehole deformation. Taking account of this situation, we developed a new gage which allows us to measure the tangential deformation of borehole wall. For the installation of the new gage into boreholes, the cementing or gluing is not required so that it is reusable and allows quick installation. We tested the new gage by a laboratory experiment and examined how it could measure accurately borehole deformation caused by packer inflation.
2. A NEW DEFORMATION GAGE
Fig. 1 shows the deformation gage developed in the present study. The gage has a part of two thin beams made of phosphor bronze and strain gages of four in total are attached on both sides of the beams. There is a short leg at the center of each beam. When the gage is pressed against the borehole wall somehow, the two legs of the deformation gage contact with the borehole wall, and afterwards they move following the tangential deformation of the borehole wall.