Practical application of the dual core-bit tool for drilling stress record cores to stress measurements at Kamioka mine, Japan

We have developed prototypes of dual core-bit tools which practicalizes a newly proposed rock stress measuring technique called "stress record coring method". The method is based on the diametrical core deformation analysis (DCDA) proposed by Funato and Ito (2017). When the core is cut from the bedrock, it is released from the in-situ stress and expands slightly in the radial direction. Using this principle, the in-situ stress can be determined from the elliptical shape of the core cross section. However, the DCDA only provides the difference between the maximum and the minimum in-situ stress, and the magnitude of each cannot be determined. This is because the core diameter d0 before expansion is unknown. To solve this problem, we proposed the coring procedure using the "Dual core-bit tool" so that the shape before expansion is retained in part of the core. The dual core-bit tool consists of an outer core bit, an inner core bit, and an inner core barrel, and has a mechanism for switching the rotational drive from the outer bit to the inner bit. After digging a groove (outer groove) in the borehole bottom by the outer core-bit, cut a smaller diameter core by the inner core-bit in a same way as a normal coring. As a result, the shape before expansion remains in the upper part of the core, and the shape should expand due to stress relief in the lower part deeper than the outer groove. To confirm the practicability, we made prototypes of the dual core-bit tools and applied them to stress measurement at Kamioka Mine. Four drilling trials were conducted, and two of them succeeded in obtaining cores that can be used for stress evaluation.

This study is a part of the Feasibility Study of Supercritical Geothermal Power Generation funded by NEDO.