We have proposed a new concept in which the in-situ stress state is determined by integrating the hydraulic fracturing data and the deformation of core samples. The integration leads to improve practicality and compensate disadvantages compared with the case using each data solely for the stress determination. The redundant data set allows us to determine both of the stress magnitude and orientation in multiple ways, and the validity of results can be confirmed from their consistency. In order for applying this concept to the stress measurement at deep depth, we have been developing a downhole tool of the Deep Rock Stress Tester, DRST, composed of double packers, a high pressure pump and fluid tanks. This tool must have an outer diameter smaller than the inner diameter of a drill pipe, since the tool runs on wire through the drill pipe for carrying out the in-situ test of hydraulic fracturing in a small hole drilled additionally at the bottom of a borehole. The small hole is referred to as a baby hole. Such a way seems complicated but it should minimizes a lime for tool selling overall while avoiding the risk of trouble in boreholes such as the tool gelling stuck. Upon the success to demonstrate the prototype tool in a borehole of BOO m deep with PQ size, the 2nd type one available for boreholes with HQ size was developed, and it was applied actually to the stress measurements using a borehole of 2000 m deep. As a result, we succeeded to estimate the in-situ stress from the deformation observed for the core sample of the baby hole. However, any fracture could not be induced by the pressurization tests, and this problem was considered to coma from insufficient performance of fluid pressure applied to last intervals. Considering those results, we started to develop a new tool with higher performance, and the development has been completed recently resulting in the 3rd type tool to be applicable under the environment of high pressure up to 30 MPa and high temperature up to 100 degC.

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