Development of borehole-jack fracturing technique for in situ stress measurement

• We developed a new technique and apparatus for measuring in situ stress.
• This technique is named “borehole-jack fracturing”.
• The principle of this technique is described by the relationship between jack pressure and rock stress around the borehole.
• Two-dimensional stress state is determined by re-opening pressures and orientations of the fractures.
• The feature of this technique is to evaluate the initial stresses without using the fluid in a deep borehole.

In order to measure crustal stresses at great depths of more than 1000 m from the ground surface, we have been promoting research and development of borehole-jack fracturing technique. When a borehole wall is loaded by a borehole-jack, a pair of new fractures will be induced oppositely in parallel to the borehole axis. After unloading, if the same place on the borehole wall is loaded again by the jack, the pair of fractures will be opened again. Two principal stresses and the orientation of crustal stress in the plane perpendicular to the borehole axis are determined by the re-opening pressures and the orientation of the fractures respectively. This technique is similar to a hydraulic fracturing from the viewpoint of analysis. The features of this technique are that it is possible to produce a pair of axial fractures in an arbitrary direction, possible to measure a displacement of the fracture opening, and, as a result, it is possible to determine the re-opening pressure accurately. This paper describes our results of numerical analyses, laboratory experiments, and field tests.