孔内載荷による大深度岩盤応力測定の現状と問題点

Abstract

The hydraulic fracturing technique as a method for determination of in-situ stresses has the advantage over other stress measuring methods in that it can be used at considerably greater depths from a point of access. This is of great significance in earthquake regions where stresses at hundreds or thousands of meters are of interest. Despite of the increasing use of hydraulic fracturing, however, there are uncertainties associated with the interpretation of the resulting data. In particular, confidence in the calculated maximum principal stress is less than in the minimum principal stress, although the former is often of great moment. Furthermore, the equation established to calculate the maximum principal stress is unclear. More recently, dry fracturing techniques have been developed and examined as the new potential method for determination of in-situ stress at great depth, through studies both in the laboratory and in the filed. In dry fracturing, the borehole wall is subjected to shear load as well as normal load, which are induced by internal pressurization of the probe. The state of the art of rock stress measurement by hydraulic fracturing is reviewed and a comparative study is made of two kinds of dry fracturing techniques, which have been developed as the potential method for stress determination. The general review is followed by individual summaries of each method and the main characteristics of its validations and applications.