Experimental verification of the dual bit coring method developed for stress measurement applicable in deep and high temperature environment

When rock is cored at depth by drilling, anisotropic expansion of the core occurs radially with the relief of anisotropic rock stresses. Based upon this phenomenon, we propose a new method to estimate magnitude and orientation of the maximum and minimum stresses S_Hmax and S_hmin assuming a vertical borehole. We have verified the principle of the method by laboratory experiments, and developed a special tool for coring which records stress state in the shape of core. The tool has co-axial large and small core bits, and each one of the bits works sequentially from large to small ones. Then the core expands partially to have elliptical cross section due to stress relief but in the other part maintains circular cross section before expansion. By comparing the shape at each section, we can measure how much the core expands in all direction, and the measured amount and distribution of expansion allows us to estimate each magnitude and orientation of SHmax and Shmin. We have tested the tool in field experiments successfully.