A New Method of Rock Stress Measurements on Stress Record Core Obtained by the Dual Bit Coring and Its Laboratory Verification

When rock is cored at depth by drilling, anisotropic expansion occurs with the relief of anisotropic rock stresses. The core diameter variation with core roll angle is given theoretically as a function of rock stresses. Based upon this phenomenon, we have proposed a method in which the rock stress originally subjected to the core portion before drilling is estimated from cross sectional shape of the core measured on ground after retrieving (Funato and Ito, IJRMMS, 2017). We have successfully tested the method at deep seismogenic zones in South African gold mines.

However, the original method of Funato and Ito (2017) can measure the difference between the maximum and minimum stresses Smax and Smin in a plane perpendicular to the borehole but not each one of those stress components separately. The reason for the limitation is come from the difficulty to measure the original core diameter d0 before expansion. If d0 is known, each one of Smax and Smin can be estimated from the measured maximum and minimum core diameters dmax and dmin. To realize this, we modified the way of core drilling. First, a core bit cuts a circular groove into the exposed surface of rock at the bottom of a borehole. Next, another bit cuts another circular groove with a smaller diameter until the groove reaches a depth which is few times larger than the first groove depth. In this case, the upper part of the secondary cut-out column should not expand anymore, since the stresses has been relieved there already by cutting the first groove. Therefore, the upper part of the second column should be maintained at d0 in diameter. Contrary to this, the lower part of the second column should expand to have dmax and dmin in response to the relief of rock stresses. In order to verify the new concept, we carried out laboratory experiments. Being subjected to biaxial compressive stresses Smax and Smin, the specimen of 200 mm cube was drilled vertically to get a core by using each one of two coring bits with 50 and 32 mm inner diameters, respectively. We detected successfully the values of d0, dmax and dmin from the core. The magnitudes of Smax and Smin estimated from those detected values were very close to those actually applied to the specimen.

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