Experimental and Numerical Study on a Two-stage Coring Method for Stress Measurement: Application to Deep and High-Temperature Geothermal Wells

We discuss the possibility of recording the state of in-situ stress in the shape of a rock core that is sampled using a modified core drilling method. This concept is based on a phenomenon that a cylindrical rock-core expands radially into a slightly elliptical shape along its transverse cross-section when the rock sample is relieved of its in-situ anisotropic stress by drilling. The magnitudes and orientation of in-situ stresses can, in principle, be estimated from the circumferential variation of core expansion. However, it is not possible to evaluate the expansion from a core collected in a conventional way, with a single drilling bit, since it will be in its expanded state when extracted and lacks information about its required pre-expansion dimensions. To overcome this problem, we propose an innovative way of core drilling referred to as Dual-bit Coring. This approach results in a single core that has two signature transverse cross-sections: the first has an elliptical shape resulting from core expansion and the second records the initial circular shape prior expansion. Then, the amount of expansion can be estimated based on the differences between the shapes of those cross-sections. We propose practical drilling tool for carrying out Dual-bits Coring. The tool works mechanically and does not require any electric devices sensitive to high temperatures so that it is expected to function even in deep and high temperature rocks at geothermal fields, such as those encountered in supercritical geothermal systems.