Tool Development and Field Test of the Stress Measurement Method with Dual Bit Coring Applicable to Geothermal Fields

ABSTRACT: The Dual Bit Coring (DBC) is the method to record the in-situ stress state into cross-sectional shapes of a boring core. The magnitudes and orientations of the in-situ stresses in a plane perpendicular to the borehole can be estimated from the measured core diameters upon the principle with which the core expand radially in an elastic manner as it is relieved from the in-situ stresses. We developed a downhole assembly to realize DBC which contains small and large diameter bits. A core sample is cut out in three stages with changing those two bits. We developed another downhole tool to detect the core orientation. They are designed to operate in temperatures more than 200 °C at depths of a few km considering application to geothermal fields. We simulated the three-stage coring by numerical simulations and confirmed the core to have cross sections of an expanded-elliptical shape and a circular shape prior to expansion in the lower and middle sections, respectively. We tested the tools in field tests and estimated the in-situ stress state successfully. 1. INTRODUCTION Rock core samples obtained by drilling have slightly elliptical cross-sections instead of circular ones, when the in-situ stress state is anisotropic (Funato and Ito, 2017). Such elliptical shapes arise as the core expand radially in an elastic manner when it is relieved from the in-situ stress. The amount of expansion is proportional to the stress magnitude. The core expands most and least in the orientations of the maximum and minimum stresses, respectively, that act along the plane that is perpendicular to the drill hole, resulting in the elliptical cross-section. We applied the phenomenon of core expansion to measure the in-situ stress state. To do this, we proposed a novel coring method referred to as Dual Bit Coring (DBC) that 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 to expansion. Then the amount of expansion can be estimated as the difference between the cross-sections prior and posterior to expansion, and the stress magnitudes and orientations can be estimated from the circumferential distribution of the core expansion.