Continuous depth profile of the rock strength along a borehole based on drilling performance parameters

In-situ rock strength with depth under the ground/seafloor is a critical parameter for various studies in geosciences. The measurements on rock/core samples, however, hardly have been done with success due to lack of drilled cores and sufficient knowledge about the in-situ conditions such as pressure and temperature. Recently, a new indicator of the strength, equivalent strength (EST) was proposed developing previous mechanical parameter, Mechanical specific energy, which is converted only from drilling performance parameters; drillstring rotational torque, bit depth and drillstring rotational per minute. The data processing method was applied to the drilling data taken from International Ocean Discovery Program (IODP) expeditions and Indian National Gas Hydrate Program Expedition 02 (NGHP-02). The calculated EST was used for comparison with other physical properties and interpretation of geology, indicating the validity of EST. In order to correlate EST with rock strength measured by conventional laboratory experiments, drilling experiments using a high-speed rotary-shear apparatus were performed in the laboratory. Metal drill bits with diameter of 20 mm, fitted to the apparatus, were manufactured, and a standard rocks and sediments were drilled at a rotational speed of 0.01 to 10 RPM under axial load of 0.2-1.0 kN (equivalent to 1.6 to 8.1 MPa). Drilling torque and penetration speed at each condition were measured. The mean EST calculated based on the recorded drilling data shows comparable values to the uniaxial rock strength of the test samples. No significant influence of rotation speed on EST was found in the experiments.