Experimental Evaluation of the Influence of Drill String Axial Compliance on Rotary Drilling Performance in Hard Rock Formation

Improving drilling performance in hard formations remains a key challenge for mining, geothermal, carbon storage, and petroleum applications. This study experimentally investigates the effect of axial compliance, introduced through a Belleville spring stack above the drill bit, on rotary drilling performance, compared against a conventional rigid drill string. Tests were conducted on fully characterized Fine-Grained Gabbro (UCS approximately 152 MPa) using a Large-Scale Drilling Simulator, at 60 and 120 RPM across a WOB range of 5–15 kN with a bit configured of six PDC cutters. Performance was evaluated through the Rate of Penetration (ROP), depth of cut (DOC), torque, and Mechanical Specific Energy (MSE), with axial displacement measurements characterizing drill string vibration. The compliant configuration consistently outperformed the rigid configuration, with the average ROP improved by 15% at 60 RPM and 25% at 120 RPM, while the average MSE was reduced by up to 27%. These gains began once the WOB exceeded a threshold level and grew stronger with rotational speed, indicating the improvement is driven by the number of oscillation cycles rather than displacement magnitude alone. These results show that axial compliance can meaningfully improve drilling performance relative to a rigid drill string under the tested conditions.