Large-Scale Experiments of the Borehole Instability on Shale Formation Influenced by Drill Pipe Rotation

This study explores the effects of field drilling operations on wellbore instability, specifically in shale formations. While earlier research extensively addressed the physicochemical and mechanical underpinnings of wellbore instability, the direct impacts of drilling operations, such as drill pipe rotation, remain relatively unexamined. The paper introduces an innovative device, integrated into a true triaxial cell, which mimics the downhole conditions and the drill pipe rotation. This enhanced setup facilitates a comprehensive analysis of how drill pipe rotation influences shale instability, considering variables like rotation speed, weight on bit, drilling fluid pressure, and borehole microcracks.

Results indicate that drill pipe rotation intensifies wellbore enlargement, with higher rotation speeds exacerbating the collapse. The weight on the drill string is also a pivotal factor, as a greater weight results in a larger collapsed wellbore diameter. Among the considered factors, drilling fluid density has a lesser impact compared to rotation speed and weight on the bit. Nonetheless, the presence of microcracks, often caused by the impact of drilling tools, has a profound effect, leading to a potential enlargement of the wellbore diameter by over 40%. The study also charts the temporal and depth-related variations of borehole collapse under the influence of drill pipe rotation. Among the analyzed factors, their significance in inducing collapse is ranked as: microcracks > weight on bit > rotation speed > drilling fluid pressure. The collapse severity is most pronounced in the direction of minimum horizontal stress, a phenomenon elucidated by examining the stress distribution around the wellbore's circumference. In essence, this research offers vital insights into the often-overlooked role of drilling operations in wellbore instability, bridging a knowledge gap in the field.