Axial Load Peristaltic Transfer Mechanism of the Drillstring to Improve Penetration Rate

Abstract

Summary During the drilling of long horizontal wells, the significant frictional resistance generated by the wellbore walls poses a challenge for the drillstring to efficiently transmit load to the drill bit, which eventually reduces drilling efficiency and restricts the extension distance achievable. Inspired by the structure and movement principle of an earthworm, we propose an earthworm-like load transfer method for the drillstring to address this issue. Specifically, the proposed method involves the installation of a pulse generator and multiple vibration subs within the same drillstring, decomposes the drillstring into multiple sections and modulates it to creep like an earthworm, thus facilitating load transfer. Experimental studies and numerical simulations were conducted in this paper to explore the fundamental mechanisms of earthworm-like crawling, aiming to enhance the efficiency of load transfer within the drillstring. The experimental results suggest that adopting earthworm-like excitation can increase the load transfer efficiency of the drillstring by 36–52% compared to conventional drilling methods. However, if the drillstring experiences helical buckling, there is a significant decrease in the efficiency of load transfer. Meanwhile, a dynamic model of the drillstring, considering the 3D wellbore trajectory, multipoint excitation, Stribeck friction, and penetration rate, has been developed. The simulated results from the proposed model align well with the experimental results obtained before the drillstring buckling, with an error of less than 5%. The simulation results for a 1000-m drillstring indicate that the earthworm-like excitation significantly enhances the efficiency of load transfer compared to conventional drilling methods. This improvement is attributed to the increase in the proportion of reverse-motion drillstring segments by 35.8–40.25%, which will greatly reduce the instantaneous total vector frictional force of the entire drillstring.