Nonlinear rub-impact characteristics of drillstring-wellbore and wear quantitative evaluation method

Aiming at complex vibration behavior of drill string on drilling efficiency and safety in drilling engineering, this paper has been conducted on the nonlinear rub-impact characteristics and quantitative evaluation method of drill string. Firstly, a nonlinear dynamic model of the drill string was established, comprehensively considering the nonlinear contact between the drill string and casing, drilling fluid buoyancy, and damping effects. Then, the influence of driving speed, eccentricity, and drilling fluid density on the vibration response of the drill string was analyzed. Finally, a visualization method was proposed to characterize the random contact and collision between the drill string and casing. Based on this characterization, a dimensionless rub-impact index (RI) was defined to quantitatively evaluate the nonlinear rub-impact behavior between the drill string and casing. The results show that excessive high and low driving speeds can induce complex and unstable lateral motion trajectories of the drill string, accompanied by random and periodic rub-impact, significantly increasing the possibility of destructive lateral impacts between the drill string and casing. An increase in eccentricity leads to more frequent rub-impact between the drill string and casing. Specifically, when the eccentricity increases from 0.01 m to 0.02 m, the number of rub-impact occurrences rises sharply from 98,256 to 148,920, an increase of 51.6%. Moreover, although increasing the drilling fluid density complicates the vibration modes of the drill string, it effectively suppresses lateral vibrations, thereby protecting the wellbore. The visualization method can intuitively characterize the nonlinear rub-impact characteristics induced by the lateral vibrations of the drill string. The dimensionless rub-impact index (RI) based on rub-impact characterization accurately reflects the severity of the drill string-casing rub-impact. It can serve as an effective tool for assessing the degree of rub-impact. This facilitates a deeper understanding and analysis of the vibration characteristics of the drill string. Provide valuable references for drilling vibration control and optimization of drilling parameters.