The visualization of rub-impact characteristics of drill string on casing

Field experience revealed that a whirling drill string in the wellbore would make severe impact and rubbing against the casing and lead to uneven and random wear of casing. The casing wear due to vibrational impacts of the drill string was also one of the major causes for casing failure, but it was always ignored in the previous studies on casing wear. This work aimed at modelling and intuitively displaying the rub-impact characteristics of drill string and the induced wear on casing. Taking into consideration of the damping of drilling fluid, bit-rock interaction together with the geometric and contact nonlinearity, a drill string dynamic finite element model was established and then solved with the node iteration and the Newmark method. The whirl orbit, whirl velocity and collision characteristics of the drill string were then obtained. Based on these results, a three-dimensional visualization method was established to describe the dynamic features of drill string. The results indicate that the drill string presents a strong nonlinear dynamic behavior and makes frequent impacts against the wellbore, which appears mainly in the low side of borehole and is more serious in the hole section where BHA is located. The hole curvature and surface rotary speed have important influence on the casing wear induced by impact-rub action. A large hole curvature will lead to more serious rub-impact action of the drill pipe on the casing, and a rotary speed closer to the natural frequency of BHA will obviously increase the wear risk of casing.