Numerical analysis of whirl for drill string with aluminium alloy drill pipes in deep vertical well

Abstract

In this study, a new finite element (FE) model using beam-shell composite elements is established. The model combines the advantage of computational speed for beam elements with the contact details of shell elements. The thermal instability of aluminium alloy materials and geothermal gradient are considered. Besides, kinematic contact model, Coulomb friction model, and initial imperfection are considered in this model. The drill string with aluminium alloy drill pipes of Songke-2 (SK-2) well is selected to simulation. The calculation results are consistent with the failure mode of the aluminium alloy drill pipes at engineering site. The effects of aluminium alloy drill pipes position, geothermal gradient, and rotational speed on the whirl characteristics of drill string with aluminium alloy drill pipes are analyzed. The results show that the closer the aluminium alloy drill pipes are to the drill bit, the more obvious the whirl phenomenon of the drill strings is. For deep wells with a maximum temperature of 170 °C, the temperature gradient has limited effect on the whirl state. The rotational speed has a significant impact on the whirl. When the rotational speed reaches 80 rpm, backward whirl occurs at the bottom of the borehole. Moreover, when the distance from the drill bit is different, the drill string will exhibit different whirl states. This work provides a new approach for modeling the dynamics of drill strings, selecting different element types based on the importance of different parts. Geothermal gradient is also considered in this model, which can help to calculate drill string dynamics for ultra deep drilling and geothermal drilling.