Frictional Heating of Casing Due to Drill String Rotation – Finite Element and CFD Simulations

Abstract

A drill string in a wellbore always contacts a casing. However, in a curved section of casing the contact force between the drill string and the casing can be significant. Friction between the casing and a rotating drill string tool joint in contact with the casing generates a heat source at the interface between the two objects. The generated heat energy is a function of rotational speed of the drill string, side force and friction coefficient between the hard-banding layer covering the tool joint and the casing. Heat partition between hard banding layer and casing depends on the thermal properties of both. When there is no mud circulation, e.g. due to a pack-off in the annulus or lost circulation, and the contact region stays in the same section, the resulting temperature increase can lead to degradation of the mechanical strength of both the drill string tool joint and the casing. In addition, the casing strength reduction can facilitate casing wear, which may lead to leak and tool joint heating can lead to heat checking cracks or mechanical strength weakening which may result in a parted drill string due to brittle or ductile fracture. When there is no mud circulation, rotation of the drill string leads to mud angular rotation inside and outside the drill string. Convection heat transfer occurs due to mud rotation and convection heat transfer coefficient depends on mud flow regime. CFD simulations were performed to compute the convection heat transfer coefficient. Two and three-dimensional steady state and transient finite element simulations were performed to compute the temperature distribution in the casing and the drill string tool joint when there is no mud circulation. Results show that, when there is no mud circulation, conduction through the drill string and casing has the highest impact on the maximum temperature generated due to frictional heating. Two graphs are plotted, one shows the steady state temperature versus side load at different rotational speeds while the other shows casing yield and ultimate stresses degradation versus increase in temperature. Both graphs can be used by drilling engineers at the well design phase to select the appropriate rotational speed of drill string to avoid failure when there is no mud circulation. Novelty of this paper is in thermal analysis of a tool joint hard banding layer rubbing against casing. In the analysis the convection heat transfer through mud rotation is involved.