Friction-Induced Wellbore Instability Due to Drill String

Abstract

In this chapter, we study numerically friction-induced wellbore instability due to frictional interaction between drill string and rock materials using torsional pendulum system. The classical Amontons–Coulomb (AC) friction laws are widely used for explaining the variety of sliding- and rotation-related phenomena, yet the AC laws fail to explain stiffness dependence of stick-slip motion. In recent times, the rate and state friction (RSF) model has found widespread applications for understanding the phenomena related to sliding of rock surfaces. The RSF model, which is basically modified form of the aforementioned AC laws, has not yet got any attention for studying the friction-induced wellbore instability. The RSF laws state that friction of hard surfaces such as rocks and metals at high (~MPa) normal stress depends on current slip velocity as well as nature of the sliding surfaces. The literature review reveals that rotation of drill string causes stick-slip vibration, thus potential initiation of the failure process in surrounding medium. We use linear and nonlinear stability results to discuss a critical stiffness above stick-slip behaviour of the rotating system disappears. It is also demonstrated in the numerical simulations that stick-slip motion could also be eliminated by increasing rotational velocity of the drill string.