A Stick-slip Analysis Based on Rock/Bit Interaction: Theoretical and Experimental Contribution

Abstract

Abstract The purpose of this paper is to analysis bit stick-slip motion by using rock mechanics considerations coupled with field bottom hole data. The data was collected using an instrumented PDC bit. This new tool enables to measure accelerations, Weight-On-Bit (WOB) and Torque-On-Bit (TOB) directly above the bit. Tests were conducted in Montrose (Scotland) at about 700 m depth in a consolidated Devonien sandstone. During several hours of testing, the sensitivity to operating and different structural parameters was evaluated. The effect of stick-slip on Rate Of Penetration (ROP) was quantified: ROP is really affected by this dynamical phenomenon which appears above a certain eight-On-Bit threshold. Two stages rule the drill string behaviour. The stationary behaviour (small oscillation of the rotation bit speed around the mean value) is described by modal analysis. Experimental frequencies are in agreement with the theoretical natural ones. A perturbation of the operating parameters (generally an increasing of the WOB) makes the system bifurc towards stick-slip. The dynamical system is now governed by a non linear autonomous boundary condition, linked to the rock/bit interaction. Rock mechanics considerations justifie such a law. Numerical simulations converge on stick-slip, typical limit cycle experimentally exhibited. The last analysis is focused on an analysis of "drill-off test". In this operating stage, the rock/bit interaction function changes. Numerical simulations, justified by clearly experimental evidence, show the benefit effect of such a test, which stabilizes the system. A discussion on the effect of the stabilizers and bit type follows as well as how to attenuate this non linear phenomenon. Introduction This paper deals with drill string torsional oscillations, even known as stick-slip motion of the bit. In this case, the rotary speed of the bit oscillates with a large amplitude, even when the rotation speed is constant at surface. Sometimes, the bit speed oscillates so much that it can be at rest for a while, as if it has been plugged into the formation. The bit can rotate at two times the surface speed, few instant later. This is called "stick-slip". The study is applied to PDC (Polycristalline Diamond Compact) bits although some analogies to roller bits could be pertinent. A lot of studies have been devoted to lateral and axial vibrations despite it is difficult to quantify the rock/bit interaction in some instability cases (bit whirling or bit bouncing). Bit bouncing is more specially specific to roller bits. Moreover, some real bit design improvements reduced the tendency of the bit to whirl1. Rock/bit interaction is often considered as one of the major parameter which induces stick-slip but the fundamental mechanism of self-excited torsional vibrations is not yet explained using rock mechanics considerations. We have chosen such a description, justified by theoretical and experimental investigations. Severe torsional drill string vibrations can affect drillpipe and also PDC bit life detrimentally. Empirically, these vibrations are associated with accelerated PDC bit wear. It is possible that this accelerated wear results from torsional vibrations, particularly because it is theoretically possible for the bit actually to spin backward at some point during a cycle2. These vibrations affect the Rate Of Penetration (ROP) and can occur more than 50 % of the total time of a classical rig step3. The genesis of stick-slip has to be really understood to improve the drilling performances.