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 bottomhole 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 Weight-On-Bit threshold. Two stages rule the drillstring 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.