Modeling and analyzing the movement of drill string while being rocked on the ground

Automatic rocking drill string on the ground has proven to be capable of improving slide drilling performance. To make best use of the rocking technology and avoid potential risks, this paper proposes a model to calculate the response of drill string to the surface rocking motion. Friction between drill string and wellbore is finely incorporated in the model to simulate the redistribution of friction during rocking and friction-induced stick-slip motion. A finite difference method with second-order accuracy is used to solve the numerical model. The model was first used to demonstrate that commonly-used static torque and drag models are not applicable for directly determining the rocking depth or required surface torque. The influences on weight transfer and tool face stabilization of surface rocking parameters are investigated. The simulation results indicate that greater rocking velocity produces smoother weight transfer and rocking depth determines the range of weight on bit. Rocking velocity should match rocking depth to reduce the fluctuation of weight on bit. In the design of rocking depth, reactive torque and relationship between weight on bit and reactive torque should also be taken into consideration. The simulated results are in good agreement with field practice mentioned in relevant literature and field measurements. Therefore, the proposed model and solving method are useful for determining rocking parameters and further improving the efficiency of automatic rocking technology.