Recognition and Mitigation of the BHA Lateral Vibration Chatter Mode

Abstract

Abstract Lateral vibration modeling of certain BHA (bottomhole assembly) designs has shown great sensitivity to the proximity of stabilizer blades. This paper will explore the nature of the vibrational dysfunction that we call BHA chatter. A frequency-domain model that has been field-proven shows how this dysfunction occurs, its rotary speed dependence, and mitigation methods and results. A frequency-domain BHA lateral vibration model will be used to illustrate the role of the nodal point constraint in the determination of the dynamic side forces acting at stabilizer and LWD tool blade borehole contacts. These contact forces may be strong functions of rotary speed and BHA contact spacing, especially if the spacing is close. The relationship between the dynamic contact forces and vibration index model results will be described in relation to the solution to the lateral vibration model. Examination of the dynamic contact side forces associated with these vibration indices reveals the nature of the dysfunction that the indices represent. The contact forces that push a stabilizer blade to be constrained within a borehole include both static and dynamic components. The static forces are well understood. However, the dynamic forces are difficult to visualize as they are generated by the BHA in motion, and a well-formulated dynamic model is required to evaluate these forces. Dynamic dysfunction at a particular location along the BHA is revealed by examining the underlying states of a dynamic model. Charts of the dynamic contact side force quantify the amount of dysfunction. Vibration indices comprising the integrated, length-averaged bending strain energy provide a good summary of the overall BHA response that is particularly useful in BHA design investigations, but it is the dynamics of the individual contact points that drive the dynamic response of the model. Examples of field BHA designs illustrate both good and bad drilling results, generally in agreement with expectations based on the modeling. The literature is full of references to the whirl mode of lateral vibrations. This is commonly accepted. Chatter is a different mode that occurs primarily in response to the spacing of blade contacts. It is difficult for two blades that are close together to simultaneously serve as nodal points. A lateral vibration wave propagating along the BHA may generate alternating wall contact at the two blades, causing a dynamic chatter dysfunction.