BHA Lateral Vibration Chatter Impedes Application of Weight to Bit

Abstract

Abstract Lateral vibration modeling of certain BHA (bottomhole assembly) designs has shown great sensitivity to the proximity of stabilizer blades. In a previous paper, the BHA chatter vibration mode was presented, including the basic theory and three field examples. In each case, lower drilling rates were associated with higher levels of BHA dynamic dysfunction as determined using a lateral vibration model. One case clearly illustrated a reduction in effective weight applied to the bit, determined using bit dull grade results. This paper will further explore the mechanisms in which BHA dynamic contact forces can act to reduce the effective weight on bit. The contact forces that push a stabilizer blade to be constrained within a borehole include both static and dynamic components. 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. In drilling, we simply see the effects as worn and damaged drillstring components. A frequency-domain BHA lateral vibration model represents lateral vibration flexural waves propagating in the BHA. This model can be used to calculate dynamic side forces acting at stabilizer and LWD tool blade contacts with the borehole for reference excitation forces. These contact forces may be strong functions of rotary speed and BHA contact spacing, and they typically create a non-trivial level of residual drag that impedes getting weight to bit. The physics of dynamic contact forces are similar in some respects to static contact forces. Superposition of the dynamic modes illustrate that, although an individual component may vanish momentarily, additional modes and phases can constructively combine such that the total force summation does not vanish. The net result is that dynamic forces can generate significant lateral forces that are similar to static contact forces. Static effects are known to impede application of weight to bit in deviated and horizontal wells, and small ledges can prevent a stabilizer blade from moving forward. Dynamic effects from improper stabilizer placement are potent sources of lateral dysfunction that can have a similar effect. BHA dynamics have the potential to both create borehole irregularities and resist forward motion resulting from contact forces between drilling tools and the borehole.