Predicting the Build/Drop Tendency of Rotary Drilling Assemblies

Abstract

Summary Today, the majority of rotary bottomhole assemblies (BHA's) for directional control are designed through practical experience and trial and error. This approach can produce satisfactory results when a great deal of local experience can be drawn on. It can prove costly, however, during drilling in a new area because of the increased number of trips and correction runs. This paper demonstrates how a BHA model can be used to predict the directional inclination tendencies of rotary assemblies, thus limiting the uncertainty associated with the traditional BHA design techniques. The technique is demonstrated on data from 17 bit runs from three wells on the same platform in the Gulf of Mexico. Predicted tendencies from BHA descriptions alone proved to be accurate (to an error of ±0.1°/100 ft [±0.03°/10 m]) in more than half the cases. The uncertainty of other predictions appeared to depend on the hole gauge. The distance taken for a BHA to reach a stable build/drop rate after the start of a bit run depends on the length of the BHA. This factor must be taken into account in the prediction of BHA performance.