Optimizing Pendulum Bottom-Hole Assembly Based on New Straightening Capability Evaluation Index

Borehole deviation is an important issue of vertical drilling, and the pendulum bottom-hole assembly (BHA) with one stabilizer is often applied to straighten it. The traditional design and application method mainly takes the bit side force (pendulum force) as the evaluation index of straightening capability. The positive influence of bit side force is often over-emphasized while the negative influences of the other factors are often neglected. To improve the design and application method of pendulum BHA, the equilibrium tendency method to predict the build-up rate (BUR) of a steerable BHA is introduced to directly predict the inclination change rate of pendulum BHA for the first time, and then taking it as new evaluation index of straightening capability to optimize the pendulum BHA. The main influencing factors and laws of straightening capability of pendulum BHAs in three boreholes are analyzed and compared, and new understandings have been obtained for the first time. The research results have shown that predicting the inclination change rate by the equilibrium tendency method and taking it as the evaluation index of straightening capability can improve the design and application of pendulum BHA. The straightening capability of pendulum BHA significantly changes with stabilizer position, weight-on-bit (WOB), bit anisotropy index and borehole diameter. Compared to the pendulum BHA (optimal stabilizer position is about 20 m, recommended specific WOB is below 0.35 kN/mm) in 215.9 mm borehole, pendulum BHA in larger boreholes can properly increase the stabilizer position and specific WOB, but pendulum BHA in smaller boreholes must properly decrease the stabilizer position and specific WOB. It is suggested not to apply the pendulum BHA in small borehole, because the allowable WOB for deviation control is often too small to meet fast drilling requirement.