A Method of Modeling the Directional Behavior of Bottomhole Assembles Including Those With Bent Subs and Downhole Motors

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Brett, J.F.
Gray, J.A.
Bell, R.K.
Dunbar, M.E.

Abstract
This paper presents an analytical model that has successfully predicted the inclination directional behavior of rotary drilling bottomhole assemblies, including assemblies that contain bent subs and downhole motors. The model has been installed as part of the Engineering Simulator for Drilling (ESD), and combines the effects on directional behavior of bottomhole assembly configuration, hole geometry, rock drillability, bit type and operating parameters such as bit hydraulics, weight-on-bit and parameters such as bit hydraulics, weight-on-bit and rotary speed. The model combines previous work dealing with bottomhole assembly analysis and bit mechanics (including side-cutting behavior), with original work defining the manner by which bits directionally penetrate.

The model's computation scheme depends on a microcoded version of a finite element bottomhole assembly behavior program running on an array processor. The system provides solutions of the finite processor. The system provides solutions of the finite element routine every one to six seconds. Solutions are required at this frequency in order to obtain stable directional predictions at 30 times faster than real time.

The paper also shows that comparisons of the model results with field data are good in areas where formation affects are benign. Comparisons of model predictions with field results in areas of significant formation directional affects are also shown. These comparisons are useful in quantifying the effect of the dip and strike on directional behavior, and in defining areas where future work is needed.