General Formulation of Drillstring Under Large Deformation and Its Use in BHA Analysis

Abstract

ABSTRACT The paper presents the basic formulations adopted in a 3-D BHA program called DIDRILSM*, for drillstring deformation under general static downhole loadings. These formulations account for large deformation effects, and have been simplified for fast solution using the finite difference method, while retaining second order terms. An explicit formula for the bit side force is presented. In addition to previously demonstrated effects of WOB, TOB, and borehole curvatures on the bit side force, this paper demonstrates some non-linear effects on the bit side force, which increase with large deformations, mainly due to increased borehole curvatures. For 2-D borehole with less than 5°/100′ curvature, the nonlinear effects on the bit side forces are in general small. The nonlinear effects are seen to be insensitive to WOB, TOB, and hole clearance for the examples considered. Effects due to these latter variables on the local force/moment distributions may be more pronounced.