An Improved Modeling Program for Computing the Torque and Drag in Directional and Deep Wells

Abstract

ABSTRACT In directional and deep wells, the torque and drag generated by contacts between the drillstring and the borehole play major roles in the following operations of the well: planning, drilling (trouble diagnosis and prevention), and casing running/setting.They also affect the completion/cementing operation. This paper discusses the shortcomings of the "soft-string" model, which has been widely used in the drilling industry. The soft-string model ignores the effects of drillstring stiffness, stabilizer placement, and borehole clearance. Consequently it generally shows reduced sensitivity to local borehole crookedness and underestimates the torque and drag. An improved modeling program is presented here that combines a BHA (bottomhole assembly) analysis in the stiff collar section, coupled with an improved soft-string model for the remainder of the drillstring. Case studies are presented to demonstrate the difference between this improved model and the soft-string model; and also to justify the approach of the improved model. For deep vertical wells, spiraling of the well can result in very severe torque and drag. Simplified formulas are derived, and examples are given to demonstrate the exponential increases of torque and drag as the well depth increases. In essence, deep vertical wells with spiraling tendencies should be handled as directional wells.