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

Abstract

SPE Member 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 planning, drilling (trouble diagnosis and prevention), and casing running/setting. They also 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. Introduction A. Challenges In Directional Drilling: As oil and gas exploration becomes more (expensive due to more severe environments deeper offshore, artic areas), there is an increasing urgency to reduce the total drilling, completion, and production cost of a well in order to develop a reservoir more economically. This will reduce the influence of price fluctuation of the world's crude oil on the exploration and development programs of the operating companies. Directional drilling is now regarded as one way to achieve this objective. Directional drilling is increasingly being regarded as an effective means to minimize overall development and production cost of an oil field, particularly for the following situations:Drilling multiple directional wells from the same platform or rigsite, particularly in offshore and arctic areas, to reduce rig cost;Drilling "horizontal" wells to improve production drainage, avoid water coning, and develop very thin reservoirs. While the outlook on directional drilling is very positive, there are many unique technical problems that need to be resolved in order to further reduce the total cost of a directional well. The following two problems are particularly significant:Trajectory prediction and control;Torque and drag generated by the drillstring's eccentric deformation and the consequent contact between the drillstring (or casing) and the borehole. A comprehensive drilling program must therefore address the following issues in an integral manner:planning/prediction/control of well path; P. 407