Vibration and Rotation To Extend Coiled-Tubing Reach

Abstract

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 106979, "Vibration and Rotation Considerations in Extending Coiled-Tubing Reach," by K.R. Newman, SPE, CTES, prepared for the 2007 SPE/ICoTA Coiled Tubing and Well Intervention Conference and Exhibition, The Woodlands, Texas, 20–21 March. In recent years, there has been increased use of coiled tubing (CT) to drill shallow wells or to drill laterals from existing wellbores. Often the weight on bit (WOB) that can be applied with CT is limited as a result of friction between the CT and the wellbore, making some drilling operations impossible. Two methods proposed to reduce this friction are CT vibration and rotation. The full-length paper presents research performed to determine the effectiveness and practicality of these two means of friction reduction. Introduction The US Dept. of Energy (DOE) funded research in microhole drilling with CT. A portion of this funding was devoted to researching methods for mitigating downhole friction to enable CT to be used for extended-reach-drilling applications. Small-diameter CT (used for microhole drilling) helically buckles easily when in compression. Once helically buckled, additional wall-contact forces (WCFs) are caused by CT contact with the wellbore. These additional WCFs increase friction between the CT and the wellbore. This friction increases exponentially with the compressive force in the CT until no additional force can be transmitted to the bit. This situation is known as helical lockup. When the axial force reaches the helical-bucking load (HBL), the additional WCF caused by buckling far exceeds the WCF resulting from weight. Methods To Increase WOB. Geometry Changes. Increasing CT diameter or decreasing hole diameter is an effective way to increase WOB. Increasing CT wall thickness may increase WOB, especially in vertical wells. Lubricants. Various lubricating fluids have been used to reduce the friction coefficient between the CT and well tubulars or the wellbore. These lubricants have been somewhat successful in cased-hole workover applications. However, in drilling applications, the drilling fluid must turn the downhole motor, maintain well control, and carry the cuttings out of the hole. It is difficult to design a drilling fluid that will perform all of these functions and also provide significant lubrication. Tractors. Downhole tractors are available for CT-drilling (CTD) applications. Tractors provide the WOB so the CT can remain in tension. However, a downhole tractor adds additional deployment logistics, risk, and expense. Currently, tractors are not available in sufficiently small diameters to perform microhole drilling. Rollers. Various types of rollers have been developed that can be attached to the CT and to the drilling bottomhole assembly. The time and logistics associated with attaching and removing rollers make them undesirable.