Sidetracking Technology for Coiled Tubing Drilling
Authors: Leising L.J. , Hearn D.D. , Rike E.A. , Doremus D.M. , Paslay P.R. 
Published: June 2014

Summary
Coiled tubing (CT) drilling is a rapidly growing new technology that has been used for shallow new wells and reentry applications. Averaged over the last 4 years, the number of wells directionally drilled with CT has doubled each year from 3 wells in 1991 to over 50 in 1995 (Fig. 1).
A new market has evolved as being a major application for CT drilling. This market is through-tubing drilling. The motivation driving through-tubing drilling is low cost rigless reentry when overbalanced and safe drilling with Christmas tree and tubing in place when underbalanced (for reduced formation damage). The lower cost of mobilization of a coiled tubing unit (CTU) to an offshore platform or Arctic wellsite vs. a rotary rig provides additional economic incentive. In addition, the ease of drilling 4-3/4-in. and smaller boreholes with CT is an advantage in a region which does not have an established practice of slimhole drilling.
The remaining key enabling technology for viable through-tubing drilling is the ability to sidetrack in casing below the tubing tail. The three technologies (cement sidetracking, whip stock in cement, and through-tubing whip stock) that have been developed for sidetracking are described in this paper.
A mathematical model of forces, penetration rates, and torques for window milling with the cement sidetracking technique is presented. Window milling has been a "seat of the pants" operation in the past. to the authors'knowledge. this is the first published work on the mechanics of window milling. The analysis has shed much light on the interaction between motor bending stiffness, motor bend angle. and allowable advance rates for "timedrilling."
The results from several yard tests are presented, and indicate some of the problems associated with sidetracking. The photographs of the sectioned hole/window illustrate the ledges caused downhole from minor" bottomhole assembly (BHA) changes.
The cement sidetrack technique has been successfully applied many times in the field, and the results of one of these field applications is presented.

Introduction
When faced with a production problem in the past, an operator had nodrilling options unless the tubing was pulled and a conventional reentry well was drilled through production casing. While this is done at a reasonable coston most land operations, the rig mobilization costs offshore (and under Arctic conditions) dictate that only the wells with the highest potential can justify this type of expensive reentry. In areas of high rig mobilization costs, arigless through-tubing reentry can save 50% or more over the cost of a conventional reentry.
With the advent of CT drilling. the option of through-tubing drilling has become a reality. Now, when faced with a production problem. an operator has the following through-tubing sidetracking options:–cement sidetracking (CS)–whipstock in cement (WIC)–through-tubing whipstock (TTW).
Each of these techniques is discussed individually.
Cement Sidetracking (CS). This technique is the most straightforward.