CT drilling has proven to be a commercially viable technique for drilling horizontal drainholes with over 100 directional wells drilled in 1996. The horizontal reach of these wells is much less than with rotary drilling. The various weight transfer devices and methods currently used (or conceived) to enhance the reach with coiled tubing drilling are compared in detail.
For example, larger OD coiled tubing (CT) is the most effective way to extend the drilling reach if transport of the reel is not a problem. Tractors would be very effective to extend reach, but reliability and vulnerability to hole conditions may be problematic. A rotator (second downhole motor) to rotate the CT has the potential to drill a 20,000 ft horizontal drainhole. Composite CT is shown to have much less extended reach drilling capability than steel.
The results from field testing a solid bottomhole assembly (BHA), bumper sub (thruster), and weight on bit (WOB) equalizer are presented. The field test conditions were specifically selected for the difficulty of weight transfer. The WOB equalizer provided the highest rate of penetration (ROP) by a factor of 2. The equalizer also indicated (via a change in pressure drop) to the CT operator when the downhole WOB exceeded a preset value. Stalling of the downhole motor was experienced only with the solid BHA (without WOB equalizer or bumper sub).
The first known analysis of stick-slip motion with CT drilling is presented. A method is illustrated to deduce both static and dynamic coefficients of friction from pickup and slackoff data. These are used in the analytical model to calculate the motion of the bit as it drills off and illustrate both the problems of stick-slip drilling and the theory behind the effectiveness of the WOB equalizer.
Since the introduction of CT drilling in 1991, the business has grown dramatically as shown in Fig. 1. As the drilling of 2,000 ft lateral re-entries becomes routine and 3,300 ft is shown to be achievable, the question often arises "what is the maximum drainhole length achievable." Leising and Newman went into great detail to show what is possible and how this is determined for straightforward re-entries. The purpose of this study is to indicate what can be done to maximize the drainhole length possible today and what may be done to extend the reach of CT drilling in the future. The potential is obvious; the record CT drilled well has a horizontal section of 3,256 ft. The record rotary drilled well (as of March '96) has a horizontal displacement of 26,361 ft with a hole twice the diameter of the record CT drilled well.
Many techniques have been studied with rotary drilling to extend the reach; some of these apply to CT drilling and some do not (e.g., eccentric drillpipe). Only those which apply to CT drilling are considered here. The various means of extending reach are discussed with respect to potential problems, cost, risk, and potential benefit. The primary methods as graded by the authors' opinions are summarized in Table 1. These methods are discussed below.
Current Technology to Extend the Reach of CT Drilling Larger CT/Smaller Liner. The options of increasing the CT diameter or installing a liner to reduce buckling were previously analyzed. The results are shown in Fig. 2 for a geometry similar to that of Fig. 3 (without tubing in well). Fig. 2 covers the conventional re-entry application where the tubing is pulled from the well. With no tubing in the well, the CT will buckle inside the casing. Note the increase in drainhole length possible by increasing the CT diameter or running inside a smaller casing/liner. In many cases, this is the easiest way to extend the reach. P. 677^
