This paper will identify the increasingly stringent requirements of bit and mill design associated with small holes (particularly <31/2") for use with coiled tubing. Recommendations are made on the correct slim hole bit/mill design to use when drilling a variety of different targets. In essence, this paper will show that as the bit/mill size decreases, the bit/mill choice becomes driven more by coil and downhole motor considerations; as the bit size increases, the bit choice becomes driven more by the properties of the target to be drilled.
Coiled tubing drilling and milling operations can be very fast and efficient, leveraging of coil's advantages of fast rig up, fast tripping times, and inherent live well capability. Coil has been successfully used to drill vertical and deviated wells, mill cement, fish, scale, ball valves, mill windows, in fact a very wide range of applications. This paper discusses both drilling and milling operations. However, for the sake of simplicity, the terms "drilling" and "bits" will be used generically to cover both milling and drilling operations with both bits and mills.
Much of the published information on coil drilling describes operations with larger coil units (>2") drilling what are considered large holes (>31/2"). This paper will focus to a greater extent on the bit requirements when drilling smaller holes, usually with smaller coil, for both workover and open hole drilling applications. The paper will show that in fact, it is generally more difficult to engineer an optimum slim hole drilling assembly for use with small coil than it is a large hole drilling assembly for use with large coil.
This last statement may appear contrary to the prevailing belief that slim holes are simpler than large holes. Large holes can be problematic for coil, primarily due to poor hole cleaning ability resulting from the relatively low flow rates achievable through coil. This limitation, however, is not necessarily related to the bottom hole assembly (BHA). Small holes can be problematic for coil, not because of hole cleaning concerns, but because of the limited torque output from the smaller down hole motors, the limited rigidity of small diameter coiled tubing and a more limited bit choice in the small diameters.
Drill bit and mill design is supported by an entire industry. The science behind bit/mill design is well developed. This paper will not reiterate the established theory; it will instead focus on the bit requirements that are unique to coil drilling. It will show that in some circumstances, coil actually introduces few additional considerations and the bit/mill selection process is very similar to the process used for conventional rigs. However, for smaller coil in smaller holes, correctly modeling the coiled tubing prior to selecting the bit is generally critical and greatly influences the bit design.