Large Diameter Coiled Tubing Drilling
Authors: Elsborg C.C. , Graham R.A. , Cox R.J. 
Published: December 2007

(*now with Dowell Schlumberger Ltd.)

Abstract
Coiled tubing drilling has come of age through 1995 and 1996 with the beginnings of technical and economic successes utilizing coiled tubing as a medium to drill underbalanced wellbores. With the development of field reliable downhole tools, coiled tubing drilling is now being accepted by many operators as the technology of choice in the growing underbalanced drilling market, where technically feasible.
The obvious next step is to find out the limitations, both technically and economically, for the application of this technology. The bulk of the early development centered around the use of 50.8 mm coiled tubing and 73.0 mm bottom hole assemblies. The advent of 60.3 mm coiled tubing provided more leverage and propelled the development of 89 mm bottom hole assemblies to drill 120.7 mm holes. Now, the use of 73.0 mm coiled tubing has provided the basis to drill with 120.7 mm tools in hole sizes up to 158.8 mm.
This paper examines the pro's and con's of the use of these larger diameter tools in one of the more popular hole sizes used in the Western Canadian Basin, both from a technical and economic perspective. Some case studies are offered and discussed for the observations noted. The potential for this application and a projection as to where it is heading is also discussed.

Introduction
Coiled tubing underbalanced drilling has reached the first plateau where the gap is being bridged from technical to economic success. Reliability and repeatability is being established as the database of wells drilled with coiled tubing grows. It is also important to understand that the industry is only beginning to come to terms with the intricacies of underbalanced drilling as a viable technology for reservoir management.
This paper looks at the objectives for development of a high porosity, high permeability sandstone reservoir in Southern Alberta and the rationale behind the selection of coiled tubing. Drilling technology as a vehicle to meet those objectives. The theory behind the technology is examined for technical feasibility through the use of circulation and drag models and the methodology employed is outlined. Two case studies will be examined where 156 mm hole was drilled out of 177.8 mm casing and economics of a cross-section of wells will summarize the outlook for application of this technology.

Project Objectives
Underbalanced coiled tubing drilling and horizontal technology was selected as the most suitable technique to improve the economic benefits of the project. The Glauconite "A" field required the application of this technology to attempt to achieve the following:–confirmation of the technical feasibility and benefits of underbalanced, large diameter coiled tubing drilling.–reduction of formation damage by maintaining underbalanced conditions 100% of the time while drilling. - improvement of oil productivity while minimizing drawdown to reduce the potential for water coning.–reduce drilling problems associated with high porosity and permeability, and an underpressured reservoir (i.e. differential sticking).–cost effective underbalanced horizontal drilling compared with overbalanced horizontal drilling.

Technology Development
The use of 73.0 mm coiled tubing for drilling has found application in two main hole sizes: 156 mm hole size out of 177.8 mm casing and 121 mm hole size out of 139.7 mm casing. The former has expanded the application to one of the more popular hole sizes for horizontal drilling where downhole tools are well developed and readily available. The latter has provided an alternative to the use of 60.3 mm coiled tubing with improved hole cleaning and weight on bit characteristics.
One of the disadvantages of the larger coiled tubing sizes is the relative decrease in cycle fatigue life. P. 187