Coiled-Tubing Drilling In Perspective
Authors: Byrom T.G. 
Published: June 2014

Distinguished Author Series articles are general, descriptive representations that summarize the state of the art in an area of technology by describing recent developments for readers who are not specialists in the topics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and present specific details only to illustrate the technology. Purpose: to inform the general readership of recent advances in various areas of petroleum engineering.

Summary
Coiled-tubing (CT) drilling is a technology of the current decade. The question that many operators have regarding CT drilling is, "Should we consider using CT drilling in our operations?" CT drilling has a considerable number of advantages over conventional drilling methods in certain applications, but it also has a number of limitations that an operator must be aware of. The decision to use CT for drilling must be based on its capabilities because conventional economics seldom favor its use as a drilling method. CT does have unique capabilities that make it excel in true underbalanced drilling and in certain re-entry applications.

Introduction
Openhole drilling with CT technology is truly a product of the 1990's. In1990, Fultz and Pitard1 showed that it was a feasible concept, and Ramos et al.2 reported on its use in a successful producing-horizontal-well case history in 1992. The technology was conceived, was born, and grew within this decade, and its growth and successes in this brief time interval are astounding. CT offers a number of advantages and unique capabilities over conventional methods for openhole drilling and likewise has a number of disadvantages and limitations.
CT Capabilities.Drill and trip under pressure.Fast trips.Continuous circulation while tripping pipe.Continuous, high-quality two-way telemetry between surface and downhole for real-time data and control.Slimhole capability.Small location size.Portability.
CT Limitations.Cannot rotate.Limited fishing capabilities.Small diameters.Limited reach in horizontal laterals.Low circulating rates.High circulating pressures.Short tube life.High maintenance.High daily costs.Limited availability of high-capacity units.
Despite these limitations, there are a number of applications where the unique capabilities of CT make it a better choice than conventional methods. Ingeographic areas where it has proved itself, before recent oil price declines, there were waiting lists of several months for the available CT-drilling units.

CT-Drilling Equipment
A common misconception is that a CT-drilling unit is a small self-containedunit that drives up on location ready to drill with a minimum of rigup involved. In truth, the typical CT-drilling unit must have much of the same equipment that a conventional drilling unit has (e.g., circulating pumps, mudtanks, solids-removal equipment, mud-mixing facilities, well-control equipment). It commonly requires about five to six truckloads of equipment and possibly more if the drilling operation is to be done underbalanced. Also, CT-drilling units with 2 3/8-, 2 7/8-, or 3 1/2-in. outer diameters (OD's) are not exactly the small unit most operators are accustomed to seeing on location for typical well-service jobs. These are necessarily high-capacity units with more associated equipment. Newer units built specifically for CT drilling are designed to overcome some of these mobility problems. Some of these units also have conventional pipe-handling capability, which greatly enhances their utility. Others are being constructed primarily as small drilling rigs that also have CT-drilling capability.

Downhole Equipment.
The downhole equipment for CT drilling can be simple or quite complex. For a simple vertical-drilling project, it may consist of a bit, a downhole motor, and a few drill collars. It is much more elaborate for a directionally drilled well.
A typical directional bottomhole assembly (BHA) consists of a bit; abent-housing mud motor; a steering tool to sense and transmit the directional data; an array of optional sensing devices, such as gamma ray and other logs, bottomhole-pressure (internal and external), weight on bit, bit torque, temperature, and vibrations; and an orientation tool to change the direction of the bit (Fig. 1). All sensing and control devices are in continuous two-way communication with the surface through one or more electric and often hydraulic lines inside the CT.
CT Capabilities.Drill and trip under pressure.Fast trips.Continuous circulation while tripping pipe.Continuous, high-quality two-way telemetry between surface and downhole for real-time data and control.Slimhole capability.Small location size.Portability.
CT Limitations.Cannot rotate.Limited fishing capabilities.Small diameters.Limited reach in horizontal laterals.Low circulating rates.High circulating pressures.Short tube life.High maintenance.High daily costs.Limited availability of high-capacity units.
Despite these limitations, there are a number of applications where the unique capabilities of CT make it a better choice than conventional methods. Ingeographic areas where it has proved itself, before recent oil price declines, there were waiting lists of several months for the available CT-drilling units.

Downhole Equipment.
The downhole equipment for CT drilling can be simple or quite complex. For a simple vertical-drilling project, it may consist of a bit, a downhole motor, and a few drill collars. It is much more elaborate for a directionally drilled well.
A typical directional bottomhole assembly (BHA) consists of a bit; abent-housing mud motor; a steering tool to sense and transmit the directional data; an array of optional sensing devices, such as gamma ray and other logs, bottomhole-pressure (internal and external), weight on bit, bit torque, temperature, and vibrations; and an orientation tool to change the direction of the bit (Fig. 1). All sensing and control devices are in continuous two-way communication with the surface through one or more electric and often hydraulic lines inside the CT.