A New, Integrated, Wireline-Steerable, Bottom Hole Assembly Brings Rotary Drilling-Like Capabilities to Coiled Tubing Drilling
Authors: Anderson D.R. , Dorel Alain , Martin Roy 
Published: December 2007

Abstract
Coiled Tubing Drilling (CTD) is becoming more widely used as the technology develops. CTD has wide applications in reentry and multiple branch drilling. Underbalanced drilling with coiled tubing is used as a means to increase productivity by reducing formation damage and to increase the rate of penetration. The technique is also used to reduce problems associated with lost circulation and differential sticking. A new bottom hole assembly (BHA) has been designed to be compatible with current and future CTD techniques.
The first generation of BHAs were often cumbersome devices which were slow to orientate, difficult to handle and had limited data transmission to the surface. Those BHAs which rely on mud pulse telemetry have been unable to operate in real time in the aerated fluids sometimes used in underbalanced drilling operations. New bottom hole assemblies have been developed that include wireline telemetry to allow the tool to be used with aerated fluids. Some of the new BHAs incorporate hydraulically powered orienters. In some, hydraulic fluid is pumped through umbilical lines inside the coiled tubing. These lines, together with a wireline cable represent a substantial restriction in the coil and may be difficult to replace when a problem arises. Another approach has been to generate hydraulic power within the tool using electric power conducted through a wireline cable. These hydraulic orienters can turn through approximately one revolution before the direction has to be reversed.
This paper describes a new, integrated system which uses wireline inside the coil to orientate the power section of the BHA electrically. The same wireline is used to transmit data from the sensor package at 100k bit/s compared to a few bit/s of mud pulse telemetry. The electrically operated orienting tool allows adjustments as fine as one degree in either direction or continuous rotation of the power section of the BHA. This feature of the tool permits a rotational element to be introduced into what is normally a sliding operation. This benefits directional stability and rate of penetration by reducing tortuosity.
The tool incorporates state of the art pressure sensors for tubing and annular pressure measurements required for control of the underbalanced drilling process. Gamma-ray, casing collar locator and direction and inclination sensors are located closer to the bit than in conventional BHAs. The surface data acquisition system is based on existing wireline and MWD technology and displays a full suite of drilling data for the coiled tubing driller.
Results from testing and case histories are presented to illustrate the benefits of the tool.

Introduction
Coiled tubing drilling is a young technology in the early stages of development. Apart from a few experimental wells, coiled tub in drilling began in the early 1990s with 4 wells drilled in 1991. This increased to 250 wells in 1995 and growth is expected to continue.
The reasons for using coiled tubing to drill are many and various; the smaller footprint makes it ideal for restricted onshore locations or small platforms offshore; for simultaneous operations with two rigs offshore; for drilling small diameter holes; sidetracks can be performed through existing production tubing without the need to remove it from the well; depleted formations, in which differential sticking or lost circulation has been a problem can be drilled underbalanced; formation damage can be reduced and rates of penetration increased by using underbalanced drilling techniques; underbalanced conditions can be maintained during drilling since no connections have to be made; work in wells where hydrogen sulphide may be present is performed more safely with coiled tubing than with jointed pipe; reduction in pipe handling enhances the safety of the crews.
At first, tools for use with coiled tubing drilling were derived from those used for conventional or rotary drilling. As operators became more familiar with the technique, coiled tubing was used to drill in increasingly complex environments. This in turn required the development of new tools. The bottom hole assembly described here represents the next generation of downhole tools for coiled tubing drilling.
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