Drilling and Underreaming Simultaneously: A Cost-Effective Option
Authors: Dewey Charles H. , Miller Gregory C. 
Published: December 2007

Abstract
Recent technology advances have made underreaming a reliable element in today's drilling plans. Details on the proper use, reliability and economics of one underreaming option, underreaming-while-drilling, are given. Field data demonstrate how this advanced technique has made drilling operations more efficient and economical.

Introduction
Underreaming is used in the drilling of wells to open the borehole to larger than the internal diameter (I.D.) of the last string of casing. This is done to reduce casing program costs (smaller sized casing can be used from top to bottom of hole) and/or accomplish large bore completions at formation depth to improve production.
Although underreaming has been used for nearly 50 years with varying success, technology advances over the last decade have made it an economical, reliable component of today's drilling plans. In fact, it has become so reliable that many drilling programs call for extensive underreaming. Some programs even specify drilling and underreaming simultaneously, or underreaming-while-drilling (UWD), an increasingly useful option.

Underreaming-While-Drilling Overview
Underreaming has become a necessary process in the drilling engineer's arsenal. Successfully applied it helps to hold down overall well construction costs by enabling optimized well design. Today's underreaming tools can reliably cut longer intervals and harder formations at faster penetration rates than previous technology. They also operate successfully in the most difficult downhole environments, including high temperature, high pressure and high angle.
Advanced design and field testing have led to an underreamer capable of performing its task while drilling (Fig. 1). Placed in the bottom hole assembly (BHA), this tool can be activated or deactivated as needed, thereby eliminating a separate trip into the hole for underreaming prior to setting a string of casing. A single trip into the hole for both drilling and underreaming not only further controls well construction costs, but also can eliminate the risk and time associated with tripping long drillstrings (Fig. 2).
The optimized well plans and reduced costs enabled by the UWD technique are desired particularly in high-cost drilling environments, such as:–the deep offshore (e.g., the UWD technique is being applied regularly on rigs operating in deep water offshore Louisiana),–high-pressure/high-temperature (HPHT) wells, or–multistring wells.
This might include underreaming to enable a minimum clearance casing program, building a hole with smaller than usual surface and intermediate casing strings, or underreaming to maximize hole size across the production zone.
Unfortunately, experience with previous underreaming technology has led some engineers to be concerned about several areas when considering UWD. Among these are:–downhole failure of underreamer assembly components,–interference with drilling operations (e.g., angle corrections, drilling out of float equipment, etc.),–BHA vibration and imbalance,–irregular hole gauge,–inadequate hole cleaning and slow penetration rates, - incompatibility with directional tools,–hole size and angle limitations.
Over the last decade, however, technological innovations have been applied to reduce or eliminate most of these issues. Of the more than 300 underreaming jobs run since 1985 with a purpose-built UWD tool, over 60 have been true UWD scenarios.

Underreaming-While-Drilling Innovations
A mechanically robust yet customizable underreaming tool has been designed to reliably enlarge hole while drilling at increased rates and over long intervals (Fig. 1). Most hole sizes and angles can be accommodated.
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