With almost 45% of wells drilled globally employing some form of hole opening technology, borehole enlargement has become commonplace. However, for many reasons it can still be very demanding, from both planning and execution points of view, particularly in the deep water basins being exploited today.

To address the needs of these challenging applications, a variety of tools have been developed by many service companies. There are two primary borehole enlargement methods involving eccentric and concentric tools. Contained within these two general categories are many options: fixed cutter, roller cone, mechanical, hydraulic, hydro-mechanical, bi-center and so on. Factor in the different drive types available (rotary, positive displacement motor steerable, rotary steerable, coiled tubing) and the potential borehole enlargement options can quickly lead to a confusing, even circular, decision making process.

With so many options, an important question must be answered by the drilling engineer: How does one select the appropriate borehole enlargement option for a specific application? With continually increasing costs and more difficult wells being drilled around the world, there is little room for error. Several factors must be weighed and evaluated before making the correct decision. Depending on the type of well being drilled and the formation characteristics, one must choose the optimal BHA and drilling parameters to obtain a successful borehole enlargement result.

This paper reviews the decision making process that can lead to the best chance of success in many borehole enlargement applications. Understanding the application is critical in delivering a successful borehole enlargement operation. Based on results obtained over many years of industry experience with applications involving all types of borehole enlargement technologies available, a practical guide to hole opening solutions has been derived. It takes into account critical factors, including economics, borehole quality, equipment availability (including rig capacity), drive type, drilling parameters, the type of well being drilled, and formation characteristics.

After a brief introduction and a description of currently available technologies, this paper will review the decisions that are made in determining which borehole enlargement tool to use and how to apply these in practice. This allows the drilling engineer to have a practical view of the available options and their versatility for proper tool selection.

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