Guide to Borehole-Enlargement-Tool Selection

Abstract

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 142431, ’A Practical, Application-Based Guide to Borehole-Enlargement- Tool Selection,’ by J. McCarthy, SPE, J. Rebellon, SPE, S. Barton, SPE, and R. Rambhai, National Oilwell Varco, prepared for the 2011 Brazil Offshore Conference and Exhibition, Macae, Brazil, 14-17 June. The paper has not been peer reviewed. Borehole enlargement (BHE) has become commonplace. Still, it can be very demanding from planning and execution points of view, particularly in deepwater basins. To address the needs of challenging applications, a variety of tools has been developed. The two primary BHE methods use either eccentric or concentric tools. Depending on the type of well that is being drilled and the formation characteristics, the engineer must choose the optimal bottomhole assembly (BHA) and drilling parameters for successful BHE. Understanding the application is critical in delivering a successful BHE operation. Introduction BHE technology has enabled exploiting complex and unconventional reservoirs. Heavier and larger casing strings are set at greater depths, allowing operators to obtain improved recovery rates from the reservoirs. Each application must be analyzed thoroughly to provide the best design. Proper selection of BHE technology is critical. Improper selection of BHE technology can lead to complete loss of the well. For this paper, a BHE tool is one that can expand a borehole beyond the diameter at which it can pass through. Typically, this means creating a borehole larger than the inner diameter of the casing already in place. BHE tools are defined by their pass-through diameter and their expanded diameter, or drill size. BHE tools can pass through casing (or other restriction) to create a larger-diameter hole than the restriction and then can be retrieved safely from the well. To select the appropriate BHE technology, the engineer must understand the intricacies of the application and configure the drilling system to provide the best results in an economical and safe manner. Several factors affect this selection, some of which are set before the operation begins. Factors such as economics, rig capabilities, borehole quality, and drive type or system used to drill the well must be reviewed during the selection process. BHE Technology Fig. 1 shows the two primary categories of BHE tools: eccentric and concentric. Tools can be placed in the drillstring at two locations (at the bit or up in the drillstring). A third category of tools, the hole opener, often is thought of as a BHE tool, but these tools cannot generate a borehole larger than the minimum restriction of a well, so they do not fit the definition used here. However, they are sometimes used in conjunction with BHE tools, as a secondary cutting structure to provide a solid platform for the BHE device. Many hole-enlargement products are available, each of which fits a particular application. The simplest are the fixed-cutter designs.