Through-Bit Logging: Applications in Difficult North Sea Wells
Authors: Denney Dennis 
Published: February 2015

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 92256, "Through-Bit Logging: Applications in Difficult Wells Offshore North Sea," by John Runia, SPE, Shell EPT-R; John Boyes, SPE, Precision Energy Services; and Robert Jan Lodder, Shell EPE, prepared for the 2005 SPE/IADC Drilling Conference, Amsterdam, 23-25 February.
Through-bit logging (TBL) is a data-acquisition method that uses the drillstring and bit as a conduit into the wellbore. Two case studies demonstrate obtaining data in difficult and trouble-some well situations in the North Sea. Recent developments include a new type of side-entry subassembly for surface deployment as well as provide a fishing method in case the logging tools become stuck during TBL operations. Compatibility with rotary-steering and measurement-while-drilling (MWD) operations is under development.
Introduction
Data acquisition in oil and gas wells has evolved over the last century. Wireline logging is mature, and logging while drilling (LWD) is developing rapidly, giving the end user options to perform data acquisition during the drilling phase, in open hole after drilling, or subsequently in the cased hole. All options have advantages and dis-advantages, and a balance must be struck between cost, quality, risk, and timing of data acquisition.
In the new system, the logging tools are conveyed through the drillstring and pass into the open hole through a specially designed bit, with formation-evaluation data acquired with either wireline or memory tools—in the latter case, while tripping drillpipe out of the well, thus improving rig efficiency.
Acquiring Log Data
Fig. 1 shows how the logging tools are lowered inside the drillstring, passing through the bit into the open hole. The TBL bit shown in Fig. 2 is a conventional polycrystalline-diamond-compact (PDC) -bit design modified with a removable central section leaving a 2½-in. passageway. During nor-mal drilling operations, the central section (insert) is latched into the bore of the bit. When required, the insert can be released outwardly with a running tool attached to the bottom of the logging string. The running tool releases the insert and carries it (locked to the logging string in a fail-safe manner) into the open hole below the bit. Bit performance is similar to that of the standard PDC designs because, as Fig. 3 shows, the outward form is identical to the original, and the internal latching mechanism does not interfere with basic bit functionality.
There are two methods of running the tool. For simple once-off memory-logging applications, the intention is to pass the logging string through the bit, then log in memory mode back to surface while tripping pipe out of the hole. The more sophisticated two-way tool enables a retrievable/reversible-latching method. After logging, the logging tool can be pulled back through the bit (and back to surface), closing the bit for further drilling or additional logging. Currently, the tool is run as part of a check trip or wiper trip [removing any restrictive bottomhole-assembly (BHA) items, such as mud motors and LWD tools].