A new cement integrity tool has been developed, the Segmented Bond Tool (SBT), which affords a complete cement bond analysis in one logging pass. An array of transmitter receivers, mounted on six pads, are utilized to extract quantitative measurements of the cement bond in six 60 degrees segments around the borehole. In addition, by effectively mounting the transmitters and receivers in contact with the casing wall through the use of pads, many of the problems common to the standard mandrel-type tools are effectively eliminated. Experimental laboratory tests indicate that angular resolution of the tool is 60 degrees as expected and that channels as small as 15 degrees can be detected.
From conception of this new generation CBL to final product, several rigid parameters were demanded. Briefly these are;
Ease of interpretation.
Eliminate ambiguity of conventional bond logs by qualitatively depicting the bond condition around the casing (i.e. channeling).
Complete data acquisition in one logging pass in all casing Sizes4–1/2 "to 13–5/8 " in both vertical and deviated well bores.
Be unaffected by gasified wellbore fluids, fast formation and slight tool decentralization.
The purpose of this paper is to determine the success of the SBT in fulfilling these goals. Presented here are four Canadian examples from widely varied areas having casing strings cemented with various slurries.
References and illustrations at end of paper.
As discussed in Lester's Paper(CWLS 1989) and as summarized below, the evolution of cement bond logging has resulted in four distinct tool types:
Standard Tool - Utilizes a single transmitter and dual receivers usually mounted at 3 and 5 foot intervals, with the amplitude measured at the 3 foot interval. Attenuation is calculated from this amplitude. The VDL is derived from the 5 foot spacing (Pardue et. al. 1962). The data collected is effected by tool centralization, well bore fluid condition, transducer variations, fast formations, and calibration problems.
Compensated Tool - Dual transmitters and two or three receivers are utilized to "compensate"for the inherent problems associated with the standard bond tool (Gollwitzer and Masson, 1982). Transducer variations are reduced by compensation and fast formation effects are curbed by closer transmitter/receiver spacing. Centralization, however is critical and the log data is significantly deteriorated in gasified wellbore fluids.
Pulse Echo Tool - Pulse echo tools employ an array of eight transducers in which reflection characteristic are generated at eight "points" around the casing wall (Froelich et. al. 1981). This was the first tool design which could detect channeling in the casing to cement area. Advanced tools are able to provide a more complete cement analysis, however two logging passes were required. Tool response is subject to wellbore fluids and centering problems similar to the standard tool.
Pad Mounted Transmitter-Receiver Tool - By mounting the transmitters and receivers on pads which then ride along the casing wall in the logging position, many of the factors which effect the earlier tools can be eliminated. The Segmented Bond Tool (SBT) utilizes transducers mounted on six pad sections as well as an improved 5 ft. VDL section. The pad section provides attenuation measurements around the casing in 60 degree segments averaged over 3 ft of vertical depth to accurately define channeling.