Unsolicited. This document was submitted to SPE (or its predecessor organization) for consideration for publication in one of its technical journals. While not published, this paper has been included in the eLibrary with the permission of and transfer of copyright from the author.
The Cement Bond Log is a very useful and common tool used in the Petroleum Industry. It is also a very poorly understood tool and in many cases a mistrusted one.
With the increasing price of oil, a larger number of borderline wells are becoming economical. Many wells, which in the past would not come on stream due to the presence of an aquifer, are being completed every day. The successful completion of this type of well requires a hydraulic seal between the aquifer and the potential production zone. The Cement Bond Log is the only tool that can be used to indicate the effectiveness of this hydraulic seal, without damaging the seal or the reservoir.
The fundamental problem with this tool is that it is difficult to interpret accurately and it also has innate sources of error which are generally not compensated for. Initial errors made in calibrating the tool can result in completely erroneous cement bond indications; either optimistically or pessimistically. A badly centralized sonde is extremely serious-and results in pessimistic CBL indications.
Besides these sources of error, there are many ways to improve the potential for a good hydraulic seal, and there are also circumstances where poor CBL indications can still be acceptable. By using neat cement and good cementing procedures, the potential for good hydraulic seal is greatly improved. When a cement with additives, such as a gel, is used, the CBL seldom shows as good a seal as neat cement.
This paper describes some of these problems in depth and suggests that they should be taken into account in the interpretation.
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