The presence of cement behind casing can easily be identified by the current generations of cement evaluation tools. However, there are a few important questions to be answered in which many tools fail; is the cement attached to the casing, does any micro or macro annulus occur over intervals of interest and is the cement attached to the rock to prevent micro or macro fractures?
Many tools are incapable of answering these questions. Analysis of the cement bond tool data is very crucial for forthcoming decisions on a well. Therefore, it is important to identify, cement qua/iv with a capable tool.
In this paper, case histories are addressed in both, poor and fair cement jobs behind casing. In the field examples, the effect of tool centralization on the log quality, the means and ways of identifying micro and macro channels, and possible remedy based on correct cement log evaluation and analysis are addressed.
Many logging tools are available in the oil industry market to evaluate cement quality behind casing. Most of the available tools can easily identify the presence or absence of cement in the annulus, however, the challenge is cement quality.
There are three important regions that challenges these tools:
Cement attachment to the casing. This factor was deemed important during the 1960's through to the 1980's.
Cement body quality and if micro fracture is generated within the cement body after it is set. This point was ignored for many years.
Cement - formation attachment.
Old cement logging tools concentrated on the attenuation to reflect qualitatively the cement-casing bond, whilst the VDL is still the tool that reflects the answer to the other two points mentioned above.
There are many reasons for the generation of micro and/or macro annulus over the cement body or its attachment to the steel or formation(2,3). Some of these factors can be controlled which producing better cement bond, while other factors are difficult to control.
The most important thing for any cement bond tool, in order to acquire reliable data, is to be centralized inside the casing. This situation is very difficult to achieve especially when the hole is deviated by more than 10 degrees. Since the VDL section is located at the bottom of the cement bond tool, it is easier to centralize and, therefore, can acquire reliable data.
All cement tools acquire travel time data, which indicates if the tool is centralized or not(4). Travel time is the time for a pulse to travel from the transmitter through the mud, 3 feet along the casing, through the mud again and then to the receiver. Therefore, the position of the first amplitude peak (EI) will change or dampen depending on how much the tool is centralized. For example, if the tool is eccentered by 118th inch the percentage of the amplitude signal will be dampened by 25%. This results into better cement quality indication even though there might not be good cement behind the casing.