Cement bond logs (CBL's) are typically used to approve or condemn cement jobs. This "report card" has caused disputes between the cementing and logging companies for years because typically the CBL is interpreted independently from the cementing operation. Thus, a bad "report card" is usually translated into a bad cement job, which is often not the case. This paper illustrates the value of integrating cementing with bond logging to give a clearer picture of the actual quality of the cement downhole. Logs should be used as a tool to understand the cement quality and determine what improvements need to be made to enhance cementing job success.
Four examples are presented which show that factors such as 1) borehole conditions, 2) cement job design, 3) displacement practices and 4) tool selection and calibration all have an effect on the bond log interpretation. However, in all cases, integrating the log response with a thorough cement evaluation results in a more accurate analysis of the cement quality, and a better educated decision on the necessity for remedial work.
Complete isolation of production intervals from non-bearing intervals, water sands and thief zones is vitally important for successful exploration and development wells. Much care needs to be taken when engineering the cementing design and placement program necessary to obtain proper isolation. A few days after the cement is placed, a wireline company may run one or more cement bond logs to measure cement quality. Essentially, these logs represent the "report card" for the cement job. The tool calibration and logging parameters are determined by the cement properties measured in a laboratory. However, the true properties of the cement in the wellbore may be much different than those measured in the lab. This discrepancy is due to a number of environmental, operational and downhole parameters which are seldom incorporated into the cement design and lab test programs. Ignoring these factors can result in a log interpretation that is not representative of the true wellbore cement quality. This can lead to an ill-advised, unnecessary and costly decisions about remedial (squeeze) cementing. Integrating a thorough cement job evaluation with the log interpretation produces a much better picture of the true cement quality. It also allows for a more comprehensive decision about the squeeze cementing, this can save time and money!
From the cement designers point of view, the factors usually considered when designing a cement slurry and lab test program are: