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Cement Bond Logs (CBL) and/or Cement Evaluation Logs are usually conducted to evaluate the quality of the cement bond behind a cemented casing or liner before a well test or production operation is performed in the well. These logs may also be carried out during workover operations to evaluate the integrity of the casing before sidetracking the well. In many countries, regulatory authorities require that cement bond logs be conducted in every well after cementing operations to investigate the presence and quality of the cement bond behind the casing. This is performed to evaluate the hydraulic and mechanical seal of the cement. There is still a prolonged discussion in the industry regarding the validity and reliability of such cement evaluation logs in determining the quality of the cement bond behind pipe. There is a faction which believes in the cement evaluation logs and uses them regularly while another does not believe in these logs and hence do not utilise them at all. The CBL, which records the amplitude and transit time of a sonic signal travelling along the casing, has been around for many decades. The measurement principle of the CBL tool is well known and understood. The new generation cement evaluation tools, such as the ultrasonic scanner tool, use an ultrasonic transducer to send an ultrasonic signal inside the casing and estimate the acoustic impedance of the cement behind the pipe. The measurement principle and processing of such a cement evaluation log is more complicated and can be sometimes difficult to understand. Furthermore, depending on the input parameters used, such as the impedance of the wellbore fluid, a cement evaluation log can give contradicting results. For example, a log which has been processed using a certain mud impedance showing very poor cement quality can be reprocessed with a different mud impedance and produce another log which indicates better cement quality. Therefore, it is important to understand how such a log is processed before making a decision to take any remedial action based on the apparently poor quality of the cement bond indicated by the log. In this paper, ultrasonic cement evaluation log examples illustrate the complex task of evaluating the quality of the cement bond. Likewise, even the old CBL can be affected by the properties of the borehole fluid. A heavy mud in the well can attenuate the sonic signal, leading to spuriously low CBL amplitude and hence falsely indicating a good cement bond. Cases like micro-annulus, if not properly recognised, will be interpreted as poor cement bond and may lead to unnecessary and unsuccessful attempts to squeeze cement into the annulus. The presence of a micro-annulus can be determined by re-running the CBL log with the casing under pressure. The case histories in this paper illustrate the importance of understanding how the cement bond and evaluation logs are acquired and processed, so that proper decisions can be made regarding the quality of the cement bond for any remedial action that needs to be taken. Examples are also given to illustrate the instances where total disregard of cement bond and cement evaluation logs can result in undesirable consequences leading to expensive remedial actions later. This paper presents case studies which will highlight the importance of evaluating cement bond quality behind pipe. It will indicate that cement bond and cement evaluation logs play an important role in identifying possible well integrity issues, which can result from poor cementing jobs. Therefore, due diligence and care should be taken in interpreting cement bond and cement evaluation logs to prevent costly mistakes. |
