Cement Evaluation—A Risky Business

Abstract

Summary Cement evaluation is often limited to running a cement-bond log (CBL) and making some attempt to interpret the results and determine whether there is isolation in the wellbore. Often, that interpretation is made in isolation with little or no information on what occurred during the drilling and cementing of the well, or the cement systems used. Evaluating cement in older wells where the drilling report states “ran casing, cemented same” can be particularly challenging. Cement evaluation is much more than just a CBL. Understanding the objectives of the cement job, the design limitations imposed by those objectives, and the resulting slurry and job designs are integral parts of cement evaluation. Often, the selection of a specialty cement system to meet specific job objectives can dictate how one can evaluate the cement. To properly evaluate a cement sheath, knowledge of the cement job, slurry designs, and the limitations of the evaluation technique. To attempt to perform a cement evaluation that is based solely on the log output from a CBL, or any log, invites considerable error and bias into the resulting interpretation. This paper reviews various methods of cement evaluation—from job data, casing- and formation-pressure testing through sonic and ultrasonic logging. The assumptions associated with each technique are outlined, and the discussion includes some of the limitations of the various techniques, along with cautions on how misinterpretation of the results can lead to assumptions of cement integrity that may not be appropriate. The impact of new specialty cement designs, which incorporate high concentrations of inert materials to give the set cement unique properties, is discussed. The ability of specific logging methods to evaluate the presence of these slurries is presented. Data on selected specialty cement systems in which conventional ultrasonic cement-analyzer-strength data are not representative of the crush strength of the cement caused by the incorporation of specialty materials are included. Both overview of cement evaluation and a risk-based discussion of the technique that may be most appropriate on the basis of the cementing objectives are presented. Methods of reducing risk uncertainty in cement evaluation are discussed along with the “validity” of the various data sets available to the engineer to perform a proper cement evaluation on the well. Understanding the objectives of the cement job sets the boundary conditions for the designs, and from those designs, one can determine the ability to evaluate the resulting cement placement and well isolation. Setting the evaluation methodology and understanding the type of information required to apply that methodology can improve the quality of the evaluation.