Cement Evaluation - A Risky Business

Abstract Cement evaluation is commonly thought of as running a cement bond log (CBL) and attempting to interpret the results to determine if there is isolation in the wellbore. Oftentimes 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 a CBL. Understanding the objectives of the cement job, the design limitations imposed by those objectives and the resulting slurry and job designs are all integral parts of cement evaluation. Oftentimes the selection of a specialty cement system to meet specific well requirements can dictate how the cement can be evaluated. To properly evaluate a cement sheath, knowledge of the cement job, slurry designs and the limitations of the evaluation technique must be understood. To attempt to perform a cement evaluation 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 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 boutique cement designs, which incorporate high concentrations of inert materials to give the set cement unique properties, is discussed. The ability of specific logging techniques to evaluate the presence of these slurries is presented. Data on selected boutique cement systems where conventional UCA strength data is not representative of the crush strength of the cement due to the incorporation of specialty materials is included. An overview of cement evaluation, and a risk based discussion of what technique may be most appropriate based on the cementing objectives is 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 the ability to evaluate the resulting cement placement and well isolation can be determined. Setting the evaluation methodology and understanding the type of information required to apply that methodology can improve the quality of the evaluation.