Advances in Cement Evaluation Tools and Processing Methods Allow Improved Interpretation of Complex Cements
Authors: Frisch G. , Fox P. , Hunt D. , Kaspereit D. 
Published: December 2007

Evaluation of cement bonding and zonal isolation is a challenge that the oil and gas industry continues to face as wells are drilled deeper within more hostile environments. The complexity of this task has increased as these wellbores have more challenging trajectories and being drilled in formations for which there is little drilling and completions experience. In addition, cement slurries have become more complex with the addition of inert gases, micro-spheres, non-traditional liquids, and many other additives designed to improve the cement sheath quality. These slurries require non-traditional interpretation methods to effectively evaluate the cement sheath because older methods do not yield accurate results in these situations. This paper will present information concerning the existing cement evaluation logging tools, basic interpretation techniques, and an overview of the new, advanced methods for existing tools available from a variety of vendors in the industry.

Progress is continuously being made in the development of more effective cement evaluation tools and evaluation techniques. Standard cement evaluation logging tools consists of two major classes, sonic and ultrasonic. The standard cement bond log, segmented bond log, and radial bond log are all part of the sonic logging family. The ultrasonic family consists of tools with either a rotating transducer or a stationary array of transducers. This paper, however, will not focus on the hardware but will focus on the interpretation of available measurements and on facilitating optimized decisions using measurements from both families.

Advanced interpretation methods discussed in this paper broaden and refine previously published methods in order to effectively evaluate wellbore conditions with the commonly available cement evaluation tools. The original processes developed in the early 1990s now incorporate a statistical variance mapping display for both the sonic and ultrasonic tools. The resulting variance image from the ultrasonic tools allows detection of minor changes in cement or fluid composition and aids in the interpretation of the pipe-to-cement bond. This technique provides a robust answer product helpful in diagnosing zonal isolation and highlighting channeling and quality of materials behind pipe for all cement compositions. It is also possible today to process and interpret non-standard sonic data, such as refracted monopole and flexural dipole from logging tools not specifically designed for cement evaluation.

Correct application of the newer interpretation techniques described in this paper can lead to fully evaluated cement sheath quality and distribution behind pipe. Several examples using the new advanced interpretation methods will be presented including comparisons (1) between a scanning ultrasonic tool and a radial bond tool (2) a sequence of evaluations using a cement bond log combined with an ultrasonic tool before and after several multiple remedial squeeze operations, and (3) also a comparison between two different scanning ultrasonic tools and a segmented bond tool. The final example shows a successful use of the new technique in a completion using titanium casing.