Borehole geometry and casing centralization are major contributing reasons for contamination of cement slurry by drilling mud and spacers during its placement in the annulus of an oil or gas well. This paper investigates the effect of fluid contamination on the mechanical properties of the annular cementby evaluatingcement sheath strength with the help of electromagneticacoustic transducers (EMAT). Correlation is presented by analyzing their influence on the shear horizontal (SH) guided acoustic waves in an experimental lab setup and then in the field.

Contamination of aqueous and non-aqueous fluids during the cementing process affects the strength and the elastic properties of the set cement. In certain situations contamination may prevent the cement slurry from setting or reaching the ultimate mechanical strengththat is needed to ensure an acceptable hydraulic seal.

As a part of the work covered in this paper, different levels of cement contamination are created in the lab model using standard ordinary Portland Cement (OPC) slurry. Continuous monitoring records property changes in the contaminated cement and the impact upon compressional and shear transit times. The shear slowness measurement is then combined with analytical and numerical modelling for a thorough analysis.

The shear measurements from the experimental test cell are used to characterize the response of the new sensor that produces shear horizontal (SH) waves on the casing. The SH waves respond only to annular material with shear rigidity and solids characteristics. Consequently, a decrease in the acoustic impedance of contaminated cement will not prevent the SH measurements from identifying solid material in the wellbore annuli. The laboratory model was initially used to demonstrate the impact of the cement's elastic properties and the corresponding effect on shear wave attenuation.

The SH response was successively validated in a customer well. After processing the SH attenuation and correlating it to the modelled acoustic response, a comparison between field measurements and lab-generated data was performed. The comparison provided quantification of the actual in-situ contaminated cement condition.

The cement shear properties offer an additional piece of information for drilling and cement engineers to evaluate cement sheath and validate well isolation objectives.This evaluation and validation become even more important when coupled with cement placement software as an integrated technique to identify deficiencies, improve modeling mathematics, and better understand a more complete picture of cement placement and wellbore isolation.

Cement integrity engineers have the task to interpret all the information at hand to properly assess cement isolation objectives. The shear wave attenuation concept discussed in this paper, combined with design, testing and job execution data, represents a unique source of information that enables increased confidence of decisions related to wellbore isolation. This novel and comprehensive approach will be helpful for evaluating and improving current cementing practices.

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