Cement Evaluation with Ultrasonic and CBL/VDL Combination showing Microcannulas.
Figure shows responses from the ultrasonic and CBL Cement bond log /VDL logs run in a well-cemented with a 12 lbm/gal cement. The logs were run at two wellhead pressures: zero and 2000 psi. The cement bond index at these pressures is not similar indicating the presence of microcannulas in the cement. The acoustic impedance of the cement solids is between 3-5 MRayls. The cement map from the ultrasonic tool shows dark regions with good cement and light regions having poor cement.
Challenges to Cement Evaluation
The cement evaluation methods should be able to describe cement properties irrespective of its density, compressive strength, or quality. The acoustic logging devices have to be carefully used for the evaluation of low density, low compressive strength cement such as foamed cement, cement with sponge-like microspheres, cement containing hydrogen for control of annulus flux, or any cement containing gas percolation. The compressive strength of the lightweight cement is low. Hence, the acoustic impedance of the lightweight cement is slightly more than that of the drilling mud. The devices should also be able to work in high mud weights and thick-walled casings used in ultra-deep sea environments. The tool parameters have to be correctly set for such challenging cement evaluations.
As environmental concerns continue to become more dominant, the use of more sophisticated cement evaluation tools will become important.
