Variable Density Log (VDL)
Variable Density Logs (VDL) and Cement Bond Logs (CBL) are conducted after the primary cementing operation for evaluating the quality of the cement bond behind a cemented casing or liner. It is a sonic tool that is run on wireline and uses a combination of transmitter and receivers to measure sonic wave arrival for assessment of cement bond quality. The principle used is that when no cement is bonded to the casing, the pipe is free to vibrate, whereas when the casing is bonded to hard cement, casing vibrations are attenuated proportionally to bonded surface.
A bit and scrapper run is recommended before the CBL-VDL log to remove any cement or scale from the casing wall. The tools should be properly centralized and the wellbore should be fluid-filled for accuracy of results since gas or air bubbles induce inaccuracies.
Radial tools were developed to overcome some limitations of conventional CBL-VDL tools. The Radial CBL-VDL tool permits a more accurate evaluation of cement distribution. It uses more sensitive transducers and evaluated the cement quality around the circumference providing precise location of partial bond and channeling.