We have already indicated that after each casing string is installed a pressure test is administered to make sure the casing does not leak. These tests must be held for a specified period of time and have a criterion for how much pressure loss can be allowed for the test to be passed. If the test failed, then the test might be repeated, and if failed again, remedial action is necessary, starting with investigating to find the source of the failure. Another test that is commonly run after the last (production) casing string has been installed is the Cement Bond Log.
Cement Bond Log
A cement bond log is derived from an acoustic wireline tool lowered into the hole. The tool provides a continuous measurement covering the entire depth of the wellbore looking at the quality and completeness of the cement bond. The acoustic waves from the tool will generate a ringing if there is empty annular space behind the pipe (no cement) and a more muffled sound signal if the annular space is full of cement. If the log indicates any ringing noise, there are likely integrity issues related to the cement job at the depth of the ringing signal. You interpreted an example cement bond log earlier in this course as a test for good wellbore integrity.