The following general procedures can be followed before running cement bond logs in wells.
First make sure the well is ready for Logging:
Allow cement to cure for a sufficient time to give its full compressive strength. As a general practice, the cement curing is done for 72 hrs, before this is called the green cement, if logging is done, the log may show poor bonding. For further details check with the cement manufacturer.
The well bore must be circulated thoroughly with a fluid (either sea water/brine/mud) of uniform weight. Travel times are affected by the type of fluid in the hole. If the fluid changes between two points, the travel times may change, causing difficulty in both interpretation and quality control.
One must be prepared to run the cement bond log under pressure to reduce the effects of micro annulus. Micro-annulus may be caused due to many reasons, but the presence of a micro annulus does not necessarily deny the cement's ability to form a hydraulic seal. If the cement log shows poor bonding, logging the well again with pressure applied, will make the casing expand a little and reduce the micro-annulus.
Parameters recorded during the CBL Log:
Amplitude (mV) - This curve shows how much acoustic signal reaches a receiver and is an important indicator of cement bond. Record the amplitude on the 3 foot spaced receiver.
Travel time (μs) - This curve shows the amount of time it takes an acoustic signal to travel between the source and a receiver. For free pipe of a given size and weight, the travel time between points is fixed as per the tool manual supplied. The Field Operation manual will be able to provide exact values of travel time for a free pipe of given size and weight. Travel time is required as a quality control measurement. Record the travel time on the 3 foot spaced receiver.
Variable density (VDL) - Pipe signals, formation signals, and fluid signals are usually easy to recognize on the VDL. If these signals can be identified, a practical determination for the presence or absence of cement can be made. VDL is logged on the 5 foot spaced receiver.
Casing collar locator (CCL) - Used to correlate the bond log with cased hole logs and to match casing collars with the collars that show up on the VDL portion of the display.
Gamma ray - Used to correlate the bond log with other open hole logs.
Make sure you calibrate the tool in free pipe at the shop before the job. Include the calibration report along with log. Run sufficient number of inline or overbody centralizers (Gemcos) as required.
In inclined well, there may be a need for more centralizers. Complete log header with casing/cement data, tool data, gate settings and tool diagram showing all the centralizers. Set the amplitude gate so that skipping does not occur at amplitudes greater than 5 mV. Record amplitude with fixed gate, and note position on log.
During logging, if one cannot spot the free pipe, carry out the logging using shop calibration or the previous well calibration.
Logging speed should be approximately 30 ft/min or as per the tool manual. Log repeat sections to verify the working of the tool. Both main and repeat sections must match with allowable tolerances.
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