This paper describes a witnessing procedure based on a review of procedure based on a review of experience acquired during cement band logging operations by ARTEP (French Petroleum Research Association) Petroleum Research Association) members. The aim is to provide an answer, on the wellsite, to the following two questions: - are the anomalies observed on cement logs genuine and significant ? - if so, what are the chances of success of any remedial action ? The described operational procedures and standard presentation have now been successfully implemented in hundreds of cases where the Oil Company witness has been helped in the interpretation of a CBL/VDL or a CET to determine the potential risk of a leaking barrier. The paper also highlights the cash savings linked to sound log quality control, when one pays exclusively for qualified and pays exclusively for qualified and reliable data. Some case histories of insufficiently studied cement bond logs are presented.
Logging companies are not held responsible for any interpretation of data recorded with their own equipment. In terms of cement evaluation, this attitude is taken for granted, mainly because this logging is an acoustic, non destructive technique whereas cementing a well is primarily hydraulic (to prevent fluid primarily hydraulic (to prevent fluid communication between two zones). The fact that the witness belongs to the Drilling department and is not familiar with logging services which will be eventually charged to the Geology department contributes to this lack of coherence. This situation may partially explain the lack of partially explain the lack of confidence shown by the Oil Companies in this service. The CBL/VDL/GR/CCL is perhaps not dead yet because it is also valid for correlation before shooting, and for environmental reasons a well cannot be abandoned without running one. As it is still going to be run, it must be made to tell us more, faster. Cement evaluation is the only logging service where a decision has to be taken just after data acquisition. As very little data post-processing is involved to decide whether the pipe-to-formation bonding complies with the pipe-to-formation bonding complies with the sealing requirements, the log is first and foremost visual. This means that a strict operational procedure and unique log presentation must be observed, which is adapted to most cases.
Dual signal detection (using fixed and floating gates) is always recommended. The fixed gate detection (To delay) allows one to read an accurate amplitude in the 0-10 mV range where the good bond cut-off is expected. See Fig. 1. The floating gate detection (Tx) helps to check which acoustic peak has been measured: amplitude and transit time are read for the same peak, which is important for instance in case of fast formations (tight formations whose velocity is faster than steel). See Fig. 2. Throughout the log, the smallest amplitude of the two detections will be considered although the Tx amplitude is only qualitative in fast formations and does not guarantee a good bond. Microannulus disappearing with adequate pressure, CBL/VDL is run under pressure and this pressure is different for each job.