Making Sense of Why Sometimes Logs Do Not See Cement in the Annulus

Abstract

Abstract Ultrasonic and sonic logs are commonly used to evaluate the quality of cement placement in the annulus of a pipe and its potential to perform as a barrier. In some cases, we observe that the log response is in conflict with the expectations on the outcome of the cementing job that was executed without any major issues. The log sometimes does not see the cement! This apparent disagreement if not resolved could potentially lead to costly and unsuccessful attempts to remediate the cement sheath. We highlight reasons why sometimes logs may not see the cement and implement new workflows that not only recognize these factors but also account for it in the cement sheath evaluation. The ability to recognize the existence of a wet or dry micro-annulus is critical for cement sheath evaluation and is discussed through two new interpretation workflows using ultrasonic and sonic logs. Firstly, we introduce a new ultrasonic (TIGHT model) interpretation workflow that improves discrimination of light-weight solids from displaced muds and enables identification of dry micro-annulus situations. Secondly, we introduce a new integrated workflow that combines sonic amplitude / attenuation with ultrasonic measurements in the acoustic impedance (AI) space (Sonic vs ultrasonic acoustic impedance). Through this approach, we demonstrate how we can identify cement that is well bonded and discriminate from those with a dry or a wet micro-annulus. The workflows were applied to data acquired in wells where we compared the results of conventional interpretation approaches as well as those discussed in the two workflows. The gap caused by the detachment of cement from casing (de-bonded cement), either wet or dry micro-annulus, can affect the log responses differently and sometimes significantly. The effect of micro-annulus on logs can be recognized which then facilitates correct assessment of the cement sheath behavior. We note that the log response in the presence of a dry micro-annulus may be different to that for a wet micro-annulus. In the case of a dry micro-annulus, even with relatively smaller gaps between cement and casing, conventional approaches to interpretation of the sonic and ultrasonic logs can completely miss "seeing" the cement. We observe that the integration of ultrasonic and sonic measurements through these workflows has the potential to improve the quality and reliability of cement evaluation when cements de-bond from casing. The ability to do so with existing ultrasonic and sonic technology not only helps improve confidence on cement sheath evaluation but also has the potential to reduce unwanted and costly remediation decisions.