Application of New Interpretation Workflows that Improve Cement Evaluation in Presence of Microannulus

Abstract

Abstract In many cases when using traditional cement evaluation techniques, we observe disagreement between the ultrasonic and sonic measurements on the quality of cement sheath. In some instances, both measurements can lead to a pessimistic assessment of the cement sheath in spite of a well-executed cementing job. To improve the robustness of the cement evaluation, new interpretation workflows were tested in North Africa to investigate if the apparent "disagreement" can be explained and result in an improved cement evaluation. The data acquired in wells in North Africa was processed and interpreted with traditional workflows and compared with new interpretation workflows that were recently made available. The new workflows include a) an ultrasonic workflow that improves the discrimination of lightweight solids and through its detection of the presence of a microannulus also provides the ability to discriminate a dry microannulus situation and b) a novel way of comparing sonic and ultrasonic measurements in the acoustic impedance space. The workflows have been applied in many wells and have been seen to improve the robustness of the cement evaluation that is consistent with the expectation for a cementing operation. In case studies from data acquired in two wells in North Africa, we observed that presence of a wet or dry microannulus affected the ultrasonic and sonic responses differently, which the traditional evaluation techniques failed to address adequately—in some cases resulting in a pessimistic assessment of the cement sheath. The new workflows enable not only the detection of the presence of a microannulus but also allow discriminating between a wet microannulus and a dry microannulus. This improved ability to diagnose debonded cement behaviour in situ has helped us arrive at consistent, robust, and reliable evaluation of the cement sheath. The examples presented in this paper demonstrate the successful application of new interpretation workflows for ultrasonic and sonic measurements that improve the confidence of cement evaluation, especially when cement has debonded from casing. This ability also has the potential to reduce unwanted and costly cement remediation work.