Analysis of Varying Microannulus Size on Cement Evaluation
Authors: Bazaid Abdulaziz , Jawed Rehan , Alatigue Mohammed , Kumar Apoorva 
Published: May 2024

In many instances, it has been observed that the results obtained from logs, such as ultrasonic, flexural, and sonic logs, often do not align with the expected outcomes of a cementing job, and in some cases, they do not detect cement altogether. This discrepancy is frequently attributed to the presence of what is known as microannulus. Microannulus may be present for several reasons, such as variations in temperature or pressure during or after the cementing process, poor mud removal from the casing wall, difference in hydrostatic pressures during a cementing job and while logging, volumetric behavior of cement during the setting process against formations and overlapping pipe sections. This microannulus gap may be filled with either liquid (i.e., a wet microannulus) or empty space/gas (i.e., a dry microannulus). If effects of microannulus on cement logs go unnoticed it can lead to incorrect assessments of the condition of the cement which in turn can result in expensive and unsuccessful intervention efforts.

To successfully identify microannulus, we employed a combination of advanced techniques, including flexural attenuation, ultrasonic pulse-echo technology, third interface echo (TIE) analysis, and cement bond log-variable density log (CBL-VDL) measurements. The study included cross-plot analysis of sonic and ultrasonic data across several intervals of varying size of dry and wet microannulus in multiple wells to observe the effect of different size of microannulus on sonic and ultrasonic data. Also, the inferences were confirmed by taking pressure pass across corresponding intervals. As the microannulus size increases, even sonic measurement was affected by a dry microannulus, and even ultrasonic measurement was affected by a wet microannulus.

Throughout our study, several wells were studied with varying sizes of dry and wet microannulus affecting the sonic and ultrasonic measurements by varying degrees in pressure and non-pressure pass. It was observed that dry microannulus at the casing-to-cement interface has a very strong effect on the ultrasonic measurement but has minimal effect on traditional sonic measurement if the microannulus is of small size. On the other hand, the sonic measurement is very sensitive to wet microannulus compared to ultrasonic measurements, which is less affected. In some instances, we observed large dry microannulus that had a significant impact on the ultrasonic data, making it appear as if there is gas filled annulus. Conversely, in wells with smaller dry microannulus, the ultrasonic measurements indicated that the annulus had azimuthal cement with large liquid pockets which shows how one can observe a totally liquid- or solid-filled annulus without any traces of gas even in the case of dry microannulus.

The multi-well study helps to understand the condition of their cement better and take necessary remedial measures. This work also helped in preparing a guideline for quick identification of both dry and wet microannulus of varying size with or without presence of pressure pass which would help interpreters for evaluating cement bond quality.