Measured Properties of Annular Cement Compared With Ultrasonic Cement Logs

Abstract

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 204015, “A Comparison of the Measured Properties of Annular Cement With Ultrasonic Cement-Evaluation Logs,” by Dave Gardner, SPE, Hans Joakim Skadsem, and Katherine Beltrán-Jiménez, SPE, Norwegian Research Centre, et al. The paper has not been peer reviewed. In this work, different measurement methods have been used to investigate the quality of the cement sheath inside two cemented “sandwich” sections, including high-resolution ultrasonic cement-evaluation logs, sample analysis, mechanical loading, and fluid-seepage measurements. The examined sections were recovered from a North Sea well during a permanent plug-and-abandonment operation. The measurements have been analyzed with an aim of describing in detail the spatial variations in the cement properties and relating them to the logs. The data analysis has improved the understanding of the cement-sheath mechanical properties, seal quality, and response of the ultrasonic cement-evaluation logs. Introduction A well drilled at the Valhall Field was permanently abandoned in 2018 after being in production since 1985. To install the surface plug, the 9⅝-in. and 13⅜-in. casings were both cut and retrieved to surface in sections simultaneously. After the well abandonment, two of the cemented casing-in-casing sandwich sections were sent to shore for further analysis, including relogging. The deepest section, recovered from the interval between 251 and 263 m, was named Fish 11. A visual inspection of the section showed complete annular cement coverage at both ends but with signs of mud contamination, especially at the casing/cement interface and primarily on the narrow side of the annulus. The 9⅝-in. casing had an eccentricity of 70% compared with the outer casing and equivalent to the standoff created by the 9⅝-in. casing-collar thickness. The uppermost sandwich section covering the top of the cement was retrieved from 119 to 131 m and was named the transition joint. The eccentricity of the inner casing compared with the outer casing was 60% for this section. The bottom of the section had complete cement coverage; however, some mud intrusions were apparent in the bulk cement as well as mud contamination between the casing/cement interface. The 9⅝-in. casing was originally cemented in two stages because of the presence of a weak formation. Both the transition joint and Fish 11 were retrieved from the upper part of the section, which was cemented in a second stage by bullheading slurry from the surface and down the annulus. In this operation, the denser slurry was displaced from above the spacer fluid and the oil-based mud was used to drill the 12¼-in. section into the formation. The authors speculate that the mud displacement during the bullheading operation was instantaneous, with no backflow. The fact that the sections were cemented by bullheading may have influenced the properties of the cement, particularly close to the top of the cement.