Challenges and Solutions: Exploring Limitations of the Traditional Three-Media Model in Coated Casing Barrier Assessment Using Pulse Echo Tools

There is always a need to evaluate cement bond quality behind casing strings. This is to confirm effective placement of circumferentially distributed hydraulic seal behind casing as required by well integrity life cycle cementing zonal isolation requirements. Historically this has primarily been achieved by ultrasonic pulse echo (PE) and sonic pad device tools. Cement bond evaluation tools however face significant limitations in assessing downhole cement bond quality when the casing is externally wrapped with an anti-corrosion coating layer. The challenge to evaluate presence of cementing zonal isolation barriers in such scenarios is further highlighted when exotic light / ultra-light cement slurries are used. Currently, no ultrasonic logging service provider is known to offer a model accounting for four media logging (i.e. mud in hole – steel casing – anti corrosion coat – cement sheath behind casing). It is believed that the impact of coated casing in the accurate evaluation of cement behind such casing strings is still not well understood within the industry. Its presence predominantly results in the underestimation of the measured acoustic impedance of cement behind the pipe, indicating potentially invalid free pipe assessments or suggesting poor cement quality presence in otherwise excellently bonded pipes. These findings could suggest misleading conclusions and subsequent erroneous and costly decisions, including invalid cement squeezes, side-tracks, or potentially even unnecessary well abandonment. This paper introduces an approach tailored for accurate acquisition and interpretation of cement bond data behind FLINT coated and Fusion Bonded Epoxy (FBE) coated casings; thus, facilitating informed decisions in well construction and completion projects.