Pushing Ultrasonic Logging Limits: First Cement Evaluation in 26-Inch Od, 1.25-Inch Wall Casing

Abstract The growing complexity of offshore well designs has led to the use of increasingly large conductor casings above 22 inches in diameter with wall thicknesses greater than 1 inch. While these configurations meet demanding structural requirements, they present major challenges for cement evaluation. Conventional CBL-VDL sonic tools are ineffective in such environments, while ultrasonic measurements are heavily affected by signal attenuation and limited energy transmission through thick steel. These limitations become particularly critical in offshore wells where confirmation of cement presence is required before drilling can proceed. This paper presents a case where a 26-inch conductor was set 120 m shallower than planned, with no visible cement returns at the seabed. Without confirmation of cement around the shoe, drilling and blowout preventer (BOP) installation could not continue, creating a significant operational risk and potential cost escalation. A new cement evaluation approach was implemented to overcome these limitations. The solution combined low-frequency, high–signal-to-noise-ratio ultrasonic transducers, a large rotating logging head to minimize standoff effects, custom-engineered centralizers for optimal tool positioning, and enhanced data processing algorithms that reduced turnaround time from 12–24 hours to just four hours. This approach delivered the first-ever successful cement evaluation in a 26-inch OD, 1.25-inch-thick casing—confirming cement presence behind the casing and around the shoe. This verification enabled safe BOP installation and continuation of drilling without the need for costly re-spudding or abandonment. This achievement extends the operational limits of cement evaluation in extreme casing environments, improves decision-making in high-risk offshore scenarios, reduces rig time, and lowers the environmental footprint of drilling operations.