Integrated Approach for Cementing Dual-Casing Running System for Deepwater Wells

To improve drilling operation efficiency in the riserless tophole section, a new casing running system was introduced in offshore worldwide Eni operation. In this new system, the hole sections for the conductor and surface casing are drilled in one run, followed by running both casing strings in one trip, requiring a single cement job. This system uses a minimum of one trip in the hole compared with conventional techniques used for running casing and cementing in the riserless phase. The cementation of the dual casing has similar objectives to normal surface casing cementation: stable wellhead with cement returns to seabed, no shallow flow in case of shallow drilling hazards, and good cement integrity at the casing shoe to continue drilling operations with minimum wait-on-cement time. The cementation of the dual casing is challenging for several reasons: the cement job has to cover two concentric annuli, the cement returns are passing through the unconsolidated formation with a high risk of losses because there is no conductor casing set below the seabed, possible shallow flow, large formation washouts close to the seabed depth, and the requirement to develop sufficient compressive strength in a short period of time at very low temperatures to bear the weight of both casing strings and the wellhead assembly. In offshore deepwater wells, the casing running system has been used several times in water depths from 600 and 2200 m. Initial cement design used conventional lightweight extended lead cement with a neat tail and its drawbacks. The use of this system caused losses close to the seabed and resulted in an unstable wellhead requiring top-up jobs. Careful engineering and planning between the service company and the operator resulted in the development and execution of two successful cement techniques. The first technique consisted of Low Temperature Optimized Particle Size Distribution Cement, followed by neat tail slurry. Because shallow drilling hazards were present, shallow water flow slurries were designed for the job. The second technique consisted of a foamed lead cement followed by a neat tail slurry. Both cement designs fulfilled the job objectives. The dual-string running technique, combined with an efficient cementing technique, enabled a 40% time reduction in the riserless phase of the well operations while safely achieving the well integrity objectives.