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When wells have come to the end of their lives, it becomes necessary to plug and abandon them and return the seabed to its original condition. In the UK sector of the Central North Sea (CNS), an operator managed 6 fields comprising a total of 30 subsea wells in 7 clusters, required to be plugged and abandoned. These fields are among many that are coming to the decommission stage, with the over-riding requirement from the UK government being that of no leakage of hydrocarbons to the environment or between separate permeable geological zones. The operator standards required the placement of two cement barriers of a minimum of 100-ft each for zonal isolation. The preferred route was to find the annular portion of the barrier by interpretation of ultrasonic imaging tool in combination with the cement bond (CBL) wireline logs used for cement evaluation service, then to set a 500-ft plug inside the casing opposite that zone. In the case that no barrier quality cement was identified in the annulus, section milling of the casing was undertaken to expose 100-ft of formation over which cement was placed. A number of challenges were faced to design the cement slurry prior to the logging results. The setting depth may only have been confirmed a few hours before the cement job. To cover the possible setting depths and temperature ranges, laboratory testing consisted of performing temperature sensitivity tests on base slurries designed with a wide temperature range retarder, but still optimizing the system to minimize wait on cement (WOC) time. A specialized high magnesium resistance (HMR) cement system that provides long-term zonal isolation and protects against cement degradation was identified as being best solution. The HMR cement is a blend of blast furnace cement and fly-ash, which reduces the cement permeability and limits the effect of alkaline brine corrosion. Optimal plug placement was also required for long-term isolation. Specialized plug placement software that accounts for in-pipe and annular contamination, and fluid interface matching during pulling out of the plug was utilised. The slurry design and emplacement best practices will be summarized in this paper. These subsea wells have been successfully plugged and abandoned by laying temporary, primary, secondary and environmental cement barriers by several different methods: inside casing, across section-milled windows, multi-annular, through scaled production tubing and through coiled tubing according to each particular well's condition. Success ratio was exceptionally high with all the long term barrier themselves being flawlessly placed and verified without any repeat job being required.

Deepwater cementing becomes increasingly challenging as drilling operations move to greater water depths and to more remote locations. Understanding these challenges and mitigating the associated risks in time has become very important tasks. Before the first exploration well for an operator offshore Tanzania was spudded in a water depth of 2580 m, an extensive analysis was performed to determine all the risks associated with cementing operations, especially in the increasingly challenging and critical surface casing section.The risks determined were low temperatures at seabed, unconsolidated formations close to seabed, potential shallow gas, and likely presence of hydrates. Because of the remoteness and limited oilfield infrastructure, logistics were also determined to be demanding. To successfully continue to drill further sections, the job objectives were zonal isolation across the shallow flow and hydrate zones, cementing back to seabed to provide a stable wellhead, and good cement around the casing shoe providing isolation for deeper formations.The cement job was designed to meet these challenges and objectives. Optimized particle-size distribution (PSD) cement slurry with low density was designed to avoid losses and still develop high compressive strength quickly at low temperatures. Low-temperature gas-migration additive and cement set enhancer were included to provide a short transition time preventing gas migration to seabed. Because of the lower cement ratio in the optimized PSD-blend, the slurry did not destabilize the hydrates during the setting process. The cement placement was simulated to confirm the job design would meet its objectives.Careful preparation and the optimized PSD-slurry system allowed executing the job meeting all objectives. Good zonal isolation and a successful leak off test were achieved, eliminating the contingency liner.

Offshore East Africa is a new frontier in terms of deepwater exploration. One of the new projects in ultradeepwater offshore Tanzania was located in Block 2 with water depths ranging from 1000 to 3000 m. Seismic data had shown the potential presence of shallow gas, shallow water flow, and gas hydrates while drilling the riserless section. Lack of local oilfield infrastructure, and the lack of pump and displace (PAD) mud availability made the project challenging because the surface section was to be completed within a matter of several days. Statoil decided to drill the Tangawizi-1 U1 pilot hole to confirm the absence of shallow hazards and avoid the need for overbalance drilling the riserless section with PAD mud. The 8 ½-in. pilot hole penetrated 600 m through the critical zone below the seabed, drilled with sea water, was then plugged back by filling the open hole with cement. The decision was made to perform the cement job through the 8 ½-in. Bottomhole assembly (BHA) containing measurement while drilling (MWD) and logging while drilling (LWD) tools. The combination of deepwater, riserless, potential shallow hazards and pumping through a BHA with significant restrictions made successfully setting balanced plugs extremely challenging. The slurry was designed following industry standards to prevent fluid influx or destabilizing the hydrates during the setting process by the use of an optimized particle size distribution (OSPD) slurry system. In addition, to successfully be placed through the BHA and avoid plugging the nozzles, this system needed to be stable and have low rheology. The pilot hole was successfully abandoned by setting two cement plugs. Cement returns were observed at the seabed by a remote operated vehicle (ROV) and no visual flow after placement was observed.