Technical Paper: Riserless Plug and Abandonment of Pilot Holes in Ultradeep Water

Society: OTC
Paper Number: 25214
Presentation Date: 2014
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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 between 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 1/2-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 1/2-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 three cement plugs. Cement returns were observed at seabed by the Remove Operated Vehicle (ROV) and no visual flow after placement was observed.

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