Riserless Plug and Abandonment of Pilot Holes in Ultradeepwater

Abstract

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.

Influence of the Characteristics of Cement Plug on well Integrity for Offshore Abandonment

“…According to the theory of elastic mechanics, this question can be simplified. For the plane strain problem, the following assumptions [7,8] are made: 1) the casing and cement plug are isotropic elastic materials; 2) the cement plug is sealed continuously and the thickness distribution along the hoop is homogeneous; 3) cement plugs installed in the wellbore, no internal effective stress; 4) When abandoned, the wellbore pressure is equal to the hydrostatic pressure, and the pore pressure in the cement plug is continuous.…”