Developing the Okwori field (offshore Nigeria) required the combination of several recent technological advances. The subsea development targeted multiple oil-bearing zones in all wells with the requirement that each zone could be operated independently. Expandable sand screen (ESS) strings were deployed within casing for up to four zones in order to prevent sand production from unconsolidated sandstone reservoirs. Independent control of all zones was made possible by using remotely operated surface controlled sliding sleeve units.
Losses experienced after perforating required the use of loss control material (LCM) for well control. Initial LCM formulation appeared to be damaging productivities. A special effort ensued to design optimal LCM pills with regards to the specificities of this project. This paper presents the completion issues associated with the project, the optimization of the LCM pills, and the design and placement aspect of the remedial treatment associated with this type of completion. The evolution of the completion efficiency for each zone throughout the learning curve is presented and discussed.
After discovering Okwori field in 1972 offshore Nigeria (Figure 1), several operators studied field development options, but none felt in a position to ensure sufficient economic returns. The inherent subsurface difficulties coupled with the status of offshore technology and economical constraints would not permit at that time the development of such a complex project. In 1998, Addax Petroleum Development (Nigeria) Limited (APD(N)L) acquired this asset, and subsequently engaged actively in studying its own development plan which the Nigerian Petroleum Authorities sanctioned in 2002.
The highly faulted and compartmentalized nature of the Okwori reservoirs required dispersed well surface locations, hence a subsea development with each well being individually tied back to a central floating production separation and offloading vessel (FPSO) (Figure 2). In order to make the Okwori development economic, each well needed to intersect several oil-bearing reservoirs. The well trajectories were carefully planned to target these successive horizons. The wells were cased and cemented to isolate all reservoir sands. Local regulation stipulates that each reservoir must be able to be tested and produced individually. Therefore, an inner completion string had to be designed to allow complete independent control of all the producing zones from surface.
In the Niger Delta region, oil is produced from weak to unconsolidated sandstones. Due to the "interventionless" nature of the subsea wells, a sand exclusion system compatible with the required full selectivity was chosen for the Okwori development. Expandable sand screens (ESS), were therefore installed inside the 9 5/8″ casing. An inner completion with packers and remotely operated sliding sleeves was then installed before the well was tested and hooked up to the production vessel.
The Okwori development required highly productive wells to meet the set financial goals. The integrity of the wells was also an important concern since any well intervention would take place at a huge expense due to semisubmersible rig mobilization costs. Under these conditions, the different steps taken to minimize formation damage were keys to the project success. Curing the losses after perforating without damaging the formations and restoring the original near wellbore permeability of each zone were major sandface completion issues.
The complexity of the Okwori field can be best summarized by the large number of reservoir layers and fault-delimited compartments resulting in numerous potentially hydrocarbon-bearing pools. More than one hundred of these fault-dip closures were mapped from two vintages of 3D seismic surveys whilst about thirty pools were penetrated by six wells drilled prior to the start of field development (Figure 3).