Ofon Field Development: Challenges of Monitoring a Complex Turbiditic Field

Emmanuel De-Pierrefeu, Daniel Ekpenyong, Nnamdi Amechi, and Renaud Perret-Du-Cray

The Ofon Field was started in 1998 and developed with water injection. It is currently producing from 17 producers and 7 injectors. All the data acquired during the 1998-2004 period and the production behaviour of the field showed an increasing complexity of both the static description and the dynamic behaviour.

A redevelopment of the field is planned for 2009 with 39 new wells and an additional injection capacity for massive pressure support.

This paper summarizes all the monitoring actions that have been done so far on the field. It will review the information provided by these monitoring actions, and then explain the new needs that appeared and how they will be handled.

Pressure monitoring:

• RFT data provide important source of information, with the wells progressively drilled until two years after start-up and lately (2003) with four new wells. They proved some communication across most parts of the field, giving argument for a unique oil pool.
• Regular build-up pressure measurements have given very valuable data on pressure support from the injection, even though it gave a much more complex picture by proving different dynamic trends for nearly all the wells.

The combination of the two sets of pressure data is the main tool for optimising injection allocation.

Test data for GOR and water monitoring:

• GOR of the producers does not always follow a clear relation with pressure around the well and distance to gas cap. Indeed secondary gas caps are present all over the field.
• GOR hysteresis: few wells have very quick GOR response to choking and others much longer.
• Despite the low water production, few water breakthroughs already occurred, some of them still unexplained.
• The salinity of the produced water allows sometimes to identify the source of water.
• Few PLT and RST have been run to help understand the pressure evolution and water or gas breakthrough, in wells producing from several sands bodies considered to be in communication in a common geological layer.
• Ofon proved to be more complex than initially anticipated dynamically, with clear pressure barriers between wells, not expected in high permeability sands, complex sedimentology and production far below bubble point with large volume of free gas inside the reservoir.
• In order to better optimize the production until massive re-injection comes on-stream in 2009 and to optimize the future development, a new strategy has been defined for monitoring of the field with new tools:
• New seismic data (3D OBC) to try to explain the pressure barrier seen by dynamic synthesis and be able to forecast them. This is of course of the ultimate importance because of the development choice of injector/producer combination.
• Time lapse seismic acquisition (4D OBC) to monitor evolution of saturation front, water and secondary gas-cap.
• Tracer campaign that was not seen at the beginning as a need for Ofon is now considered to provide some valuable data, to explain any future unexpected water breakthrough and prove the communication.
• Interference test, between injectors and producers, on the current producers and injectors. This is also planned for the future wells.