During the summer of 2004 an experimental OBS survey was acquired and processed by CGG over the Girassol field operated by Total Angola, offshore West Africa. Five ARMSS nodes (Autonomous Reservoir Monitoring Seismic System) were deployed in this area at a water depth of 1300 meters. The primary objective of this trial was to verify the operational sequence and performance of this new generation of 4C recording equipment. There were also a number of secondary objectives which were to benchmark the recorded data with a view to infilling the streamer acquisition in this difficult environment and to evaluate the added value of recording four components. In this paper, after a short description of the recording system and the acquisition layout, the results of the processing are compared with streamer data acquired previously in the same area. The 4D capabilities of the node technology are assessed. A comparison of the results proves that nodes could be used in infill areas to complement streamer data.
Deepwater hotspots, such as Angola, Brazil and the Gulf of Mexico, are becoming increasingly congested with production infrastructure. Operators wishing to monitor production at these sites are faced with the impracticability of a time-lapse seismic survey (4D) of the whole area using the conventional surface streamer method. Ocean Bottom Seismic technologies offer an attractive solution for acquiring infill time-lapse seismic data as a complement to streamer acquisition to give a full picture of the target zone. As an experiment, data was acquired in the deep water of Angola with five autonomous nodes and analysed with respect to potential future industrial applications.
Figure 1 shows the node in operating mode. The sensor head incorporates three fixed geophones in a Galperin arrangement. Tilt measurements are performed by three accelerometers. A vibrating system is included to ensure optimum coupling with the seafloor at the layout stage and a hydrophone is in direct contact with the water to complete the 4C system. A separate case contains the full recording system including batteries and the data storage unit, which can operate non stop for forty-five days at water depths of 3,000 meters. The system provides a high degree of vector fidelity through a decoupling of the sensors to the case unit via a fit-for-purpose umbilical wire. The compactness of the autonomous system compared to the cable system allows placement adjacent to existing seabed facilities while its innovative design removes the need to plant the nodes, thereby reducing layout time and cost. Once deployed, the nodes remain in place and can be positioned quite close to production infrastructure. Repeatability is ensured as the nodes can be repositioned at the same location.
The node pilot project conducted over the Girassol field in the summer of 2004 included layout, acquisition and recovery. The aims were to validate the new node technology with a view to its future deployment for industrial applications and to assess its 4D capabilities with respect to the fidelity of its seismic signal compared with previous high-resolution streamer surveys.
