Ocean Bottom Node Seismic Imaging — New Technology Deployed in Deepwater Nigeria

McCloy, John; Iyiola, Sunkanmi; Bee, Michel; Smith, Gordon; Page, Chris; Berg, Eivind

In late 2009, Chevron Nigeria Ltd. began the acquisition of an ocean bottom node (OBN) seismic survey over Agbami producing field, offshore Nigeria, in water depths of about 1500m. Production started in 2008; current daily production is around 250,000 BOPD. The reservoir structure is an elongated anticline in shape with dips up to 40 degrees. Though previous streamer surveys were acquired, the OBN survey was designed as the baseline for a future 4-D program. It was the largest OBN survey recorded in the industry and first in Chevron. The data quality clearly shows that the results met expectations, with value added over the previous seismic volumes. Imaging at the reservoir level showed remarkable improvement and better clarity at the shallow. Seafloor recording provides a unique capability in providing an additional mirror migration of the down-going wavefield. The multi-azimuth nature of the acquisition allows for further investigation of individual azimuth sectors. All of this additional data collected in the field aids in better understanding of the subsurface geology. The OBN project is a good example of what can be achieved with a true partnership between the geophysical contractors and an E&P company. Fugro Seabed Seismic Systems (previously SeaBird until sold) acquired the OBN surveys and CGGVeritas processed the data. As the technology is new and processing workflows are still evolving, there was extensive collaboration amongst the R&D staff of all companies. Challenges in the OBN data processing were identified. These issues included residual multiples, S-wave noise (Vz) contamination on the vertical geophone, and poor imaging on the steep dips of the reservoir structure. New processing developments were implemented; wavefield separation and up/down decon were improved. The 3-D SRME workflow was improved upon, and adapted to also apply on both the up-going and down-going (mirror) data. Extensive velocity modeling was performed, bringing a remarkable improvement in the reservoir imaging. Fugro re-acquired the survey in 2012, ensuring the nodes were laid at the same coordinates; all acquisition specifications were repeated. 4-D analysis followed, optimizing field development plans. This presentation will begin with an overview of the P-Wave processing, followed by an in-depth look at the unique processing steps for OBN data. The velocity model building workflow will be reviewed and a comparison shown of the different volumes.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013