A large-scale validation of OBN technology for time-lapse studies through a pilot test, deep offshore Angola

Abstract

In early phases of offshore field development, repeated marine seismic surveys recorded with towed streamers have been successfully processed to image changes in reservoir fluids through time and hence have contributed to important reservoir development decisions such as elaborating future production and injection plans. In such cases, the negative impact of surface or underwater obstacles on the final image has been mitigated using undershooting strategies. Nevertheless, even in the most favorable cases, this has been achieved at the expense of 4D repeatability because short-offset illumination is missing in the undershoot survey and the azimuthal distribution within the offset classes becomes inconsistent with the base streamer data. As the field development proceeds, especially in deep offshore contexts, a complex network of in-sea and subsea installations is progressively put in place and the Health, Safety, and Environmental (HSE) issues related to seismic vessels or recording cables coming close to production facilities can no longer be neglected, making the conventional streamer or OBC acquisitions challenging, or simply not feasible. As a result, industry is proposing a “sea-floor receiver” solution with ocean-bottom nodes (OBN) for imaging and monitoring reservoir production below infrastructures. OBN are autonomous seismic recording systems deployed on the seafloor with remotely operated vehicles (ROVs). Previous tests and studies (Boelle et al., 2005; Ceragioli et al., 2006) have shown that OBN technology may be appropriate for reservoir monitoring. Here we present the results of the first large-scale OBN project completed by Total E&P Angola, between September 2008 and April 2009, over the deep-water Dalia Field, Block 17, with the aim of validating the use of this technology for 4D seismic reservoir monitoring.