4.5 The Valhall LoFS Project


Guest Contributors: Olav Barkved and Tron Golder Kristiansen (Section 4.5.1); and Einar Kjos, Kent Andorsen and Nirina Haller (Section 4.5.2)


Watch every detail that affects the accuracy of your work


Arthur C. Nielsen, Businessman, (1897–1981)


Te Valhall permanent seismic array (Life of Field Seismic; LoFS) was the first full-field permanent reservoir monitoring system to be installed, and paved the way for similar monitoring projects.


4.5.1. Brief History of Valhall LoFS


Te first version of a permanent seafloor seismic array was made at the Foinaven Active Reservoir Mangement (FARM) system in 1995 in the UK sector of the North Sea. In January 2000, BP rolled out the LoFS concept to seismic contractor and manufacturing companies, in order to engage them in the creation of a potential market for permanent seismic cable technology, and a solution that included equipment from Oyo GeoSpace was selected. Te Valhall field in Norway was then identified as a good candidate for trying out this type of technology. It was thought that the new system would be a major factor in improving recovery and add significant value to the field. During the summer of 2003, 120 km of seismic cables were trenched into the seafloor covering an area of 45 km2


. A total


of 2,400 receiver units, each with 4C sensors (three geophones and one hydrophone), recorded ~52,000 shots to create one LoFS survey. Te four-component data make it possible to create images of both PP data (conventional data measuring P-waves being reflected from subsurface interfaces) and PS data (P-wave down being converted to S-waves at the interfaces and propagating back to the seabed receivers again). Te cables are connected to a recording system at the central platform. Te system records continuously and is controlled from onshore, and data can be sent onshore via the optical network. No part of the subsea system is exposed above the seafloor, thus avoiding any conflict with fishing activity in the area. Eighteen LoFS surveys were acquired between 2003


and 2015, with survey 19 shot in 2017. Survey 16 used re- deployable cable to expand the area imaged by the LoFS array, and surveys 18 and 19 rely upon re-deployable nodes rather than the permanent array, which has served well beyond its intended life and has now become less reliable. Nodes also provide flexibility in that the area covered may be adjusted. Te high number of 4D surveys can be displayed as a matrix, as shown in Figure 4.41. Te matrix presentation was chosen as a simple communication tool. It is accessible from a website and is updated by scripts when new data arrives. Tese matrices have been produced for each generation of processing sequence and are also therefore an efficient tool to visualise improvement in seismic imaging. Each displays


177


Figure 4.41: Matrix image of 55 4D difference maps. Top row: difference between first monitor and base survey, second monitor survey and base survey etc. Second row: difference between second monitor survey and base survey, third monitor survey and base survey etc. These matrices are described in the text. At Valhall, water injection start-up is typically characterised by an acoustic impedance increase as gas comes back to solution when pressure increases.


Figure 4.42: The Vallhall field is in the Norwegian North Sea.


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