Using the system, BP can determine how injected water is moving through the reservoir and “see” through the gas cloud which covers the field.[REMOVE]Fotnote: “First Permanent Seismic Cables Installed at Valhall Field”, June 2003,http://www.rigzone.com/news/oil_gas/a/6910/. Other advantages include learning more about the causes of well collapses, enhancing the placement of wells and providing improved surveillance of seabed subsidence.
Seismic surveying has been important for the Norwegian oil industry from the start, with geological structures being mapped during the 1970s by two-dimensional (2D) methods. A breakthrough occurred in the late 1980s with the extensive use of 3D technology, which was progressed to 4D by adding time – in other words, repeating surveys to check sub-surface changes. This approach provides a better picture of how reservoirs are being drained, and lays the basis for the optimum positioning of production and injection wells.
Also known as seabed seismic, the four-component (4C) solution is the latest advance in seismic technology and involves shifting the receivers from the water column to the seabed. That makes it possible to use geophones in addition to hydrophones to pick up the seismic echoes, and to register shear waves as well as pressure waves.
On Valhall, a network of cables comprising more than 10 000 sensors was laid during the summer of 2003 over 45 square kilometres to cover roughly 60 per cent of the field. Plans call for the whole area to be incorporated in the system, which currently consists of over 100 kilometres of cables in parallel lines buried one metre down in the seabed.
When conventional 2D or 3D marine seismic surveys are shot, streamers – long cables fitted with hydrophones – are towed behind a vessel to pick up echoes from the sub-surface. The 4C technique acquires the same data but, as mentioned above, also collects shear waves whose motion is at right angles to the direction of the sound waves. These differ from pressure waves by not propagating through water, which prevents them being picked up with streamers near the sea surface. Nor do they register oil and gas in the same way.
Traditional seismic surveys have been a challenge on Valhall because of a gas deposit which lies like a cloud over the reservoir, making it difficult for pressure waves to penetrate.[REMOVE]Fotnote: Barkved, Heavey, Kjelstadli, Kleppan and Golder Kristiansen: Valhall Field – Still on Plateau after 20 Years of Production . Paper for Offshore Europe, 2003, 4. Utilising shear waves, which pass straight through the gas, allows a better picture of the underlying reservoir to be obtained.
The LoFS makes it possible to combine 4D and 4C technology on Valhall. Seismic is shot at intervals of a few months, and the resulting data used to create ever-better reservoir models.
Moreover, streamers being towed at the sea surface move with wind, waves and current. The buried cables are stable and therefore provide a more accurate picture of the sub-surface.[REMOVE]Fotnote: Overvåker Snorre og Grane News at offshore.no published April 10, 2013 http://offshore.no/sak/37403_overvaaker_snorre_og_grane.
From 2D to 4D
A single streamer is towed behind the survey ship when shooting two-dimensional seismic, increased to multiple parallel streamers for the 3D technique. That makes it possible to cover larger areas faster and more cheaply. At the same time, the sub-surface is imaged three-dimensionally. With 4D, surveys are repeated over time to permit studies of such aspects as oil migration through the reservoir as well as changes in pressure and temperature. Production and injection wells can therefore be drilled closer to the optimum, improving the field’s recovery factor. Permanent systems on the seabed are a further development of 4D seismic.
Results from the LoFS surveys are transmitted via fibreoptic cable to land for analysis. Huge volumes of data call for a stable broadband connection able to to carry several terabytes. Valhall was connected in 1999 to the Nor Sea Com 1 fibreoptic cable, which supports data communication with both Norway and the UK.[REMOVE]Fotnote: Paulsen, Gard, Oslo, 2005, Informasjon over Nordsjøen: Telekommunikasjoner på norsk sokkel. Transfer capacity was greatly improved in 2011, with a dedicated cable from the new process and hotel (Valhall PH) platform to Lista in southern Norway. Both mathematical and physical models are used to analyse the material. Powerful computers and advanced digital tools help to analyse the big quantities of survey data. This process generates 3D and 4D images of the reservoir, which can in turn be called up on workstations or reviewed in three dimensions.
Acquiring 4D seismic data using only streamers towed behind a survey ships is very expensive, and such investigations are therefore generally conducted at intervals of several years.[REMOVE]Fotnote: Geo Expro, September 2004, 27. With a permanent system for collecting seismic data installed, the cost per survey is reduced and opportunities are provided for more frequent repetitions. Surveys with the aid of the LoFS have been carried out once to three times a year on Valhall since November 2003. And they can be done at short notice if need be.
One reason for installing the LoFS on Valhall was the start of water injection by BP centrally on the field in January 2004, which was extended to the northern flank in 2008.[REMOVE]Fotnote: Business Impact of Frequent Time-lapse Observations Around Valhall Water Injection Well . EAGE conference paper, June 2010. The purpose of this programme has been to stabilise reservoir compaction and consequent seabed subsidence on Valhall, and to improve recovery.
Using the LoFS and 4C surveys, BP can check in detail how the water behaves in the reservoir, whether it is taking new routes through the rock, and how oil is drained around a well. Regular surveys allow the effect of water injection to be monitored over time, and thereby permit it to be adjusted and optimised as required. Information acquired from the LoFS surveys also provides BP with an improved basis for placing new wells in a more secure manner.
Thanks to their work on the LoFS, BP and the Valhall Unit received the Norwegian Petroleum Directorate’s annual prize for improved oil recovery (IOR) in 2003. This award is presented to companies, projects or individuals who have distinguished themselves through their work with IOR on the Norwegian continental shelf. Its presentation in 2003 honoured the field licensees for their commitment to a project involving large up-front investments to optimise reservoir exploitation on the Valhall field.[REMOVE]Fotnote: IOR award 2003: the NPD’s award for improved oil recovery goes to Valhall. News at NPD web site published January 13, 2004. http://www.npd.no/en/news/News/2004/IOR-award-2003-the-NPDs-award-for-improved-oil-recovery-goes-to-Valhall/Measures to combat Valhall subsidenceLicensees on Valhall and Hod