Time lapse seismic proves worth as a tool to boost field output

Paul Boughton

If one is looking for clear evidence that 4D (time lapse) seismic is worth its weight in gold as a tool to boost field output, they one need look no further than the Gullfaks field in the Norwegian sector of the North Sea. Jeremy Cresswell reports.

Thus far, the application of 4D (time lapse) seismic 4D at the now mature Gullfaks field on block 34/10a in the Norwegian sector of the North Sea has helped increase production by 60 million barrels, raising the recovery rate to 60 per cent, with the prospect of squeezing the reservoir for another 40-plus million barrels, thus pushing the recovery rate to 70 per cent by 2030.

Operator StatoilHydro is confident that this can be done and said so at the 2008 World Petroleum Congress. Reidar Helland, head of the company's petroleum technology unit pointed out that the additional 60 million barrels recovered to date equated to more than one year's production from the field.

"This gives you an idea about the great accuracy of 4D seismic surveys," Helland said at a session on improved oil and gas recovery from mature fields.

Basically, 4D technology makes it possible to confirm the existence of oil deposits that have not been identified through established reservoir simulation models.

Migration of hydrocarbons

Repeated collection of 4D seismic data allows changes in the migration of hydrocarbons within a reservoir over time to be recorded. It is a very powerful tool as it enables the accurate positioning of infill production and waterflood/gas lift wells to achieve a better sweep of the reservoir than would otherwise be possible.

Seventeen wells have been drilled at Gullfaks based on 4D seismic data since StatoilHydro and other leading lights like BP started using this technology in the mid-1990s. The first repeated 4D seismic technology was introduced at Gullfaks in 1996.

Helland told the conference: "Oil has been struck in all of these wells - in other words, an excellent accuracy. This experience clearly shows the importance of employing new technology.

"Building reservoir simulation models is highly resource-demanding. Using 4D seismic technology, we get a more precise model which provides a better basis for the search for oil."

It is admitted that pushing the recovery rate at Gullfaks to 70 per cent will be a tough call, but StatoilHydro has taken many of the steps necessary to make this possible.

Data volume

Gullfaks has not been the only field using 4D seismics on the Norwegian continental shelf. The technology has been successfully employed in the Heidrun field, though Gullfaks stands out because of its great scope, great data volume, and the number of data sets that have been entered.

At Heidrun, in 2004, StatoilHydro ran a 4D survey over the southern part of this large field, following the time-lapse seismic project performed from 1986 to 2001.

This earlier work established time-lapse seismic as a useful technology for identifying remaining oil at Heidrun and contributed to the planning of four new wells in the field. The purpose of the 2004 repeated seismic survey was to particularly improve reservoir understanding and drainage strategy in the Fangst and Upper Tilje reservoir units.

Like StatoilHydro, BP is an enthusiastic user of 4D and, in January 2008 for example, Scottish firm Reservoir Imaging Ltd (RIL) was awarded major contracts from Shell UK and BP for 4D work for the year.

The objective was for RIL to advise on the implementation of all aspects of 4D seismic surveys from initial design and planning to quality control, either on board vessels or remotely from onshore.

The company' has developed a method of isolating all the contributing elements to a 4D seismic survey so that they can be accurately measured for comparison purposes with previous and future surveys over the same reservoir.

Crucial variables

Project information on crucial variables such as the acquisition technology, survey geometry, and the steering and feathering of the towed streamer recording equipment is logged for future reference so that any subsequent survey can repeat the same parameters.

Additionally, a GIS (Geographic Information Systems) database approach provides oil companies with a key resource to ensure the repeatability of their 4D seismic surveys carried out over time.

For example, BP will be applying RIL's know-how to Clair, a multi-billion barrels heavy crude resource located west of the Shetland Islands.

Indeed, during 2008, Wavefield Inseis set its teeth into a contract for the provision of a source vessel to integrate into a 'life of field seismic' project over Clair.

The company said that the project uses a permanent array of variant 4C seismic sensors that were buried into the seafloor above the reservoir in 2006 so that high quality 4D data sets can be efficiently acquired during the production life of Clair.

Clair, which is being prepared for phase two development based on a fixed steel jacket production platform, with the possibility of at least one further phase, is a neighbour of the field where BP's experimenting with 4D started west of Shetland in the early 1990s.

Indeed Foinaven is the first example of a 4D project launched before first oil and where the baseline survey was acquired as a dedicated 4D reference before production.

his was especially important for this field as it was developed based on suites of subsea production wells tied back to an FPSO - in this case a converted former Russian submarine tender.

This combination happens to make difficult routine re-entry into production wells to obtain production log information at reservoir level.

Foinaven, now more than 10 years in production, is a very good example of 4D seismic earning its keep. In many ways it has set a mould for others to follow.

The population of subsea wells tied back to floating production, storage and offloadings is rising fast, so making time lapse seismic all the more important as a production management tool.

West Africa and Brazil are examples of prime markets now opening up.

For example, in September 2008, Shell reported that it had concluded a 4D seismic acquisition campaign over its hugely important Bonga field located on block OML118 offshore Nigeria. The field covers about 60 km2 and has an average water depth of 1000 metres (3300 ft). The Bonga field was discovered in 1996, with Nigerian government approval for its development given in 2002.

The Bonga field began first production in November 2005. The field is produced via a FPSO vessel. The field produces both petroleum and natural gas; the petroleum is offloaded to tankers while the gas is piped back to Nigeria where it is exported via an LNG plant.

Shell said the data acquired during a 76-day survey would help assist in its location of wellsites and help to maximise production from the field over its lifetime. The formal reserves estimate is 600 million barrels of oil, though this could climb to more than 1 billion over time and with careful management.

"Having produced the field for two years, it was very important to get this picture if we are to achieve our aspiration of operating the field to the benefit of all stakeholders including the Nigerian government and Shell," the company said.

Survey contract

Through Petrobras, Brazil has become a major user of 4D and, in late 2004, the company awarded WesternGeco what was then the largest 4D survey contract ever awarded.

The company was to acquire and process seismic data across the Marlim field using its proprietory Q-Marine technology over the Marlim complex, offshore Brazil. The 1520sq km involved the Q-Technology vessel Western Pride, towing 10 x 6000m cables with 50m streamer separation.

In Q2 2008, Petrobras awarded Petroleum Geo-Services (PGS) a $251 million contract to undertake the largest high density 4D (HD4D) marine seismic survey campaign ever shot.

They cover sectors of the prolific Campos Basin and headline grabbing Espirito Santo Basin where a crop of pre-salt multi-billion barrel oil discoveries demonstrate massive hydrocarbons potential of 70 billion or so barrels.

Strategic milestone

Jon Erik Reinhardsen, PGS' president and chief executive officer, said at the award: "This contract is an important strategic milestone to PGS, as we are providing the most sophisticated technology in the business, with the newest vessel in the industry, in the area where the worlds most significant recent discovery is made."

At the time of writing, that programme was under way. It comprises five surveys, totaling 4945km2, plus there is the proposition of a further 120 days of data acquisition. Ramform Sovereign (see Fig.1.) is carrying out the work.

Total daily production of Campos Basin is 175,000m3 (1.1 million barrels) of oil and 17.36 million cubic metres of natural gas per day.

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