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'Coil shooting' method improves imaging of complex geology

21st February 2013


'Coil shooting' is a new single-vessel method for acquiring seismic data over a wide range of azimuths to improve imaging of complex geology. Jeremy Cresswell reports.

According to WesterGeco 'coil shooting' is a cost-effective technique towards better imaging over small to medium sized exploration areas. And the company claims that 'encouraging results' may lead to a significant uptake of the technique over the next five or so years. Basically, coil shooting is a new single-vessel method developed by the company Geco for acquiring seismic data over a wide range of azimuths to improve imaging of complex geology.

By sailing in overlapping circles, a very large number of shot/receiver positions can be created and a single source/receiver boat can produce unprecedented offset and azimuth sampling which leads to higher quality images with superior noise suppression. Typically, this might involve an array of 10 seismic and precision-steering them through a sequence of large continuously linked circles, 5-7km in radius.

The idea is not new as early trials based on circular acquisition geometries were tried during the 1980s for improved imaging of circular shaped salt dome flanks and associated faulting in the Gulf of Mexico and in the North Sea.

The techniuque was also used when 3D VSP surveys came to the fore, with circular shooting patterns used for some of the work, notably in the Oseberg field. The objective was to acquire a wide range of azimuths in order to better map and characterise the subsurface. But then it fell out of favour, a major reason being the inability of marine acquisition technologies used at that time to accurately position and steer streamer arrays.

In 2006 WesternGeco revived the idea for 3D full-azimuth acquisition and the key was its then groundbreaking Q-marine technology. Steering devices, such as Q-Fin could be used to control the position of the streamers, making it possible to maintain constant streamer separation while the vessel sailed a circular trajectory. At the same time, receiver positions could be accurately measured.

According to WesterGeco, the first field test of the coil shooting technique was performed during 2007 in the Gulf of Mexico. The first commercial survey using circle shooting was acquired for Eni in July 2008 on the Tulip project offshore Indonesia. The 563sq.km survey, using with eight streamers, each 6km long, separated by 100m, was completed in 49 days. Another milestone was a coil shooting trial in the deepwater and geologically challenging Kozlu sector of the Black Sea over acreage held by the Turkish National Oil Company (TPAO) and in Norway, StatoilHydro and WesternGeco conducted an extensive collaborative full-azimuth survey design and modeling project to evaluate multiple wide-azimuth (WAZ) configurations including coil shooting over the Heidrun field. The upshot was that, not only was coil shooting a more effective strategy than traditional wide azimuth (WAZ) acquisition, it also gave better imaging results for complex and heavily faulted Heidrun, which lies in 350m of water.

In September 2008 StatoilHydro R&D commissioned a survey resembling a 'dahlia' flower pattern pattern with 18 intersecting coils This geometry provided full azimuth, high fold data over a 2.6km x 26km target area, plus sufficient surrounding aperture to provide 3D migrated sections that can be compared with previously acquired conventional seismic data.

The Heidrun survey was completed over a 4 day period, with a coil radius of 5.6km, and 10 streamers, each 4.5km long, separated by 75m, resulting in clearer imaging, notably improved fault definition in the reservoir section, compared to the conventional survey data. In addition, the flanks of the Heidrun structure are now well imaged.

Western Geco says the single vessel coil survey technique is particularly attractive for collecting small to medium-sized 3D full-azimuth datasets. For larger surveys, like those that are now taking place in the highly active Gulf of Mexico area, using multiple vessels may be a more cost-effective option.

While it was thought that additional noise level caused by steering the streamers in circles might be a problem, benefits have outweighed this perceived disadvantage.

Western Geco says that coil shooting has demonstrated itself as an efficient and effective survey method to acquire full azimuth data over small to medium sized areas and a particular benefit is that it requires only one vessel, so helping cut costs.

The company expects to see a 'reasonable' number of coil shooting surveys this year and to increase sharply thereafter, especially as results become available from the 2009 programme.

Western Geco's work was recognised at this year's Offshore Technology Conference in Houston, by being awarded one of the coveted OTC technology awards.

Staying on the theme of being not totally new technologically but clearly very new to the marketplace in terms of application is PGS's Geostreamer technology, which it makes much of in its latest financial results review.

GeoStreamer made its technical debut in late 2007. The system was hailed as being the first ever towed streamer to successfully house both pressure and velocity sensors, so enabling delivery of enhanced resolution, plus deeper penetration of geology to provide a higher quality, de-ghosted seismic image.

PGS started equipping vessels last year and, importantly, rapidly secured contracts in the US Gulf of Mexico and the North Sea. First out of the gate in contract terms was work for StatoilHydro kicking off mid-June 2009.

The Gulf of Mexico work was announced late 2008, with PGS stating that it was to acquire a new multi-client 3D seismic survey in the area of DeSoto Canyon located offshore Alabama and Florida in the Gulf of Mexico.Covering approximately 250 deepwater OCS blocks (5828sq.km), the new programme named Discovery is focused on a lightly explored sector recently opened for leasing in the Central and Eastern GOM planning areas.

Importantly, PGS said it would acquire the DeSoto Canyon data using GeoStreamer. Indeed, this contract marks the industry's first 3D implementation of GeoStreamer yielding higher fidelity data.

PGS Atlantic Explorer started the nine-month campaign at the turn of 2008-09. BP is an underwriter for the survey; also a technical collaborator.The Norwegian group said the project has attracted wide interest from oil companies with lease holdings in the area and those wishing to acquire new acreage or those seeking farm-in opportunities. Data acquired utilising GeoStremer will be made available at the upcoming US Gulf of Mexico lease sale in 2010.

Turning to the North Sea and in the quest for ever more enhanced seismic resolution, better deep imaging, operational robustness and excellent final results, PGS has just started the first ever North Sea season for GeoStreamer.

That programme started in April and comprises the gathering of both multi-client and contract 3D data. The campaign kicked off with a 1600sq km survey for Lundin Norway based around the Luno discovery in the Norwegian North Sea. As for the upcoming StatoilHydro contract, it too is scheduled for the current summer. In this instance, the client is also the first customer for the Ramform Sterling, which is PGS' second new S-Class Ramform vessel, which is due for delivery in July 2009.







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