Arc of triumph
  from: Asian Oil & Gas
  by: Andrew McBarnet
  Friday, October 27, 2006

It may not exactly trip off the tongue, but wide-azimuth is definitely the big talking point in the marine seismic acquisition business. Andrew McBarnet explains what the excitement is all about.

In the future you can bet on plenty more public mentions of the wideazimuth word, because it is rapidly emerging as a 'must have' capability for the major marine seismic contractors as it offers a potentially lucrative new revenue stream.

The appeal of the wide-azimuth survey is that it provides a genuine advantage over the conventional 3D marine seismic survey in its ability to image complex geological structures, notably when the geoscientist's old friends, salt or basalt, get in the way of the seismic wave reflection process so that deep subsalt layers remain obscured. However, it is widely assumed that a treasure trove of untapped oil and gas reserves remains to be discovered if the imaging and poor signal to noise ratio issues surrounding salt structures and basalt layers can be resolved. The Gulf of Mexico, West Africa, offshore Brazil, and parts of the North West European Continental Shelf are usually cited as the most prospective areas should there be significant advances in appropriate technology.

Within the last decade much of the emphasis in tackling the subsalt imaging problem has been placed on improved processing of 3D seismic data. In this context, prestack depth migration was undoubtedly the major breakthrough in trying to make sense of the available data. Highly computer intensive, it provided an added value service for which oil companies were prepared to pay premium price but now more or less take for granted. Other key techniques, such as surface multiple attenuation (SRME) and wave equation modelling (WEM) of surface multiples, plus vastly improved visualization methods, have also played a part in working around the salt problem. However, there has been an increasing sense in the industry that processing and interpretation have just about reached their limit without some fresh innovation.

BP can lay legitimate claim to being considered the prime mover among oil companies in revisiting the data acquisition phase as the way forward, along with a number of the better known marine seismic contractors. The theory is that subsalt image quality is poor in locations like deepwater Gulf of Mexico because even with the available processing and imaging, narrow (single) azimuth 3D seismic surveys fail the geometry test. The data simply doesn't provide a good enough view. In shallower water, ocean bottom cable survey technology does mitigate the problem but this is not feasible in thousands of feet of water. In the past, interest in the wideazimuth domain was mainly confined to academia and research consortia with small scale experiments using ocean bottom survey techniques. The favoured solution now being developed is to shoot 3D seismic in more than one direction (azimuth) to better 'illuminate' the target, ie get a more all-round look. This approach has given rise to multi- and wide-azimuth techniques. In layman's terms the difference is that 'multi' could be seen as the entry level version while 'wide' is more the full monty.

Petroleum Geo-Services (PGS) has acquired a number of multi-azimuth 3D surveys in the North Sea and off West Africa where it has deployed its Ramform vessels to acquire the data. Instead of simply sailing in one direction as in a conventional 3D survey, the vessels have crossed the target in different directions to provide much denser data. Clearly this is a more expensive and time-consuming operation but improvements in illumination are said to have justified the effort.

In a brief period when through Petra oil company subsidiary PGS was associated with the Varg field, offshore Norway, the company in 2002 acquired two new 3D seismic surveys in different directions to supplement a previous survey on the field acquired in 1988. The old survey had been state-of-the-art in its day, but in spite of frequent reprocessing of the data during the 1990s, the data was unable to satisfactorily map the producing Varg reservoir which was complicated by thin sands and faulting due to salt tectonics. The idea of the Varg2002 project using the Ramform Viking with two sources and a spread of eight streamers was to cover the 200km² survey area twice and then combine the latest data with the old to come up with a new and more revealing dataset. The result was good. PGS said that the clarity and resolution of the data at the Base Cretaceous target horizon was greatly improved when all three azimuths were put together and processed. The company subsequently stated that the reservoir model revision proved to be the enabler for a successful drilling programme adding recoverable reserves to the field which would not have been possible without the multi-azimuth dataset.

In a similar exercise in 2004/05, PGS was engaged by BP to carry out a multi-azimuth acquisition project involving a total of 3150km² offshore Egypt on the Raven-1 discovery in the Northern Alexandria concession. In this case five new azimuths were acquired over an area previously surveyed as a single azimuth 3D dataset. Again improved illumination of complex geology resulted.

Although under-documented Western- Geco has had its successes with multiazimuth data. For example, on Wintershall Noordze acreage in the Dutch sector of the North Sea, salt structures within the pre-Zechstein fairway presented an interpretation problem even after the original 1990 seismic survey data had been reprocessed. A subsequent survey in 2003 was acquired with 5100m streamer long offsets. Two swaths were added to the main acquisition survey direction and this multi-azimuth strategy paid dividends in improved illumination.

Meanwhile VeritasDGC says it became interested in the possibilities of multi-azimuth five years or so ago when it observed some big differences in seismic image quality around and beneath salt accumulations achieved by going back over a previous Gulf of Mexico survey area in a different direction. It prompted the company to promote the benefits of re-shooting prospective areas in an orthogonal (90%) direction to earlier surveys with the expectation of coming up with a more informative integrated dataset. At the time Veritas was disappointed by the industry response which questioned the value of the investment in terms of the likely gain in illumination. Given the change in market conditions, the more intense quest for increased reserves, and the track record for multi-azimuth, that view may well change, especially as the technique is less costly than wideazimuth which among other things requires several vessels to accomplish.

Veritas was sufficiently clued into the theory and potential applications to be chosen by BP to collaborate on the first serious wide azimuth towed streamer (WATS) survey carried out in 2004/05 over the Mad Dog field in the Gulf of Mexico. In various presentations, including some forthcoming at the recent Society of Exploration Geophysicists annual meeting in New Orleans, BP described how the WATS approach was adopted for the Mad Dog field, located in water depths of 4100-6000ft varying most significantly along the Sigsbee Escarpment.

Essentially, conventional 3D streamer acquisition methodology had been found wanting in imaging the reservoir due mainly to the complex salt canopy which overlies a big part of the field. No amount of depth migration processing expertise was able to change this conclusion. A dual azimuth survey shot in directions that differed by about 60% did improve the illumination, but with significant gaps. It was at this point that BP focused on the wide-azimuth concept as the solution to obtaining the full image it felt was necessary.

In fact BP had a similar subsalt imaging issue at its Atlantis field in the Gulf of Mexico where recently installation of a production platform began. The company therefore decided to try an alternative strategy using a nodebased seismic acquisition system at the time in development by Fairfield Industries. As a result BP today is in the unique position of being able to compare and contrast two completely different ways of obtaining a wide-azimuth survey to illuminate deep water complex geology. Its conclusion for those impatient for the answer is that both approaches were found to be valid. The two methods complement each other in the sense that nodes have their place particularly in obstructed areas of a field development and where the survey size is restricted. The WATS method comes more into its own as the survey size increases. According to the company, it is a valid tool in field development work but is unquestionably more cost effective once the objective is purely exploration where node placement operations would be impractical compared with towed streamer.

For the Mad Dog WATS survey two dual source shooting vessels operated at the front and the back of the eight 8100m streamers towed by the 3D seismic vessel, the Veritas Vantage which for this exercise did not operate its normal airgun array source. Each line of the survey was shot four times offsetting the cable vessel from the source vessel by one kilometre each time. With the streamer cables separated by 125m, a shot patch of 8.1km x 4km could be synthesized. Total area of the survey was some 400km² and took around five months to acquire.

Veritas says that the main challenge of this novel method of shooting was to repeatedly move the seismic source for each shot location while keeping all the vessels properly positioned and synchronized during the process. The company says that its proprietary VerTEX multi-vessel communications system proved to be a major component in the success of the survey. Recognition from BP has come by way of a follow up six month wide-azimuth survey programme in the Gulf of Mexico. In common with WesternGeco and PGS, it intends to complete a multi-client wideazimuth project in time for the central Gulf of Mexico lease sale scheduled for 2008, additional confirmation of the perceived value of wide-azimuth as an exploration technique to image salt obstructed structures.

In some respects the most remarkable development to date in the wide-azimuth story has been the relationship between BP and Fairfield Industries which resulted in the pioneering use of nodal seismic technology on the Atlantis field. When BP began considering the use of nodes, there was not much case history to go by. In recent memory, the only commercial project was undertaken by the Norwegian company SeaBed Geophysical, recently taken over by SeaBird Exploration, in a 2005 project for Pemex since when little has been heard.

By all accounts it was almost by chance that BP got to hear of Fairfield's work on developing its Z700 node technology for water depths up to 700m as a cheaper and operationally simpler ocean bottom survey system than conventional ocean bottom cable technology. In what was a bold initiative BP encouraged Fairfield to jump straight into the Z3000 system rated for 3000 m water depths involving the use of subsea remotely operated vehicles (ROVs) for the positioning and retrieval of nodes.

Technically, BP could see that the deployment of a large number of ocean bottom seismic recording devices with a dense source effort shooting into this seabed recording array could achieve the desired broad range of full azimuths and offsets. Operationally, the over-riding priority of the system from the get go was simplicity with two vessels, Fairfield's New Venture and Canyon Offshore's Northern Canyon. In practice the four component sensors were distributed over 250km² on a 400m or so grid. This involved over 1628 node positions and for technical reasons to do with the number of available recording nodes (925) and battery life, the survey was actually undertaken in two stages. Each Z3000 node is a self contained sensor with a lithium ion battery and a very accurate clock, with no location signalling involved. The ROV lays and retrieves the nodes purely from positional data. During the Atlantis survey, the survival rate of the nodes was better than 99%, amazing given the water depths and the fact that some of the seabed was on a steep incline.

Fairfield is excited about the potential applications of both the Z3000 system and the Z700, which distributes the nodes on a retrievable rope rather than the more elaborate ROV system. The crucial benefit it can offer is the four component aspect of the nodes. Unlike any towed streamer system, this means that it immediately lends itself for use as a multi-component survey option for companies looking for improved resolution over target reservoir, and Fairfield says that its system has operational and economic advantages over any of the available options.

Furthermore, if the accuracy claims for node positioning of within 3m are correct, then the Z system is a natural for 4D seismic surveys where the key is the repeatability of the base and each subsequent monitor survey. By the same token, the company believes that its Z system could be sufficiently repeatable to obviate the need for life of field seismic systems which require even the most robust and reliable recording equipment to live underwater for many years and as a result will always be susceptible to deterioration or damage.

For the present Fairfield along with other marine seismic contractors are concentrating on extending the market for wide-azimuth seismic surveys with some expectation that the take-up by oil companies will be sooner rather than later. AOG

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