Surveys in silicon
'Because salt acts as a distorting lens for seismic waves,' says BP's John Farrelly, 'seeing through salt is like trying to look through frosted glass and describe something on the other side.'
Farrelly, BP's deepwater Gulf of Mexico appraisal manager, reports the Mad Dog trial greatly enhanced the team's knowledge of the size and shape of the field structure, and helped them to make decisions about how best to develop it. But the success came at a price. Shooting the WATS survey over Mad Dog required many passes to collect the data, using three seismic vessels. This not only took a long time, it was also expensive, and with the seismic market very busy in the Gulf of Mexico, chartering seismic vessels at a reasonable cost proved to be difficult. With two other sizable BP discoveries in the Gulf of Mexico having similar salt imaging problems to address, the team faced a challenge.
'In appraisal and development surveys the prime objective is to achieve the required data quality in order to reduce risk when it comes to the field development phase,' explains Farrelly. 'We needed to make a big cost shift in seismic while still achieving the sort of images we required. Our objective was to reduce the cost of WATS but still get the same quality data.'
Faced with the cost challenge, the BP team found a solution - without having to deploy a single seismic boat. Instead they turned to the virtual world, and 'shot' the survey first in the computer.
'With WATS,' explains Tim Summers, BP's subsalt imaging technology director, based in Houston, 'there are basically two key controls - the width of the azimuth array, and how often you shoot the lines. These are directly related to cost. By shooting first in the computer using a model of the salt and reservoir - based on our current understanding of a particular field - we can understand both these parameters, and tailor the survey to meet our subsurface objectives.'
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'Bearing in mind that it's always cheaper to spend the money on processing than it is on acquiring a dense grid of lines, we always have to look at the trade-off between acquisition design and fundamental data quality, against what we can eventually provide through processing,' he notes. 'As we reduced the WATS acquisition costs, we knew we needed to improve the processing.'
The effort proved well worthwhile. Adapting the processing toolkit for the WATS configuration has led to dramatic improvements in subsalt image quality and is providing valuable new insights that are helping in the appraisal of further discoveries.
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Adapting for exploration
If WATS works for appraising known discoveries, why not adapt it for exploration surveys where the difficulties of seeing below salt are just as great?A major drawback is cost. In exploration surveys of very large areas, the seismic quality requirements may not be so stringent as those in appraisal and development surveys, and thus the seismic budget per square kilometre is typically lower.
'The big driver in exploration is to find a way to cover the largest area we can, and obtain data of acceptable quality, all within the budget we have,' explains Summers. 'In a development survey, we might be looking to cover an area of 400 square kilometres. But exploration surveys, in contrast, might cover 10,000 square kilometres.'
By taking advantage of leading edge computer modelling techniques, the advanced seismic imaging team were able to 'see' the effects of altering acquisition parameters, and develop a version of WATS adapted for use in exploration. The result is XWATS, essentially a stripped-down version of WATS with a narrower array and lower shooting density. But although the acquisition parameters are less stringent for XWATS, the data quality is sufficient for exploration.
'We've designed the XWATS acquisition technique such that if we make a discovery we can carry out more surveys and add to the XWATS data to end up with the equivalent of a WATS survey,' explains Summers. 'This means that the WATS method can be used, added to, and infilled over the whole life of the field, from when we first acquire the exploration acreage to when we start producing from the field.'
Above and below
The idea of going back and adding data as needed is also proving to be the key to adapting multi-azimuth seismic (MAZ) for use in exploration. MAZ is used extensively by BP in the Nile Delta region offshore Egypt, where it has proved to be very successful in illuminating reservoirs hidden below the Messinian layer - a thin, but complex layer of anhydrite salt located about three kilometres below the surface that tends to scatter seismic waves (Frontiers, December 2005).In a MAZ survey, between two and six 3D surveys are recorded over the same area in quick succession, but at different angles - or azimuths - to one another. By processing the individual surveys separately and then combining the data it is possible to look at the same spot from many different angles. This has resulted in better illumination of the subsurface both below and, surprisingly, above the salt layer (see link to panel on MAZ surveys, below).
'MAZ,' says Jim Keggin, a BP geophysics advisor based in Cairo, 'is a simple, robust method - and it works.'
It is the characteristics of being simple and robust that have helped the group adapt MAZ for use in exploration surveys.
'To aid in the development of BP's Raven discovery in the Nile Delta, we shot lines at six different angles and covered an area of 600 square kilometres,' says Keggin. 'The subsalt images we obtained were spectacular. But for exploration purposes we wanted to shoot MAZ surveys over much larger areas. For these "Mega-MAZ" surveys we needed to find ways to reduce costs.'
The obvious answer was to reduce the number of azimuths shot. But how many azimuths would do? After carrying out trials, the group concluded that shooting just three different azimuths would be a good way to start.
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Groups working in other BP assets, including the North Sea, the Gulf of Mexico, Trinidad and Australia, are following the MAZ success story with interest. Some are reprocessing existing 'opportunistic' data - existing datasets where one survey has been later overshot with another survey at a different angle - to get an idea of what MAZ can do for them. And several are already reporting that they like what they see.
Having seen the results of the MAZ surveys in the Nile Delta and the WATS surveys in the Gulf of Mexico, geophysicists in other salt-prone and geologically complex areas are keen to 'have a go'.
'The critical thing is to choose the right technology for the job,' Summers advises. 'Not all technologies are applicable everywhere across BP's portfolio.'
MAZ and WATS, he points out, are just two elements that make up the seismic toolkit available to BP's assets. Other options to consider, where water depths are less than 300m, include ocean bottom cables. In deeper water, nodes placed on the sea floor may be more suitable (Frontiers, December 2005). All have their benefits and restrictions.
Seismic toolkit
Streamer-based techniques, like Mega-MAZ and XWATS, are useful for collecting data over large areas, but would be difficult to deploy in areas like the North Sea, where existing offshore platform infrastructure may get in the way. However, the techniques have the advantage that they can be scaled up from exploration quality to appraisal and development quality by simply returning and infilling the original surveys. On the other hand, cables or nodes can be very suitable for surveys covering a single field. Cables can be permanently installed to allow for 'life of field seismic', as BP first did in the Valhall field offshore Norway. This is now being done in the BP-operated Clair field in UK waters west of Shetland, and in the Azeri field in the Caspian Sea.'The choice of which technique to use is down to the subsurface challenge, the degree of detail needed, and operational requirements,' explains Summers. 'WATS is the "gold-plated" towed streamer solution where you have really serious problems with salt. In contrast, MAZ is a relatively simple but robust method that can help you to assess the situation. If MAZ shows up serious problems in imaging below the salt, you can always go back and acquire a WATS survey.'
'It's really important to maintain this dialogue,' says Summers. 'Input from around the BP world will not only improve the efficiency of subsalt imaging, it will also help us as we explore more and look deeper into the subsurface. And the deeper you go, the harder it is to see what's going on - these techniques will help us do just that.'
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