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Sun 1 Apr 2007 12:00 AM

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Another dimension to reservoir production

4D seismic technology can offer a clear view of reservoir contents, saving some of the time and money exploration absorbs and boosting recovery levels from existing sites

In the late 1990s, time-lapse four-dimensional (4D) seismic technology emerged offering energy companies the ability to revolutionise the way they carried out exploration and production (E&P) activity. However, 4D has been viewed as something of a niche technology and its use is not as widespread as the benefits it offers might otherwise suggest.

4D is essentially the comparison of the results of 3D seismic surveys repeated at considerable time intervals: time is the fourth dimension. For example, surveys from before a field starts producing can be compared with surveys from various post-production stages.

The reported benefits of 4D are numerous: the technology can help increase recovery levels over the life of a field and reduce the amounts of well intervention and infill drilling needed. The technology is particularly relevant for deep-water or challenging onshore fields, where it can help reduce time and costs for a drilling programme.

Part of the reason for the slow uptake in 4D technology is the fact that at the time it was introduced, the computer support practices required to efficiently use it were rare. Either they had not yet been invented or were not commercially available. Where they were available they often required a high level of technical expertise in data interpretation, which many energy companies did not have. However, the arrival of new computer technology has changed this, enabling data to be analysed more quickly and more accurately, providing a clearer interpretation of a given reservoir's particular characteristics.

As Shell puts it, the use of 4D "can highlight where oil has been left behind so a new well can be drilled to extract it."

These technological advances in IT mean that reservoir monitoring can now be carried out on a quantitative basis, as opposed to the previous qualitative method, giving a much more accurate, real-time indication of the state of the reservoir. The accurate data received from 4D seismic surveys means much smaller variations in the signal can now be detected on a reliable basis.

In the last couple of years, more oil and gas companies have started using 4D techniques. According to ExxonMobil: "4D reservoir simulation helps keep our reservoirs healthy and productive at lower cost."

Norway's Statoil has used it on the Gullfaks field helping it to accurately select the locations of over 30 additional wells and identify 600 million barrels of additional reserves. Statoil was planning to shoot around 8 500 km2 of all seismic on the Norwegian continental shelf in 2006 compared to 4 500 km2 in 2005. And the amount of 4D it acquires is also increasing to 1 900 km2 in 2006 from 1 800 km2 in 2005.

Statoil said its use "has helped improve the oil and gas recovery from existing fields." Shooting of 4D during 2006 took place on the Troll, Sleipner West, Sleipner East, Snorre, Tordis and Vigdis fields in the North Sea, on the Norne, Heidrun, Kristin fields and the Midgard structure in the Norwegian Sea, and on structures around the Snøhvit field.

The company has said that the use of 4D on Gullfaks, in terms of value creation, was worth around US $140 million "and the total may well exceed [that] by the time the field is exhausted." Statoil has also begun using quantitative 4D seismic. The "robust differences" seen between the survey results are attributed to fluid changes and/or changes in reservoir pressures. So the technology can be used "to monitor variations caused by production and/or the injection of fluids and gas to improve recovery." Statoil's approach for quantitative analysis is to integrate the seismic data with all other available information, from cores, wells, production and reservoir simulation.

One example was a 4D seismic map of saturation, made for a part of the Statfjord field, which showed the relative proportions of oil and water. According to Statoil, subsequent drilling "proved that this map was more accurate than that derived from conventional reservoir simulation."

Statoil is now planning to "enter the realm of seismic ‘history matching' by linking 4D seismic to flow simulation." It says that if successful this will "dramatically improve forecasts of how a reservoir behaves during production and the updating of geological and petrophysical reservoir models."

BP is also using 4D more often and says it has now become common practice in its producing fields. "The beauty of 4D seismic is that it gives you a good picture of the spatial distribution of the oil that has been produced, and changes in the pressure front in the reservoir over time," said Dave Whitcombe, seismic network leader and senior advisor in 4D, based in Aberdeen, UK.

"It is supplementing and complementing conventional production logging and really creating a whole new game in field surveillance. This enables us to understand a field's behaviour better, and to predict future production with more accuracy."

Since the late 1990s when BP began using 4D in the North Sea, the technique has proved to be a powerful aid in reservoir management. Now in the Valhall field offshore Norway, BP has taken 4D a step further, by setting up a permanent life-of-field seismic (LoFS) system recorder. This consists of a series of seismic receiver stations mounted on a cable on the sea floor, connected to the Valhall platform. During a survey, seismic data is transmitted from the receivers to the platform, and relayed from there to shore via an optical cable. Because each receiver station includes three geophones, as well as the usual hydrophone, both pressure and shear waves in the subsurface can be recorded.

"The shear waves are particularly good in helping us to see through gas obstructions in the overburden," Whitcombe explained. "Currently a source boat shoots a survey over the installation every three months. This frequency of acquisition opens up the possibility of making a ‘movie', where each 3D image forms one frame, to monitor changes and help to spot the more subtle effects in the reservoir." The quieter environment on the seabed also results in a better ratio of seismic signal to noise.

While LoFS is more costly than conventional seismic acquisition, it has positive payback over time, added Whitcombe. "Permanent 4D installations become entirely attractive in big fields with substantial remaining hydrocarbon reserves where the data has the potential to impact the siting of lots of wells."

Saudi Aramco is also using more 4D, but admits that while the possible benefits are exciting, "it has been both difficult and expensive to consistently obtain usable data." However, it expects to use the technology more often in the future. The company states that "reserves assessment and replacement are mission critical activities at Saudi Aramco. Annual reserves assessments are based on updated studies that use state-of-the-art reservoir characterisation technology operating on the most current data. With more than 250 reservoirs and a mix of reservoir types to which new technology and data can be applied, there are many ways to achieve both reserves replacement and accuracy objectives.

"4D seismic can provide detailed snapshots of fluid flow within a reservoir, providing a direct means to validate reservoir simulation results. Advances in seismic data processing, such as pre-stack time or depth migration, are being used to improve the structural imaging of many reservoirs by focusing seismic energy that arrives by complex ray paths. Innovative approaches in displaying seismic data, such as spectral colour and coherency, allow interpreters to extract finer details of reservoir faulting and fracturing."

The US Department of Energy's (DOE) National Energy Technology Laboratory (NETL) believes that increased use of 4D and other advances in seismic imaging could extend the application of enhanced oil recovery (EOR) to "thousands" of mature fields. It is currently funding research into advanced 4D technology. In 2005 it paid for research by the University of Kansas into the use of 4D imaging together with higher-resolution imaging of other seismic technologies, which created the first low-cost depiction of carbon dioxide movement through a thin, shallow oil reservoir. The research will continue until 2009.

The DOE's NETL claims that: "Efficiently designing and implementing 4D monitoring of EOR programs could significantly increase oil recovery in fields with marginally economic volumes of remaining oil. A clearer, real-time image of the subsurface at an economically feasible cost could unlock billions of barrels of oil."

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