Nearly thirty years on from its first beginnings, 4D – or time-lapse – seismic monitoring has now come of age as a proven technology. The technique has, at last, established itself as a valid and valuable reservoir management tool.
Its advantages have been most clearly seen in applications to monitor fluid movement and differentiating between the drained and undrained portions of a reservoir, typically within clastic rocks offshore. But developments promise to marry predictive computer simulations of changes within reservoir drainage with time-separated, real acoustic images.
This means that such qualitative data should soon be joined with increasingly quantitative information.
But while the future of this approach is exciting, there are challenges to overcome before its potential is likely to be realised on the global stage. With a market share of about 3%, despite the advances, 4D remains a much smaller sector than the original 3D exploration from which it sprang.
Part of the reason for this lack of take-up is that, while oil companies are prepared to discuss their technical achievements, they often appear more reticent when it comes to quantifying economic benefits to production, and then putting that analysis in the public domain.
This reluctance is understandable but the technology is unlikely to see wider uptake until the cash value – rather than the cost – of 4D has been more openly demonstrated.
Despite the lack of universal employment, the success stories are clear for all to see. In 1995, 4D seismic investigations were used on a full field – Gullfaks in the North Sea – as part of a Statoil-Schlumberger joint project.
During this work the technology's clear economic potential was demonstrated when it was used to distinguish between drained and undrained areas, identifying the remaining pockets of oil and gas.
Success at Gullfaks was first in a long line of fields such as Forties, Gannet, Norne, Oseberg, Statfjorda and Troll.
Today Petrobras’ Marlim field, in the Campos basin off the shore of Brazil, provides one of the best examples of how 4D seismic technology can be incorporated into reservoir management decisions. The use of WesternGeco’s 4D imaging capabilities at the field enabled improved dynamic and static modelling, which enabled the oil, gas and water movement through reservoirs to be monitored with greater accuracy and ensured that reservoir behaviour can be predicted with more confidence in the future.
4D seismic also proved highly effective in helping to locate bypassed reserves and optimise the placement of additional infill-drilling wells.
However, despite its benefits, it doesn’t provide the answer to all questions. Rildo Oliveira, a technical consultant and senior geologist at Brazilian oil producer, Petrobras says: "The information provided by 4D seismic must be coupled with additional information, such as saturation logs, tracers, formation tests and pressure."
Clearly then, highly competent multidisciplinary integration between geophysics, geology and engineering is required.
The players and the market
In addition to Petrobras, other industry players include ExxonMobil, ConocoPhillips, Chevron and Total, with the three big guns of 4D being StatoilHydro, Shell and BP. StatoilHydro, the market leader having acquired more than twenty 4D streamer surveys in 2006 alone, has begun the quantum leap from qualitative to quantitative imaging, moving on from simply identifying the drained portions of a reservoir to producing saturation maps.
Shell UK and BP have awarded major contracts to Edinburgh-based Reservoir Imaging for projects in the North Sea in 2008.
According to Keith Watt, managing director of Reservoir Imaging, its potential is now being realised in monitoring projects. "[It is] playing an increasingly significant role in maximisation of reserves production, particularly in mature provinces, such as the North Sea," he says.
It is a viewpoint with which Lasse Amundsen, StatoilHydro’s chief geophysicist, and Martin Landrø, professor in applied geophysics at the Norwegian University of Science and Technology, would concur. According to the industry experts, although it is difficult to quantify exactly, the economic impact of 4D seismic, particularly in the North Sea, has been significant.
They calculate that the savings for Gullfaks alone approaches $1bn and on average there is a 6% reduction in drilling costs and additional reserves amounting to an average 5% per field.
For the moment at least, the North Sea accounts for more than 80% of the global 4D market in terms of seismic survey expenditure. But if all of this continues, it may ultimately provide the showcase necessary to encourage further 4D technology take-up in the coming years.
Rising to the challenge
"Three main areas will shape 4D’s future," says maritime geo-consultant Mike Herning. "Repeatability; quantifiable benefits and combined technologies. If the industry solves those matters, and obviously they’re interlinked, the market could be huge."
There have already been advances in the area of repeatability, drawing on the likes of Geographic Information System, enhanced processing, permanent seismic installations and steerable streamers to obtain 4D acquisition at high accuracy.
Despite the successes with the Life of Field Seismic Project at the Valhall field and BP’s planned permanent installations at both the Clair and Azeri Chirag fields, on a wider scale the approach has not been widely adopted.
Although falling systems costs and new developments based on fibre-optics may make this more attractive in the future, a potential rival has emerged in the guise of steerable streamers. By ensuring that the receiver location is accurately replicated for two separate seismic surveys, this approach has yielded some good results for a number of fields, and clearly now demands recognition as an alternative to permanent installations.
StatoilHydro’s shift towards enabling quantitative information – including uncertainties – to be obtained, aims to achieve the widest integration of seismic data with other sources, such as cores, wells production and reservoir simulation.
On the basis of the company's trial on part of the Statfjord field, in which it produced a saturation map and then successfully tested it by subsequent drilling, StatoilHydro has undertaken a similar exercise at Gullfaks, which is more structurally complex.
The next move is to merge flow simulation and 4D seismic in an attempt to provide vastly improved forecasts of reservoir behaviour and make major improvements in geological reservoir models. If these developments prove as successful as is hoped, they will have a significant impact on the future of 4D within the industry.
There is a natural affinity between all 4D monitoring technologies – 4D seismic, 4D gravity and 4D CSEM (Controlled Source Electro-Magnetic) – and it is perhaps in combination that they may provide the greatest benefit. Seismic data, though higher in resolution than gravimetric or electro-magnetic information, is open to influences from saturation, pressure and temperature.
With electro-magnetic resistivity varying only with saturation changes, combining this technology with 4D seismic, for example, could open the door to distinguishing alterations in pressure from saturation, which, as Herning observed, links to the quest for the quantifiable.
The value of 4D seismic in reservoir management, increasing reserves, upping recovery and optimising infill well locations has been established. This is alongside its potential for driving down costs, reducing uncertainty and minimising the number of dry-holes.
However, for the take-up to grow and the technology become a global tool, the tipping point is likely to be how readily quantitative results follow qualitative success and how well the 4D industry rises to the challenge.