New seabed systems promise to make time-lapse seismic data more accessible.

By the mid-1990s, the oil industry had been benefiting from 3-D seismic data for at least 10 years - 10 years of ever-increasing 3-D data quality, quantity and cost efficiency. Its early manifestations included just two streamers and data processing largely based on 2-D data processing principles. The technique developed to the stage where purpose-built vessels towing up to 12 streamers were acquiring a square kilometer of high-quality 3-D data every 10 minutes. In this drive to acquire 3-D more and more efficiently, the cost on a per-vessel kilometer basis ultimately was reduced to about the same as that of 2-D data just a few years earlier.

This drive, known in the seismic business as "the streamer race" - which would have been called a price war in any other industry - eventually led to a crisis for the seismic contractors. In the mid-1990s, it began to dawn on all the players that they were almost giving away the single most important and value-adding service that the exploration and production industry would ever have: the 3-D seismic image.

Huge overcapacity, cost efficiency that could no longer be improved upon and an ever-increasing feeling that they had been taken for a ride provided the seismic contractors with the backdrop behind the next big seismic development. Within the seismic contractor community, the buzzwords of the late 1980s and early 1990s - cost efficiency, turnaround time, nonexclusive 3-D - began, slowly, to give way to others. After all, the seismic industry had managed, single-handedly, to sell itself down the river and make 3-D seismic data, the most valuable tool in the exploration and production business, a mere commodity where the cost per square kilometer was all that separated one contractor from another. Somehow, seismic data had to be returned to its rightful place in an industry that relied upon it not only to find potential oil and gas fields but to reduce the financial risk of drilling in ever-increasing water depths.

It was time for the seismic industry to focus on the value of seismic data again. If it didn't, survival was unlikely; indeed, many companies did not manage to survive. But luckily, perhaps, for those who saw the writing on the wall, there was a growing need in the exploration and production industry. No longer could production growth be guaranteed with the drill bit. No longer was it possible to keep ahead of the competition by finding more and more new fields. By the mid-1990s, it was clear that the main oil-producing basins of the world already were being exploited and that no amount of cheap 3-D could alter that fact. What was needed, as finding and lifting costs were both on the increase, was technology that would help companies exploit current assets to their maximum. Exploration started to take a backseat to reservoir management.

As unswept oil, compartmentalization, enhanced production and fluid injection became more and more important issues for oil companies, a new product emerged from the developers of seismic products: time-lapse seismic. Time-lapse seismic is more commonly known as 4-D, which is essentially 3-D with the added dimension of elapsed time.

When one is looking for very subtle differences in seismic datasets acquired at intervals over producing fields, it is critical that each dataset is acquired in as near identical a fashion as possible. If it were possible to replicate exactly every aspect of the seismic acquisition process, we could be sure that any changes within the data from one survey to the next would be caused by changes within the reservoir. But exact replication is impossible. The best we can hope for is to eliminate as many unrepeatable aspects of the acquisition process as possible.

In towed streamer seismic acquisition, there are many unrepeatable elements. The streamer's position, for example, is a function of sea state, tide and currents. And the sea state itself directly produces unrepeatable noise on the streamer because of the wave action on the surface. The source signature also is hard to replicate from one shot to the next, let alone from one survey to the next, but it is possible to use the exact same source size and geometry and position it more accurately than the streamers. This is why the first serious attempt at repeatable 3-D for time-lapse analysis over Foinaven, west of the Shetland Isles, used cables that were buried beneath the seafloor. This guaranteed that the position of the cable was the same for each phase of the 4-D project and that the cables would be unaffected by the unrepeatable noise caused by wave action at the surface. This allowed the seismic contractors to concentrate on replicating the source signature and position as best as they could. This method for 4-D was considered by the operator concerned with this first permanent installation to be the only way to achieve successful 4-D data sets. The majority of operators now feel this way.

Unfortunately, at the time, the permanent installation method was extremely expensive, and ultimately the equipment proved not reliable enough over a long period of time. Although the Foinaven system failed to achieve all of the original requirements of the project, it was still a huge success in that it proved the concept. A large cable spread was successfully deployed and buried in water depths greater than 1,640 ft (500 m),

4-D signal was achieved, and changes in the reservoir were interpreted from 3-D datasets recorded at different times.

But, like so many technological breakthroughs, this one came at a time when the industry was suffering from a low oil price, and cost cutting was the order of the day. So despite the results from this first project, no one could justify the expense. Instead, the industry sought other solutions to find a compromise between cost and repeatability. This meant looking again at the old tried-and-trusted towed streamer method.

With very careful survey planning that concentrated on duplicating streamer feather angles, it was discovered that certain fields could benefit from 4-D acquired with repeat towed streamer 3-D surveys. Special systems were even designed to help steer the streamers into the same position each time. This approach may prove useful for certain fields, which will remain the best compromise between cost, data suitability and the increased return from the field, at least in the short term. But the fact remains, and has been gradually occurring to all 4-D users, that to effectively manage most fields, repeatable seismic that eliminates as much unrepeatable noise as possible is the only way to accurately detect fluid movement in the reservoir, find undrained compartments, see small pressure changes, and monitor fluid or gas injection. It became apparent that a new, more efficient, less costly and ultimately more reliable and higher-quality system was needed in order to utilize the permanent installation approach.

The cables used in the Foinaven project were not purpose-built. They were based on a standard towed streamer system of the time incorporated into a solid cable design and put together by an ocean bottom cable company. The cable itself, all 98,000 ft (30,000 m) of it, contained only hydrophones and was designed with more than 100 connectors. But if this type of design has now been superceded, the method of deployment and burial has not. It has been fine-tuned, but the people responsible for the deployment back in 1995 are now doing it again with just as much success. This time, however, they have a purpose-built cable at their disposal.

This new system already is gaining widespread acceptance by the major 4-D users. One of these is investing heavily in it for a large 4-D installation sometime in 2003; another has conducted a 4-D pilot study with it in the North Sea, which is expected to lead to a rapid increase in the use of permanently installed 4-D systems.

This system brings many benefits to 4-D seismic. Its relatively simple construction, with no connectors and use of compact fixed geophones together with hydrophones, means that it can be mass-produced quickly at relatively low cost. Its high break strength, steel armoring, small diameter and lightness has meant companies that use the system have been able to develop a modular deployment system that makes use of some off-the-shelf technologies available in both the oilfield services and other industries. This means that mobilization and acquisition costs can be kept down. It is rated to water depths of 6,500 ft (2,000 m); thanks to expertise in building downhole tools that function in high-pressure environments, it soon will be engineered for even greater depths. The cable has hydrophones and three orthogonal geophones at each sensor station so that it provides not only conventional 4-D compressional wave hydrophone data but also 4-D combined hydrophone and vertical geophone data. Perhaps even more importantly, it provides shear wave data to help tackle a host of reservoir imaging problems.

The list of shear wave benefits is long, and the ability to obtain that information with a deepwater, cost-effective, high-quality, reliable system will make 4-D data more accessible. Oil companies can now consider the use of a permanent 4-D system for every field they are either producing or plan to develop in the future.

After years of trying to make standard marine seismic acquisition fit the bill, we are on the verge of a huge growth in 4-D seismic, brought about by new developments in permanently installed seismic acquisition systems that meet the critical criteria of repeatability, reliability and cost. This is no mean feat, especially at a time when the industry is suffering from years of incredible expansion in the towed streamer sector. That expansion was never going to provide answers to the type of questions that oil producers now need to have answered in order to properly manage their assets and cost efficiently maximize their production.
The future of seismic is 4-D. In the mid-1990s, the reality that permanently buried cables were the future of 4-D was already recognized. Now, finally, we have gone back to the future.



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