It was not so long ago that time lapse, or 4-D, seismic was a technology that kind of hovered in the background, being applied only in specific situations, such as when a reservoir began producing erratically rather than as predicted.
The application of 4-D, i.e., repeat 3-D surveys, to evaluate production and reservoir properties periodically over the life of a reservoir was considered an expensive luxury -- if considered at all.
But in today’s high commodity price environment, 4-D seismic is becoming increasingly commonplace as a reservoir management tool to maximize economic return. In fact, 4-D is gaining a strong foothold in the mainstream of seismic applications.
The bulk of the 4-D activity is in the North Sea, where many of the earliest surveys were undertaken, according to David Lumley, co-founder and current chief scientist of 4th Wave Imaging, Aliso Viejo, Calif., and a speaker at the upcoming AAPG International Conference and Exhibition in Perth.
He attributed this to the fact that oil is regarded as a national resource in Norway and the United Kingdom. There’s a pressing need for optimal recovery because the future of those countries depends on managing their oil wisely.
On the domestic front, look for 4-D to become really big in the Gulf of Mexico.
The Gulf shelf was the locale of some of the early 4-D technical success stories, but these projects failed to measure up economically for the most part. The big companies who controlled the properties weren’t particularly inclined to invest much capital in them, preferring to use them as cash cows to fund international E&P activity.
It’s a different scene today.
“Over the last decade a lot of independents have bought those mature fields and are successfully drilling new wells, and they’re thinking about 4-D,” Lumley said. “They’re not quite there yet because they’re more conservative about trying expensive new technology than the majors, but they’re looking at it as a return on investment to ‘sharpshoot’ bypassed oil.
“Apache has done a very similar thing in buying the old Forties Field in the UK and doubling its production in a few years with the help of 4-D,” Lumley said.
He predicts a gradual resurgence of 4-D on the shelf and said 4-D in deep water is really starting to kick off in both the GOM and Brazil, noting “if you’re drilling $50 million wells, it’s costly if you make a mistake where you put one; optimal placement is essential.
“There’s interest in monitoring reservoirs using 4-D seismic at the very beginning of a field’s life in this high expense environment,” he continued. “That way, you can anticipate a problem before it becomes irreversible.
“It’s risk reduction, like going to the doctor for an annual checkup rather than waiting for something bad to happen.”
Seeking the ‘Holy Grail’
Notable advances are occurring in 4-D acquisition technology to improve repeatability of surveys, which is crucial. Highly accurate positioning of sources and receivers is key to achieving the required level of repeatability.
Lumley noted there also have been advances in navigation systems and location positioning of cables. Steerable streamers that can be put in the same place as previous surveys are being refined as well.
Another acquisition technique being employed is to oversample the streamer receiver array by towing more cables and extracting the repeatable data.
“The Holy Grail, of course, is semi-permanent installations where receiver arrays are laid down semi-permanently on the seafloor,” Lumley said. “This could be nodes or ocean bottom cable. The biggest field test of this technology is currently BP’s permanent array project at Valhall Field.”
Regarding the source repeatability aspect of 4-D acquisition, new technology is being developed to tow more source arrays and have the closest source to the baseline shot position fire.
“Instead of towing two source arrays, you might tow three or four and select which ones fire,” Lumley said. Statoil is taking the lead to develop this high fidelity source technology.”
There’s a push going on to improve 4-D interpretation tools owing to the need to integrate the geology, rock and fluid physics, seismic and flow simulations to perform an accurate 4-D interpretation.
“Sometimes today we see anomalies but aren’t sure how to interpret them and don’t know what they mean,” Lumley said. “To develop these integrated tools to sort out those anomalies and help update the reservoir model, we have to get the geologists, geophysicists and petroleum engineers to work together in an integrated work space.”
Finding bypassed oil is high on the list of business drivers pushing 4-D advances, but this is only the tip of the iceberg.
Injecting various materials, e.g., CO2, water, steam, into the reservoirs to enhance production or maintain pressure is a pricey undertaking. It’s essential to be able to monitor where the injected material goes and if it’s doing the job it’s supposed to do; 4-D can prove invaluable in these situations.
Carbon dioxide storage is anticipated to be a major target of 4-D seismic monitoring technology. Because this greenhouse gas is not beneficial when released into the atmosphere, injecting it into the ground appears to be a good solution.
“CO2production is a big problem at some fields -- in Norway and Canada, for instance,” Lumley said, “and it’s going to be a big challenge at the Gorgon Field in Australia, for example, where they’re producing it as a by-product of the gas field. They’ll have to re-inject it and store/sequester it in deep geologic formations so it doesn’t escape.
“There will be a monitoring challenge there,” Lumley noted, “but if they can inject the gas and store it for a long time, it will have a big positive impact -- both financially and environmentally.”
It is noteworthy that other industries generate CO2and are seeking a place to contain it.
“A new business sector will be created over the next decade or two where some will want to get rid of CO2,” Lumley said, “and some will offer to receive and inject it into the ground for long-term storage. It will have to be monitored, and 4-D seismic is one of the tools that will be used to be sure it’s in the right place in the ground and not leaking out.
“Governments worldwide are getting serious about CO2sequestration technology,” he added, “and are starting to invest heavily in its research.”