The ROV sits on the sea bed,
ready to retrieve a node
to lifting to the back deck for
charging and retrieval of data.
There’s nothing quite like a successful field project to demonstrate the commercial viability of a particular technology.
So chalk one up for the recently completed wide-azimuth OBS (ocean bottom sensor) node survey over the BP-operated Atlantis field in the challenging deepwater Gulf of Mexico.
Atlantis was a first for deepwater nodal seismic surveys, and the successful outcome could spur demand for a host of such projects -- and other type applications.
The “at-scale” Atlantis field trial deployed -- and retrieved -- nodes coupled to the seabed at 1,628 locations spread out over 240 square kilometers, at water depths between 1,400 and 2,200 meters.
This was an unprecedented nodal implementation both in scope and water depth.
“There’s no doubt 3,000 meters is a frontier,” said Rodney Calvert, chief scientist in geophysics at Shell, “and this system is the only proven technique at the moment to that depth.”
The purpose of the Atlantis project was to acquire a new class of seismic data using Fairfield Industries’ 3,000-meter-rated node system -- wide azimuth data -- to overcome imaging problems related to illumination of the subsalt structures.
BP is preparing to reveal the first images acquired from in-house processing.
The nodes are positioned
in their cradles for easy loading
and unloading from the ROV,
seen here on deck.
An Added Dimension
Deepwater node technology is about much more than subsalt.
Indeed, deepwater-capable nodes are anticipated to be in demand for a range of applications, including 4-D, or time lapse, surveys to manage and monitor reservoir performance.
The individual nodes, which are self-contained sensors with a battery and a clock, are positioned on the seabed and retrieved by the familiar Remotely Operated Vehicles (ROVs). Ambient noise is low compared to streamer acquisition, and the seafloor coupling makes for enhanced signal levels.
“Every node survey as a by-product is a 4-D survey because of positional accuracy and repeatability,” said Steve Mitchell, vice president and division manager at Fairfield Industries. “Normal positional accuracy is always going to be better than five meters automatically; that alone makes it 4-D capable.”
High quality 4-D surveys require repeatable sources and receivers, according to Calvert, who noted the key to repeatable 4-D is to have repeatable geometry. The source issue is relatively easy to resolve.
“We can repeat sources pretty effectively with such things as GPS,” said Jerry Beaudoin, project manager of OBS technology at BP. “The issue has been -- especially with streamers -- repeatability, and the industry has struggled with this for quite some time.
“At Atlantis, we looked at the difference between the ROV’s estimate of position when the nodes were put down,” Beaudoin said, “and the ROV’s estimate of position when we picked them up.
“For three-fourths of the nodes, we had differences of five meters or less,” he said. “That’s pretty good in water depths almost to 2,200 meters. That’s a real world test of how effective this subsea positioning system really is.
“We looked at each other, and said ‘we’re there; we’ve demonstrated the receiver repeatability that’s a necessary prerequisite for quality 4-D surveys,’” Beaudoin said. “It’s not the only one, but an important one.”
It’s becoming S-O-P to think about the need for time-lapse seismic early on as operators ponder what sort of life cycle is needed for seismic in a field. Discussion is ongoing about not only what will be needed within a year but what kind of seismic technology might be needed five years out.
Time lapse is always part of the discussion (at BP), according to Beaudoin.
“It’s important to think about 4-D up front,” Calvert commented. “Often, at the start of a 3-D survey campaign you may not be sure how many 4-D surveys you’re going to need.
“Both nodes and OBC can sit on the seabed, and you have fixed geometry,” Calvert said. “But if you’re going to invest in permanently-placed OBC, which entails a huge up-front capital investment, you must reckon you’ll have four or five repeats to make it pay for itself.”
However, an operator may be able to determine what’s needed with only one repeat survey.
“If you’re not certain how many surveys you’ll need,” Calvert said, “nodes could be a way in because you don’t invest in seabed equipment -- you just rent it.”
Seeing Into the Future
In looking ahead to managing and monitoring some of its larger fields that are in deep water, BP is mulling what role nodes might play, Beaudoin said.
He noted there’s a world of difference in deploying cables in 70 meters of water at a field like Valhall (in Norway) and trying to do it in, say, 1,500 meters.
“The logistics of putting cable down -- permanent or otherwise -- in these water depths is intimidating,” he said. “The cable must be strong enough to hold together as it’s being dangled in a couple of miles of water.”
It’s widely recognized there’s an array of infrastructure on the heavily drilled shallow water shelf in the Gulf of Mexico. It is noteworthy that the seabed also gets quite cluttered -- often over the heart of the field -- in the more sparsely-drilled deepwater. Even the surface can pose obstacles in the form of floating production, storage and offloading facilities (FPSOs), tankers, etc.
The combo of infrastructure and cables make lots of folks nervous.
Indeed, the laying of cables across expensive and sensitive seafloor facilities can cause heartburn even for the most seasoned facilities engineer. Conversely, installation of new flow lines and the like can tear up an OBC.
Aside from the iron factor, there’s sometimes a need to circumvent critter-type infrastructure that may be present in the deep water, e.g., chemosynthetic colonies.
Nodes can be placed around and beside any and all of the above, enabling less risk and increased flexibility on a complex seabed. An added plus is the use of the familiar ROVs, which provide a comfort factor for the facilities people who view this equipment as a normal part of operations.
In the event the cost for permanently-placed cables drops dramatically and deployment becomes easier -- think fiber-optic cables -- this conceivably could become the “next” be-all, end-all approach to deepwater 4-D seismic acquisition. For now, however, Beaudoin envisions nodes to be the future for deepwater time-lapse seismic.
Think about the possibilities: An operator could have several deepwater fields that will require surveillance for maybe 10 years. A nodal system could be purchased or leased for the long-term and moved from field to field to field as needed. This would be a much better use of the capital investment, which would not have to be justified on the back of one field.
Such a moveable reservoir surveillance system would allow inclusion of satellite areas where there are subsea tiebacks and the like.
“A modular system like this, which is ROV-deployed, could open up a new business model for reservoir surveillance of these fields in deep water,” Beaudoin said.
The growing need for infill seismic data is another area ripe for node technology application.
“When a company develops a field, platform locations are strategically placed over the reservoir level,” said Roger Entralgo, vice president of seabed business development at CGG. “When you come in with a streamer or OBC survey, the obstruction creates a void, or area with no data,” he said, “which happens to be probably the primary area of interest for oil companies because it’s over the reservoir level.”
“We see a natural marriage of streamers and nodes,” said Mike Spradley, acquisitions marketing manager at Fairfield. “One of the markets we envision with nodes is working with streamer operations, so while they acquire streamer data they also would be acquiring data from nodes placed in the area that’s inaccessible to streamers.
“They would acquire a seamless data set,” Spradley added, “which would be merged in processing so the field becomes geophysically transparent.”
Demand for this particular node application is picking up worldwide, according to Entralgo. Many fields are at a stage where operators have to make critical production and drilling decisions, and they’re missing data in these areas and wanting to acquire data to ensure they’re making the right decisions.
In many parts of the world, such as Malaysia and West Africa, there are prolifically productive deepwater fields where gas clouds make it difficult to illuminate the geology via conventional seismic acquisition techniques, according to Mitchell.
Gas is spongy to P waves, causing attenuation and scattering, according to Calvert, whereas shear waves don’t see this. Multi-component, or 4-C, technology is a must.
While OBC has a good track record of success in seeing around and beneath gas clouds, there’s always room for something new.
“With nodes, we acquire multi-component as a matter of course,” Beaudoin said, “so they could be considered for that application in deep water.
“Deep water is the headline,” he noted. “The fact that it records multi-component data would be the next bullet under that. We record wide azimuth data as a natural by-product of the way we do this.
“These are really the characteristics of node acquisition,” Beaudoin said. “But it starts with the deep water -- that’s where nodes have the advantage.
“Anything you do in deepwater is expensive.”