Stanford scientists use ocean waves to monitor offshore oil and gas fields

A new technique for passively probing the sea floor using weak seismic waves generated by the ocean was tested at the Ekofisk oil field in the North Sea. 

A technology developed by Stanford scientists for passively probing the seafloor using weak seismic waves generated by the ocean could revolutionize offshore oil and natural gas extraction by providing real-time monitoring of the subsurface while lessening the impact on marine life.

"We've shown that we can generate images of the subsurface nearly every day instead of taking snapshots just two or three times a year," said Biondo Biondi, professor of geophysics at Stanford's School of Earth Sciences.

Currently, many energy companies use a technique called time-lapse reflection seismology to monitor offshore oil and gas deposits to optimize production and look for hazards such as hidden gas pockets. Reflection seismology involves ships towing arrays of "air guns" that explode every 10 to 15 seconds to produce loud sound pulses. The pulses bounce off the seafloor and geological formations beneath, then journey back to the surface, where they are recorded by hydrophones. The data are then deciphered to reveal details about subsurface structures.

Each survey can cost tens of millions of dollars, and as a result they are only conducted two to three times a year. Environmental groups and marine biologists have expressed concerns about the use of air guns for contributing to noise pollution in the ocean that can disturb or even injure marine animals, including humpback whales and giant squid.

The new technique developed by Biondi and Sjoerd de Ridder, a student of Biondi's who is now a postdoctoral scientist at the University of Edinburgh, is different. It exploits naturally occurring seismic waves generated by Earth's oceans that are several orders of magnitude weaker than those produced by earthquakes.

Ambient seismicity

As ocean waves collide with one another, they create pressures on the sea floor, where they generate seismic waves that then propagate in every direction. Scientists have known about this "ambient seismic field" for nearly a century, but it was only recently that they understood ways to harness it.

"We knew the ambient seismic energy was there, but we didn't know what we could do with it," De Ridder said. "That understanding has only been developed in recent years. Our technique provides the first large-scale application to harness it for oil and gas production."

The technique that Biondi and De Ridder developed, called ambient seismic field noise-correlation tomography, or ASNT, uses sensors embedded in the seafloor. The sensors, which are typically installed by robotic submersibles, are connected to one another by cables and arranged into parallel rows that can span several kilometers of the seafloor. Another cable connects the sensor array to a platform in order to collect data in real time.

The sensors record ambient seismic waves traveling through Earth's crust. The waves are ubiquitous, continuously generated and traveling in every direction, but using careful signal-processing schemes they developed, Biondi and De Ridder can digitally isolate only those waves that are passing through one sensor and then another one downstream. When this is done repeatedly, and for multiple sensors in the network, what emerges is a "virtual" seismic wave pattern that is remarkably similar to the kind generated by air guns.

Less disruptive

Because the ASNT technique is entirely passive, meaning it does not require a controlled explosion or a loud air gun blast to create a seismic wave signature, it can be performed for a fraction of the cost of an active-reflection-seismology survey and should be far less disruptive to marine life, the scientists say.

Since 2007, Biondi and De Ridder have been testing and refining their technique in a real-world laboratory in Europe. The scientists worked with the energy companies BP and ConocoPhillips to study recordings from existing sensor arrays in the Valhall and Ekofisk oil fields in the North Sea that are capable of recording ambient seismic waves.

The proof-of-concept experiment has been successful, and the scientists have demonstrated that they can image the subsurface at Valhall down to a depth of nearly 1,000 feet. "We've now shown that our technique can very reliably and repeatedly retrieve an image of the near-surface," De Ridder said. "Our hope is that they can also reveal changes in the rocks that could signal an impending problem."

Source: Stanford

The Wastewater disposal may trigger quakes at greater distance than previously thought

Oil and gas development activities, including underground disposal of wastewater and hydraulic fracturing, may induce earthquakes by changing the state of stress on existing faults to the point of failure. Earthquakes from wastewater disposal may be triggered at tens of kilometers from the wellbore, which is a greater range than previously thought, according to research to be presented today at the annual meeting of the Seismological Society of America (SSA). As an indication of the growing significance of man-made earthquakes on seismic hazard, SSA annual meeting will feature a special session to discuss new research findings and approaches to incorporating induced seismicity into seismic hazard assessments and maps.

The number of earthquakes within central and eastern United States has increased dramatically over the past few years, coinciding with increased hydraulic fracturing of horizontally drilled wells, and the injection of wastewater in deep disposal wells in many locations, including Colorado, Oklahoma, Texas, Arkansas and Ohio. According to the U.S. Geological Survey (USGS), an average rate of 100 earthquakes per year above a magnitude 3.0 occurred in the three years from 2010-2012, compared with an average rate of 21 events per year observed from 1967-2000.

"Induced seismicity complicates the seismic hazard equation," said Gail Atkinson, professor of earth sciences at Western University in Ontario Canada, whose research details how a new source of seismicity, such as an injection disposal well, can fundamentally alter the potential seismic hazard in an area.

For critical structures, such as dams, nuclear power plants and other major facilities, Atkinson suggests that the hazard from induced seismicity can overwhelm the hazard from pre-existing natural seismicity, increasing the risk to structures that were originally designed for regions of low to moderate seismic activity.

A new study of the Jones earthquake swarm, occurring near Oklahoma City since 2008, demonstrates that a small cluster of high-volume injection wells triggered earthquakes tens of kilometers away. Both increasing pore pressure and the number of earthquakes were observed migrating away from the injection wells.

"The existing criteria for an induced earthquake do not allow earthquakes associated with the well activity to occur this far away from the wellbore," said Katie Keranen, assistant professor of geophysics at Cornell University, who led the study of the Jones earthquake swarm. "Our results, using seismology and hydrogeology, show a strong link between a small number of wells and earthquakes migrating up to 50 kilometers away" said Keranen. The study's result will be presented by co-author Geoff Abers, senior research scientist at Lamont-Doherty Earth Observatory.
While there are relatively few wells linked to increased seismicity, seismologists seek to anticipate when activity might trigger earthquakes and at what magnitude.

"It is important to avoid inducing earthquakes large enough to be felt, that is, earthquakes with magnitudes of about 2.5, or greater, because these are the ones that are of concern to the public," said Art McGarr, a geophysicist with USGS.

McGarr's research investigates the factors that enhance the likelihood of earthquakes induced by fluid injection that are large enough to be felt, or, on rare occasions, capable of causing damage. The injection activities considered in McGarr's study include underground disposal of wastewater, development of enhanced geothermal systems and hydraulic fracturing. Of the three activities, wastewater disposal predominates both in terms of volumes of injected liquid and earthquake size, with magnitudes for a few of the earthquakes exceeding 5.

"From the results of this study, the total volume of injected fluid seems to be the factor that limits the magnitude, whereas the injection rate controls the frequency of earthquake occurrence," said McGarr.
Despite the increasing seismicity in the central and eastern US, induced earthquakes are presently excluded from USGS estimates of earthquake hazard. Justin Rubinstein, geophysicist with USGS, will present an approach to account for the increased seismicity without first having to determine the source (induced or natural) of the earthquakes.

The USGS is trying to "stay agnostic as to whether the earthquakes are induced or natural," says Rubinstein. "In some sense, from a hazard perspective, it doesn't matter whether the earthquakes are natural or induced. An increase in earthquake rate implies that the probability of a larger earthquake has also risen," said Rubinstein, whose method seeks to balance all of the possible ways the hazard might change given the changing earthquake rate.

But what's the likelihood of induced seismicity from any specific well?
"We can't answer the question at this time," said Atkinson, who said the complex problem of assigning seismic hazard to the effects of induced seismicity is just beginning to be addressed.

"There is a real dearth of regulations," said Atkinson. "We need a clear understanding of the likely induced seismicity in response to new activity. And who is the onus on to identify the likely seismic hazard?"

Source: Seismological Society of America

The Floating nuclear plants could ride out tsunamis: New design for enhanced safety, easier siting and centralized construction

This illustration shows a possible configuration of a floating offshore nuclear plant, based on design work by Jacopo Buongiorno and others at MIT's Department of Nuclear Science and Engineering. Like offshore oil drilling platforms, the structure would include living quarters and a helipad for transportation to the site.
Credit: Illustration courtesy of Jake Jurewicz/MIT-NSE

When an earthquake and tsunami struck the Fukushima Daiichi nuclear plant complex in 2011, neither the quake nor the inundation caused the ensuing contamination. Rather, it was the aftereffects -- specifically, the lack of cooling for the reactor cores, due to a shutdown of all power at the station -- that caused most of the harm.

A new design for nuclear plants built on floating platforms, modeled after those used for offshore oil drilling, could help avoid such consequences in the future. Such floating plants would be designed to be automatically cooled by the surrounding seawater in a worst-case scenario, which would indefinitely prevent any melting of fuel rods, or escape of radioactive material.

The concept is being presented this week at the Small Modular Reactors Symposium, hosted by the American Society of Mechanical Engineers, by MIT professors Jacopo Buongiorno, Michael Golay, and Neil Todreas, along with others from MIT, the University of Wisconsin, and Chicago Bridge and Iron, a major nuclear plant and offshore platform construction company.

Such plants, Buongiorno explains, could be built in a shipyard, then towed to their destinations five to seven miles offshore, where they would be moored to the seafloor and connected to land by an underwater electric transmission line. The concept takes advantage of two mature technologies: light-water nuclear reactors and offshore oil and gas drilling platforms. Using established designs minimizes technological risks, says Buongiorno, an associate professor of nuclear science and engineering (NSE) at MIT.

Although the concept of a floating nuclear plant is not unique -- Russia is in the process of building one now, on a barge moored at the shore -- none have been located far enough offshore to be able to ride out a tsunami, Buongiorno says. For this new design, he says, "the biggest selling point is the enhanced safety."

A floating platform several miles offshore, moored in about 100 meters of water, would be unaffected by the motions of a tsunami; earthquakes would have no direct effect at all. Meanwhile, the biggest issue that faces most nuclear plants under emergency conditions -- overheating and potential meltdown, as happened at Fukushima, Chernobyl, and Three Mile Island -- would be virtually impossible at sea, Buongiorno says: "It's very close to the ocean, which is essentially an infinite heat sink, so it's possible to do cooling passively, with no intervention. The reactor containment itself is essentially underwater."
Buongiorno lists several other advantages. For one thing, it is increasingly difficult and expensive to find suitable sites for new nuclear plants: They usually need to be next to an ocean, lake, or river to provide cooling water, but shorefront properties are highly desirable. By contrast, sites offshore, but out of sight of land, could be located adjacent to the population centers they would serve. "The ocean is inexpensive real estate," Buongiorno says.

In addition, at the end of a plant's lifetime, "decommissioning" could be accomplished by simply towing it away to a central facility, as is done now for the Navy's carrier and submarine reactors. That would rapidly restore the site to pristine conditions.

This design could also help to address practical construction issues that have tended to make new nuclear plants uneconomical: Shipyard construction allows for better standardization, and the all-steel design eliminates the use of concrete, which Buongiorno says is often responsible for construction delays and cost overruns.

There are no particular limits to the size of such plants, he says: They could be anywhere from small, 50-megawatt plants to 1,000-megawatt plants matching today's largest facilities. "It's a flexible concept," Buongiorno says.

Most operations would be similar to those of onshore plants, and the plant would be designed to meet all regulatory security requirements for terrestrial plants. "Project work has confirmed the feasibility of achieving this goal, including satisfaction of the extra concern of protection against underwater attack," says Todreas, the KEPCO Professor of Nuclear Science and Engineering and Mechanical Engineering.
Buongiorno sees a market for such plants in Asia, which has a combination of high tsunami risks and a rapidly growing need for new power sources. "It would make a lot of sense for Japan," he says, as well as places such as Indonesia, Chile, and Africa.

Source: Massachusetts Institute of Technology