A number of implications

Yes, Virginia, There Is Induced Seismicity

American Association of Petroleum Geologists (AAPG)

Maybe it will catch on as a new saying:

Induced seismicity happens.

Science is still working to understand how and why earthquakes can be induced by human activity. But scientists studying the phenomenon have no doubt that induced seismicity occurs.

This has a number of implications for the energy industry, including oil and gas exploration.

“Before 1950, induced earthquakes were like extra-sensory perception and little green men. Nobody believed they existed,” said Cliff Frohlich, senior research scientist and associate director of the Institute for Geophysics at the University of Texas at Austin.

“Although that’s not entirely true,” he added, “because when they built the hydroelectric dam by the Grand Canyon, they induced some earthquakes.”

Induced seismicity began drawing more attention in the early 1960s. The U.S. Army drilled a 12,000-foot disposal well at its Rocky Mountain Arsenal near Denver and began injecting fluid in 1962.

In the following years, almost 1,500 earthquakes occurred in the area. Most were small, many not even noticeable by the local population, but the largest was a magnitude 5.0 on the Richter scale. It shook both Denver and Boulder.

“By the late 1960s to early 1970s in my business,” Frohlich recalled, “it was established that this kind of thing can happen.”

Pressure Points

Scientists have recorded numerous surface seismic events related to water injection for geothermal energy projects.

For example, repeated quakes have occurred around The Geysers, the world’s largest complex of geothermal plants in an earthquake-prone area north of San Francisco.

Injections at the Hot-Dry-Rock (HDR) enhanced geothermal project at Basel, Switzerland, caused a series of small quakes in late 2006 and early 2007. After a three-year study, the Basel HDR project was cancelled.

“There’s been renewed interest in induced earthquakes over the past few years, for practical reasons,” Frohlich observed.

He identified three areas of concern:

♦ Geothermal energy. Enhanced geothermal systems incorporate high-pressure water injection.

♦ Unconventional gas. Shale gas development in particular relies on hydraulic fracturing and requires wastewater disposal.

♦ Carbon sequestration. Plans for carbon sequestration include pumping and storing supercritical – liquid – carbon dioxide deep underground.

Most induced seismicity appears to result from activities that alter pore pressure in areas with faults already under stress.

“If you force fluids into it, that ‘unlocks’ the fault. These fluids can be water or wastewater, or other stuff, but the most common is water,” Frohlich said.

“Studies show that most crustal rock is under stress, and if there are little faults there, they can sometimes shift if fluids reach them,” he noted.

Other activities can alter the subsurface and lead to measurable quakes. That includes production activity.

“There have been fields where very shallow earthquakes occurred within or near the fields, where people were pumping out liquids instead of injecting them,” Frohlich said.

Getting the Fracs Right

The Earth Sciences Division of the U.S. Department of Energy’s Lawrence Berkeley National Laboratory has conducted studies into and held workshops about induced seismicity.

Most of the existing research centers on geothermal operations, although Lawrence Berkeley noted “induced seismicity in oil and gas production has been observed since the 1930s.”

To date, hydraulic fracturing by the oil and gas industry hasn’t resulted in reported surface earthquake events.

Induced seismicity includes shear and tensile seismicity. When induced events have been significant, they’ve been related to shear failure.

Hydraulic fracturing is a short-term, high-pressure process designed to create tensile failure, to cause fractures by what is sometimes called a “mini-earthquake.” Frac activities for natural gas production haven’t caused detectable surface quakes.

“To our knowledge hydrofracturing to intentionally create permeability rarely creates unwanted induced seismicity large enough to be detected on the surface with very sensitive sensors, let alone be a hazard or annoyance,” a Lawrence Berkeley report commented.

But enhanced geothermal operations also use hydraulic fracturing techniques, and those sometimes have resulted in induced seismicity. And some studies have found detectable shear motion in a rock tensile failure process.

Frohlich noted that operators wouldn’t want to conduct a frac operation in an area where conditions could likely negate the effect of the hydraulic pressure.

“If the model is there’s a fault that’s likely to slip, you wouldn’t want to lose the liquids,” he said.

Barnett Case Study

Wastewater and brine disposal is more a concern for the industry.

In 2008, residents of the Dallas-Fort Worth area reported a series of small but noticeable ground quakes. Some residents wondered if activity related to Barnett Shale drilling and production caused those events.

Frohlich and AAPG member Eric Potter from the University of Texas and Brian Stump and Chris Hayward from Southern Methodist University in Dallas studied the quakes and wrote a case study paper published earlier this year.

They found that the quakes could have been induced seismicity related to saltwater disposal activities that started just seven weeks before the events began.

“We were able to locate the wells exactly, within a few hundred meters. No matter how you slice it, those events were within a few hundred meters of the wells,” Frohlich said.

More than 200 disposal wells were active in the Barnett Shale area, however, and the authors questioned why there were only one or two areas of felt seismicity.

At this stage of research, scientists are still searching for answers about seismic activity triggers and the relationship between existing geology and induced events.

“In the case of the DFW location, there was a minor fault near the earthquakes. That might have had an effect,” Frohlich commented.

He said the study did serve to eliminate the likelihood of induced seismicity related to other concurrent oil and gas operations in the Barnett Shale.

“Our study has found only earthquakes associated with disposal wells. It isn’t the drilling. It isn’t the hydrofracturing. It isn’t the production,” he observed.

Frohlich doesn’t rule out the possibility of induced events occurring from other activities, including oil and gas activity. Monitoring coverage is limited in Texas, a large state with many lightly populated areas.

“An earthquake of 3.0 or 3.5 magnitude could happen in Texas and we might not know,” he said.

That induced seismicity can and does occur isn’t in question, Frohlich noted.

“Some people say, ‘It’s impossible that we’d be causing an earthquake.’ And those people are obviously unaware of the Denver earthquakes and all the other examples,” he said.

Researchers are hoping for more tools, more monitoring, more data to study, more understanding of how existing faults and their type and orientation fit into the induced seismicity picture.

“It’s in the interest of everyone,” Frohlich said, “whether you are a producer, a citizen or a scientist, to keep doing the research.”

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What’s Shaking? Maybe More Than You’d Guess

Besides the recent attention that has been focused on oil and gas, geothermal and potential CO2 sequestration sites, there are other areas prone to produce induced seismicity.

They include:

♦ Reservoir impoundment (water behind dams).

“Almost all of the significant (recorded activity and in some cases felt activity) is associated with shear failure. These types of earthquakes can be very small or large depending on the geologic environment and available forces to cause an earthquake,” according to the DOE’s Lawrence Berkeley National Laboratory.

♦ Mining (creating cavities in the subsurface) also cause shear failure along planes of weakness – but that is usually due to relieving stress or subsidence, according to the laboratory.

♦ Tall Buildings.

According to a National Taiwan Normal University study published in the Geophysical Research Letters, the stress from the 508-meter Taipei 101 skyscraper may have reopened an ancient earthquake fault and caused tremors in a previously stable area.

On the other hand, it also has been described as a “pinprick” on the Earth and insignificant on its effect.

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