Energy development related seismicity scrutinized

A new study offers some relief and proposes some questions for natural gas extraction, but also has some bad news for fossil fuels in general. The study, released last Friday, prompted a Senate Energy hearing Tuesday.

The study confirmed what industry and others have been saying regarding gas extraction. Pushing water into the ground to fracture shale beneath has a “low risk” of inducing human-detectable earthquakes.

Wastewater injection, a popular disposal method of water returned from hydraulic fracturing, has a higher risk of causing human-detectable earthquakes the report says.

The worse news for not only natural gas, but particularly coal, is that carbon capture and storage, the study says, may have potential for inducing seismic events as well.

Under current Environmental Protection Agency rules, new coal-fired plants could not be built without carbon capture or other carbon dioxide-mitigating technology. Carbon capture storage would be necessary for natural gas plants if emissions were restricted further, but current policy is not as burdensome for gas-fired plants.

The new study comes from the National Academy of Sciences and was sponsored by the U.S. Department of Energy.

“Although induced seismic events associated with these energy technologies have not resulted in loss of life or significant damage in the United States, some effects have been felt by local residents and have raised concern about additional seismic activity and its consequences in areas where energy development is ongoing or planned,” NAS states.

Fracking off the hook?

As pointed out in the Senate Energy hearing regarding the matter on Tuesday, hydraulic fracturing has a relatively small seismic disturbance risk.

The study correlated induced earthquakes most closely with total balance of fluid pumped in and out of the ground. All the technologies the group examined — oil and gas development, carbon capture and storage and geothermal energy production — showed potential to induce earthquakes.

The key, authors of the report write, is to maintain a balance between the fluid injected and withdrawn. Hydraulic fracturing and geothermal development involve relatively smaller amounts of liquid imbalance and thus present a smaller risk of seismic activity.

Murray Hitzman, a professor of in the department of geology and geological engineering at the Colorado School of Mines, said it has been known since the 1920s that pumping fluid underground can cause seismic activity.

“Only a very small fraction of injection and extraction activities at hundreds of thousands of energy development sites in the United States have induced seismicity at levels that are noticeable to the public,” Hitzman said. “Although none of these events resulted in loss of life or significant structural damage, their effects were felt by local residents, some of whom also experienced minor property damage.”

The public, as evidenced by media reports in unconventional shale gas fields, is often disturbed when even small-scale seismic activity crops up in areas where earthquakes are not the norm.

In his testimony before the Senate, Hitzman said that of more than 35,000 hydraulically fractured shale gas wells in the U.S., only one case has been suspected but not confirmed of causing felt seismicity. Globally, one case was confirmed, “likely due to the short duration of injection of fluids and the limited fluid volumes used in a small spatial area” in fracturing operations.

Compared to other fluid injection activities examined by the group, relatively little fluid is injected during hydraulic fracturing activity.

“While the chemicals that comprise the fracturing fluid are relatively benign, the flow-back water can be contaminated with brine, metals and potentially dangerous chemicals picked up from the shale and must be disposed of properly,” said Mark Zoback, professor of geophysics at Stanford University.

Geological challenges of waste disposal

The real risk of earthquakes, the report states, comes from the injection of wastewater in disposal wells.

“The occurrence of induced seismicity associated with wastewater disposal from natural gas production, in particular, has increased significantly since the development of technologies to facilitate production of gas from shale and tight sand formations,” said William Leith, senior science advisor for earthquake and geologic hazards at the U.S. Geological Survey. “While there appears to be little seismic hazard associated with the hydraulic fracturing process that prepares the shale for production, the disposal of waters produced with the gas does appear to be linked to increased seismicity.”

A few cases of induced activity from waste water disposal have been recently documented.

“Although only a few induced seismic events have been linked to these disposal wells, the occurrence of these events has generated considerable public concern,” Hitzman said. “Examination of these cases suggests causal links between the injection zones and previously unrecognized faults in the subsurface.”

A part of the problem, Hitzman said, is that geological structure where wastewater is to be injected is not as closely scrutinized as similar activity such as enhanced oil recovery processes.

“In contrast to wells for EOR which are sited and drilled for precise injection into well-characterized oil and gas reservoirs, injection wells used only for the purpose of waste water disposal normally do not have a detailed geologic review performed prior to injection and the data are often not available to make such a detailed review,” Hitzman said.

Hitzman added presence of a geologic fault does not always mean increased potential for seismic activity. He stressed that while there have been documented cases of earthquakes from waste water injection, the majority of disposal sites do not present such a hazard.

What he worries about, he said, is the unknown potential consequences of increasing these injection activities.

Fossil fuel savior or earthquake generator?

Carbon dioxide emitters are increasingly pressured to reduce or capture emissions. Many view carbon capture and storage as the only hope for coal under increasingly progressive emissions caps.

The problem, the NAS study suggests, is that carbon capture and storage could have potential to induce seismic activity.

“Carbon capture and storage may have the potential for inducing seismic events, because significant volumes of fluids are injected underground over long periods of time,” the NAS stated.

Hitzman said that all of the current CCS projects involve small injection amounts and do not provide a full study of the problem.

“CCS differs from other energy technologies in that it involves continuous injection of carbon dioxide fluid at high rates under pressure for long periods of time,” Hitzman said. “It is purposely intended for permanent storage – meaning that there is no fluid withdrawal.”

Leith said there would be two major problems associated with CCS technology and seismic activity.

“The first type of risk is due to the possibility of a large magnitude earthquake that causes damage to structures in the environs of the project,” Leith said. “More importantly, there is the possibility that an induced earthquake rupture would breach the cap rock allowing the (carbon dioxide) to escape.”

Zoback was particularly concerned about the geological and atmospheric consequences of relying on carbon capture and storage technology to reduce carbon dioxide emissions.

“Not only would large-scale CCS be an extremely costly endeavor, there is a high probability that earthquakes will be triggered by injection of the enormous volumes (carbon dioxide) associated with large-scale CCS,” Zoback said.

Zoback said triggering large earthquakes is not his principal concern regarding carbon capture and storage. He said the large faults necessary to produce large earthquakes would presumably be detected and avoided during site selection.

“Our concern is that even small-to-moderate size earthquakes would threaten the seal integrity of the formations being used to store CO2 for long periods without leakage,” Zoback said.

He said studies suggest a leak rate from underground CO2 storage reservoirs of less than 1 percent per thousand years is required for CCS to achieve the same benefits as switching to renewable energy.

Zoback said due to limitations on suitable sites for CCS, he’s skeptical there is even enough capacity to store the levels of carbon dioxide scientists hope to contain.

“In this context, it must be recognized that large-scale CCS will be an extremely expensive and risky strategy for achieving significant reductions in greenhouse gas emissions,” Zoback said.

Looking forward

One solution for hydraulic fracturing operations is to recycle fluid instead of injecting it into waste water fluid repositories. Already, Zoback said, this is becoming increasingly common.

“This is a very welcome development,” Zoback said. “Re-use of flowback avoids potential problems associated with transport and injection flowback water or the expense and difficulty of extensive water treatment operations.”

Hitzman said it would be imperative for multiple agencies and institutions to work together in examining seismicity risks ahead of development.

“Although induced seismic events have not resulted in loss of life or major damage in the United States, their effects have been felt locally, and they raise some concern about additional seismic activity and its consequences in areas where energy development is ongoing or planned,” Hitzman said. “Further research is required to better understand and address the potential risks associated with induced seismicity.”

Source: Statejournal