Stimulating natural gas and oil production by hydraulically fracturing horizontal wells doesn't pose a big risk for triggering earthquakes that are strong enough for a human to feel, but other types of energy-related activities could make the ground noticeably shake, according to the conclusions of major government science report released on Friday.
However, even large types of man-made tremors created by drilling are extremely rare, said experts at the National Research Council (NRC), part of the private nonprofit National Academy of Sciences, which advises the federal government. The report was requested by Sen. Jeff Bingaman (D-NM), who chairs the Senate energy committee, which has scheduled a hearing for Tuesday to examine the link between earthquakes and energy technology.
The researchers examined not only drilling risks but the risk of tremors associated with tapping geothermal energy, as well as carbon capture and storage, and found that the total balance of fluid injected or removed underground was the main factor in causing earthquakes related to energy development.
"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," the researchers said.
After monitoring earthquakes that have occurred since the 1920s, human activity was shown to have been the trigger in only 154 of the earthquakes, with the majority being "moderate" or "small;" only 60 were in the United States, said the report, "Induced Seismicity Potential in Energy Technologies." The global annual average is close to 14,450 earthquakes with a magnitude of 4.0 or more, said the experts.
Most of the man-made earthquakes caused by drilling aren't caused by unconventional drilling in any case but rather by conventional, vertical drilling, as well as by damming rivers, deep injections of wastewater, or by purposeful flooding, according to the NSC.
Only two instances of earthquakes -- a 2.8 magnitude tremor in Oklahoma and a 2.3 magnitude tremor in England that both occurred last year-- were attributed to fracking.
"There's a whole bunch of wells that have been drilled...say for wastewater, and the number of events has been pretty small," said Colorado School of Mines geology economics professor Murray Hitzman, who chaired the NSC study. "Is it a huge problem? The report says basically no. Is it something we should look at and think about? Yes."
Keeping an eye on injection wells and considering potential repercussions from drilling in certain areas have to be considered before operators begin a natural gas or oil development, the report said. "There is potential to produce significant seismic events that can be felt and cause damage and public concern."
Most of the U.S. tremors blamed on man-made events were found to have occurred in order in California, Texas, Colorado, Oklahoma and Ohio, which coincidentally also are energy-producing states.
California and Oklahoma had the biggest man-made tremors that were byproducts of conventional drilling, according to the report. Colorado had the best documented case with three 5.0-5.5 man-made earthquakes attributed to an injection well. Northern California also has had 300-400 small quakes every year since 2005 that were attributed to geothermal energy extraction.
Injecting fluids deep into the ground at high pressure may trigger tremors because it changes the balance of fluids into and out of the subsurface, the NSC said. The imbalance then may impact the pore pressure of the soil, which helps to keep the earth's underground faults from shifting, Hitzman said.
Researchers with the U.S. Geological Survey (USGS) earlier this year said there had been a "remarkable" increase in the number of earthquakes in the Midcontinent, which likely resulted from drilling wastewater injection wells (see NGI, April 2). The USGS reported a six-fold increase in the number of earthquakes of 3.0 magnitude or more since 2001 from 20th century levels.
However, it's too soon to conclude that the recent rise in earthquakes would have changed the NSC conclusions, said Hitzman. If the United States begins to capture carbon dioxide from coal power plants and injects it underground, there is a potential for larger earthquakes given the amount of heat-trapping gas that would have to be sequestered, said the report. That issue needs more study.
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