Certain oil and gas operations that involve injecting water underground can cause earthquakes, but the risk from hydraulic fracturing is generally low, said a U.S. scientific report Friday.
The analysis by the U.S. National Research Council found that the most significant risk of earthquakes is linked to secondary injection of wastewater below ground to help capture remaining hydrocarbons from a petroleum reservoir.
Also, a technique called carbon capture and storage that aims to reduce carbon dioxide emissions to the atmosphere by capturing, liquefying and injecting them below ground at high volumes, "may have potential for inducing larger seismic events," the report said.
But fracking, a technique that offers the potential to unlock vast quantities of natural gas from shale formations and has come under intense scrutiny from environmentalists, was not a key risk factor for quakes that would be strong enough for people to feel, it said.
"The process of hydraulic fracturing a well as presently implemented for shale gas recovery does not pose a high risk for inducing felt seismic events," the report said.
There are 35,000 wells for shale gas development in the United States today, and only one case has been documented worldwide in which hydraulic fracturing for shale gas was confirmed as the cause of nearby earthquakes.
That case was in the Blackpool area of England in 2011. Fracking was found to have caused a 2.3 magnitude quake in April and a 1.5 quake in May, it said.
A number of possible cases have been documented in the United States and around the world -- at 20 U.S. sites and 18 internationally.
"Human activity, including injection and extraction of fluids from the Earth, can induce seismic events," said the report. "While the vast majority of these events have intensities below that which can be felt by people living directly at the site of fluid injection or extraction, there is potential to produce significant seismic events that can be felt and cause damage and public concern."
A key factor in the potential to cause a quake appears to be related to the total balance of fluid introduced below the surface and removed from it, and technologies that control this balance are best, it said.
While carbon capture and storage involves injecting large volumes of fluid for long storage periods and may cause larger earthquakes, there are no large-scale projects underway so the actual risk is difficult to assess.
"Insufficient information exists to understand this potential because no large-scale CCS projects are yet in operation. Continued research is needed on the potential for induced seismicity in large-scale CCS projects," it said.
Copyright Agence France-Presse, 2012
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