Most recent earthquakes in North Texas happened close to injection wells used to dispose of wastewater from oil and gas drilling in the region, according to new research.

A two-year study by the University of Texas at Austin also found that the relatively minor temblors happened more often than indicated in previous investigations.

The UT study, published this week in "Proceedings of the National Academy of Sciences," is the latest to suggest a link between drilling and seismic activity.

"You can't prove that any one earthquake was caused by an injection well," said Cliff Frohlich, the University of Texas researcher who conducted the study. "But it's obvious that wells are enhancing the probability that earthquakes will occur."

A U.S. Geological Survey report released earlier this year found that an increase in small temblors throughout the Midwest coincides with the injection of drilling wastewater in deep disposal wells. An Ohio state agency concluded in March that a wastewater injection well was responsible for a dozen earthquakes in Youngstown last year. And in June, the National Research Council released a report concluding that while there is a low risk of earthquakes directly tied to oil and gas drilling techniques, underground wastewater injections pose a higher risk of triggering seismic activity.

Drilling companies have said they do not believe earthquakes are linked to injection wells.

The wells store wastewater from hydraulic fracturing, a technique that involves pumping mixtures of water, sand and chemicals underground to release natural gas and oil trapped in dense rock formations.

About a third of the water that is pumped underground emerges as "flowback" water before a well starts producing. Later, the well discharges a briny, metal-laden "produced" water that comes from the formation itself.

The wastewater sometimes goes to municipal treatment plants or is saved for reuse at future sites but also commonly is injected underground.

Frohlich's research focused on seismic data from the Barnett Shale region of Texas from November 2009 to September 2011. He concluded that the most reliably pinpointed earthquakes occurring during that time were in eight groups, all within two miles of one or more injection wells.

That's a change from earlier studies that associated two earthquake groups in the area with specific injection wells.

But his data also show disparities. In some cases where drillers rapidly disposed of wastewater underground, there were temblors nearby. But in other cases with similar circumstances, there were no resulting quakes.

Frohlich speculated the difference may be that wastewater injected in some wells reaches existing faults.

Some of the earthquakes were too small to feel on the surface; others, as high as 2.5 in magnitude, were still too light to pose any real danger, the study said.

Although Frohlich's research focused on the Barnett Shale, recent rumblings in the Eagle Ford formation also have drawn attention. A magnitude 4.8 earthquake rattled central and south Texas last October -- followed by a magnitude 3.3 quake in November. Neither was linked with injection wells or drilling in the region, but they stoked fears of fracturing-tied tremors.

Federal regulations governing drilling -- including newly proposed mandates for hydraulically fractured wells on public lands -- have not tackled the issue head-on. But some states already have moved to address it. Ohio officials, for example, responded to the Youngstown quakes by imposing new permitting requirements and geological mandates on drillers.

Kevin Book, an analyst with ClearView Energy Partners, said this year that he anticipates Texas and other states that are accustomed to oil and gas exploration will have measured, "pragmatic" responses -- including moving disposal wells.

jennifer.dlouhy@chron.com ___

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