FORT WORTH, Texas — Could a rash of small earthquakes in areas with a lot of oil and gas drilling be a side effect of the North Texas energy exploration boom?
The Fort Worth Star Telegram reported Sunday that quakes in the town of Azle have thrust injection wells into the spotlight.
Crude oil wells typically produce tons of salt water along with oil, and injection wells pump that water back down into the formation to help extract more oil. The state has about 35,000 active injection wells, according to the Texas Railroad Commission.
Experts agree that injecting water into a depleting formation is rarely the cause of a seismic event.
But about 7,000 of the state's injection wells are being used for disposal. The widespread use of hydraulic fracturing to extract natural gas and oil from shale formations has increased the need for disposal wells, which are used to send wastewater deep underground.
And there is some evidence they can cause quakes.
"In a way, Texas has been a vast experiment in injection wells," seismologist Cliff Frohlich, associate director of the Institute for Geophysics at the University of Texas at Austin, told the newspaper.
Frohlich said disposal wells usually don't produce seismic events, but they sometimes can.
There are five active disposal wells in northern Parker County and southern Wise County, where more than 20 quakes shook the Azle area in November and December.
The quakes prompted a heated public meeting in Azle on Jan. 2 with the Texas Railroad Commission, which regulates oil and gas production statewide. After hearing complaints from residents, the commission voted this week to hire its own seismologist, according to the newspaper.
However, scientists are already studying links between drilling and seismic activities and could have answers long before the commission completes its search for a seismologist.
Researchers from Southern Methodist University and the U.S. Geological Survey have installed a network of seismic monitors around Azle and Reno, in northern Parker County. Their goal is to collect better data on earthquakes.
Art McGarr, an earthquake researcher at the Geological Survey who is working on the Azle project, said researchers expect to present their findings in late April.
McGarr's research shows that the total volume of fluid injected in a well can be the biggest factor in triggering quakes, not how fast it is injected.
The five disposal wells around Azle went into operation between 2005 and 2009. Three are permitted to inject up to 25,000 barrels a day. One well is limited to 15,000 barrels and another to 10,000 barrels. But all are injecting much less than their allowed maximums.