ANGELA K. BROWN,
Associated Press Writer FORT WORTH, Texas—A wastewater disposal process done after natural gas extraction — and not the drilling itself — is a plausible cause for the surprising series of minor earthquakes in north Texas, according to a study released Wednesday.
The first quakes occurred in October 2008 in Grand Prairie and Irving, and seismic equipment detected 11 more in the area over the next couple of months that were too small to be felt, according to the study by the University of Texas and Southern Methodist University. No major injuries or damage were reported in any of the events that set off car alarms and knocked pictures off walls — the largest being a 3.3-magnitude quake near Euless last spring.
Residents in Cleburne, about 50 miles southwest of Dallas, were shaken up by a series of quakes last summer, but results of that study are not yet available.
"It's pretty scary. ... The whole bed shakes," one woman told an Irving 911 operator in the early morning hours of Oct. 31, 2008, according to an audiotape of a 911 call released by Irving police.
The quakes occurred about a third of a mile from a disposal well on Dallas-Fort Worth International Airport property, and no quakes have been reported in that area since the well stopped operating last fall, according to the study. Disposal wells are much deeper and different from gas wells.
Researchers believe that when wastewater from the gas extraction process was injected into that disposal well, it could have affected a nearby and relatively inactive fault line running along the Dallas and Tarrant county lines, "reactivating it and generating the DFW earthquakes," said Brian Stump, an SMU geophysicist who worked on the study.
"We have a correlation between the time and space of the wastewater disposal and the earthquakes," Stump, who's chairman of SMU's Department of Earth Sciences, said Wednesday. "But actual gas drilling in the area didn't correlate with the earthquakes."
Airport spokesman David Magana said the wastewater well activity stopped immediately when questions about it arose last summer. The airport, which signed a gas drilling lease with Chesapeake Energy in 2006, now has one disposal well and 112 gas wells.
He said the airport has "acted responsibly and in good faith" in allowing a full investigation.
"Chesapeake Energy remains a valued partner for DFW (Airport), and we are confident that Chesapeake's ongoing natural gas production and exploration activity at the airport will be conducted with the utmost regard for safety and with as little impact to the community as possible," Magana said.
Chesapeake Energy agrees with the study's statement that natural gas drilling, hydraulic fracturing and production are not related to the minor quakes near the airport, Chesapeake spokesman Brian Murnahan said Wednesday.
"Chesapeake maintains that a direct, causal relationship between saltwater disposal wells and seismic activity in the DFW area has not been scientifically proven," Murnahan said.
Drilling has been going on for several years in Fort Worth's Tarrant County and others among the two dozen counties that sit atop the gas-rich Barnett Shale rock formation.
Drilling a gas well takes about a month, first vertically and then horizontally. Then comes a week or so of "fracking" — the hydraulic fracturing process that uses pressurized water to break through the dense, black rock and unlock the natural gas within.
Some of that water and other fluids are extracted along with the gas. After they're separated, the wastewater is disposed of in different wells. The disposal well at the southwest corner of the 18,000-acre DFW Airport property is so deep that it is beneath the Barnett Shale, Stump said.
But he said the study raises more questions, since no quakes have been reported around the 200 other disposal wells in seven north Texas counties.
"If the quakes are related to that (DFW Airport) well, we don't understand why it's only that well," Stump said, adding that researchers need monitoring equipment at other sites to get more data.