August 13, 2009
SMU's seismology group, in collaboration with scientists at the University of Texas, is preparing two preliminary technical reports that address a series of small earthquakes that began on Oct. 31, 2008, in the area surrounding the DFW Airport.
"While initial studies show a possible correlation between the earthquakes and one salt water disposal (re-injection) well used to get rid of waste fluids, it is premature to state unequivocally that salt water disposal at this well is responsible for the earthquakes," said Brian Stump, SMU seismology professor. "However, based on our current understanding, operations at the re-injection well seem to be a more probable cause of some earthquakes rather than from hydraulic fracturing, a process used in preparing the wells for gas production. But, more study is needed."
"The two technical reports under development do not address the earthquakes that have occurred in the Cleburne area," Stump said.
SMU deployed six portable seismometers in the area surrounding DFW airport after the initial Oct. 31 seismic event and collected data from Nov. 9, 2008 until Jan. 2, 2009. Earthquakes recorded during this time frame are all similar and location estimates of those analyzed place them approximately 0.5 km from a salt water disposal well (SWD) and close to a mapped fault south of DFW airport. The proximity to both the disposal well and a mapped fault suggests these events may be associated with this disposal site.
These earthquakes are located in a region with developing gas production. As part of this gas development boreholes are drilled horizontally in the producing zone, the Barnett Shale. Fluids are injected into these formations to fracture them and enhance the recovery of the gas, a process labeled as hydraulic fracturing or fracing. When gas is recovered, waste fluids are separated from the gas and must be disposed of, usually by re-injecting outside the producing zone, in this case by injecting well below the gas producing zone. The November through January earthquakes in the DFW area that have been studied coincide with one of these disposal wells.
On May 16, 2009, additional small earthquakes occurred south of DFW airport. On June 2, 2009, an apparently unrelated earthquake sequence began near the city of Cleburne, Texas -- 65 km to the southwest of DFW. Ten portable seismic instruments were obtained from the Independent Research Institute in Seismology and are currently deployed with local cooperation south of DFW airport and in the Cleburne area in order to characterize these more recent events. Since these instruments are still in the field and recording data, the details of the recent seismicity at DFW airport and at Cleburne still are under study.