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Initial Study of Barnett Tremors Points to Reinjection Well

Initial findings indicate a "possible correlation" exists between a series of recent small earthquakes in the Barnett Shale and one of Chesapeake Energy Corp.'s reinjection wells used to dispose of saltwater, officials said last week. As a precaution, Chesapeake said it has shut two disposal wells.

The seismology group at Southern Methodist University (SMU), in collaboration with scientists at the University of Texas (UT), is preparing two preliminary technical reports analyzing a series of small earthquakes that began on Oct. 31 in the area surrounding the Dallas-Fort Worth (DFW) International Airport (see NGI, June 15). Last month a 2.0 magnitude tremor shook the ground about four miles east of Cleburne, in Johnson County, according to the U.S. Geological Survey.

It's too early to pin the tremors on reinjection wells. However, Cliff Frohlich, associate director of the UT Institute for Geophysics, told NGI that it appears "there is a relationship...We have not proven it with scientific certainty, but we're looking at these as induced earthquakes." It is "extremely remote" that either gas drilling or fracturing caused the tremors, he added.

As a "precautionary measure" Chesapeake shut a saltwater disposal well at the south end of the DFW airport and another reinjection well in Cleburne, it informed the Railroad Commission of Texas (RRC). No gas production was impacted by shutting either of the two reinjection wells, the producer said. No other Barnett operators have reported shutting any reinjection wells or gas wells because of the preliminary findings.

Reinjection wells typically are at least 10,000 feet deep, while gas wells in the Barnett are typically 6,000-7,000 feet deep, according to RRC spokeswoman Ramona Nye. The reinjection well drilled by Chesapeake at DFW airport is about 13,780 feet deep. Chesapeake pumped 2.8 million bbl of seawater into the DFW well from Sept. 12 until it was shut on June 29.

According to the preliminary data on minor tremors that have occurred near DFW airport since last November, the epicenter is about 14,764 feet deep and within 3,300 feet of the DFW reinjection well, Frohlich said. That depth would exclude gas drilling or fracturing.

SMU in June deployed 10 portable seismometers in Tarrant and Johnson counties; up to four were placed in Cleburne after city officials requested the tremor study. SMU last year deployed six portable seismometers in the area surrounding DFW airport after the initial Oct. 31 seismic event and collected data from Nov. 9 until Jan. 2.

"Earthquakes recorded during this time frame are all similar and location estimates of those analyzed place them approximately 0.5 kilometers from a saltwater disposal well 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," the preliminary report said.

"While initial studies show a possible correlation between the earthquakes and one reinjection well used to dispose of waste fluids, it would be premature to state unequivocally that salt water disposal at this well is responsible for the earthquakes," said SMU's Brian Stump, a seismology professor. "Based on our current understanding, operations at the reinjection 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 added.

Cleburne Mayor Ted Reynolds said he's still awaiting results of investigations into what has caused tremors in his town. "We haven't received any data from SMU, and I think they [the scientists] are going strictly on info from the well out at DFW," he told NGI.

The preliminary report by SMU and UT explained that as gas is developed in the Barnett, boreholes are drilled horizontally in the producing zone. Fluids are injected into these formations to fracture them and enhance the recovery of the gas. Hydraulic fracturing is used in nine out of 10 horizontal wells.

"When gas is recovered, waste fluids are separated from the gas and must be disposed of, usually by reinjecting outside the producing zone, in this case by injecting well below the gas-producing zone," the report said. "The November through January earthquakes in the DFW area that have been studied coincide with one of these disposal wells."

On May 16, "additional small earthquakes occurred south of DFW airport," the report said. And then on June 2 "an apparently unrelated earthquake sequence began near the city of Cleburne, TX -- 65 kilometers 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."

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