Background
A study of seismic activity near Dallas/Fort Worth International Airport by
researchers from SMU and UT-Austin reveals that the operation of a saltwater injection disposal
well in the area was a "plausible cause" for the series of small earthquakes that occurred in
the area between October 30, 2008, and May 16, 2009.
Two Injection Methods Used in Gas Production
The incidents under study occurred in an area of North Texas where the vast Barnett Shale
geological formation traps natural gas deposits in subsurface rock. Production in the Barnett
Shale relies on the injection of pressurized water into the ground to crack open the gas-bearing
rock, a process known as "hydraulic fracturing." Some of the injected water is recovered with the
produced gas in the form of waste fluids that require disposal.
The earthquakes do not appear to be directly connected to the drilling, hydraulic fracturing or
gas production in the Barnett Shale, the study concludes. However, re-injection of waste fluids
into a zone below the Barnett Shale at the nearby saltwater disposal well began in September 2008,
seven weeks before the first DFW earthquakes occurred and none were recorded in the area after the
injection well stopped operating in August 2009.
The largest of the DFW-area earthquakes was a 3.3 magnitude event reported by the USGS National
Earthquake Information Center.
Faults Near the Injection Site
A state tectonic map prepared by the Texas Bureau of Economic Geology shows a northeast-trending
fault intersects the Dallas-Tarrant County line approximately at the location where the DFW quakes
occurred. The study concludes, "It is plausible that the fluid injection in the southwest saltwater
disposal well could have affected the in-situ tectonic stress regime on the fault, reactivating it
and generating the DFW earthquakes."
Field Investigations and Non-Felt Earthquakes
An SMU team led by seismologists Brian Stump and Chris Hayward placed portable, broadband seismic
monitoring equipment in the area after the earthquakes began. The seismographs recorded 11
earthquakes between Nov. 9, 2008 and Jan. 2, 2009 that were too small to be felt by area residents.
Cliff Frohlich and Eric Potter of UT-Austin joined the SMU team in studying the DFW-area sequence
of "felt" earthquakes as well as the 11 "non-felt" earthquakes. Their study appears in the March
issue of The Leading Edge, a publication of the Society of Exploration Geophysicists.
The SMU team also installed temporary monitors in and around Cleburne, Texas where another series
of small earthquake began June 2, 2009 – but results from that study are not yet available.
Stump and Hayward caution that the DFW study raises more questions than it answers.
Correlation Between Seismicity and Injection
"What we have is a correlation between seismicity, and the time and location of saltwater injection,"
Stump said. "What we don’t have is complete information about the subsurface structure in the
area – things like the porosity and permeability of the rock, the fluid path and how that might
induce an earthquake."
Injection Disposal is Common Practice
"More than 200 saltwater disposal wells are active in the area of Barnett production," the study
notes. "If the DFW earthquakes were caused by saltwater injection or other activities associated
with producing gas, it is puzzling why there are only one or two areas of felt seismicity."
Further compounding the problem, Hayward said, is that there is not a good system in place to measure
the naturally occurring seismicity in Texas: "We don’t have a baseline for study."
Application to Geothermal and Sequestration
Enhanced geothermal projects also rely on methods of rock fracturing and fluid circulation. Geological
carbon sequestration, an approach being researched to combat climate change, calls for pumping large
volumes of carbon dioxide into subsurface rock formations. "It's important we understand why and under
what circumstances fluid injection sometimes causes small, felt earthquakes so that we can minimize
their effects," Frohlich said.
The study notes that fault ruptures for typical induced earthquakes generally are too small to cause much damage.
Call for Collaboration
"There needs to be collaboration between universities, the state of Texas, local government, the
energy industry and possibly the federal government for study of this complicated question of induced
seismicity," Stump said. "Everyone wants quick answers. What I can tell you is the direction these
questions are leading us."
Find the full report at: http://smu.edu/newsinfo/pdf-files/earthquake-study-10march2010.pdf
View the report as posted by The Leading Edge at: http://dx.doi.org/10.1190/1.3353720
Report Authors:
- Dr. Cliff Frohlich, Associate Director, Senior Research Scientist, Institute for Geophysics, UT-Austin
- Eric Potter, Program Director, Bureau of Economic Geology, UT-Austin
- Dr. Chris Hayward, Director, Geophysics Research Projects, Huffington Department of Earth Sciences, SMU
- Dr. Brian Stump, Claude C. Albritton, Jr. Chair, Huffington Department of Earth Sciences, SMU
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| Professor Brian Stump and colleagues install seismic equipment near DFW Airport.
Image credit Hillsman Jackson, SMU. |
| (l. to r.) Brian Stump, Chris Hayward and student Ashley Howe install seismic equipment
near DFW Airport. Image credit Hillsman Jackson, SMU. |
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