A study of seismic activity near Dallas/Fort Worth International Airport by researchers from Southern Methodist University and the University of Texas at 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 Oct. 30, 2008, and May 16, 2009.
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 hydraulic fracturing,
the injection of pressurized water into the ground to crack open the
gas-bearing rock. 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, 7
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 States Geological Survey (USGS) National Earthquake
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
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.
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.
"What we have is a correlation between seismicity, and
the time and location of saltwater injection," Stump says. "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."
"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
Further compounding the problem, Hayward says, 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."
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
study notes that fault ruptures for typical induced earthquakes generally are
too small to cause much damage.
Natural Gas Production Plausible Cause of Texas Earthquake, New Study Says
March 17, 2010