By Matt Smithfirstname.lastname@example.org
— Seismologists from Southern Methodist University and the University of Texas released findings Wednesday on Dallas-Fort Worth earthquakes and their possible association with natural gas exploration in the Barnett Shale.
SMU seismologists, as they did in the DFW area, placed instruments in various parts of Cleburne last year to record seismic activity after several small quakes were recorded in the city. The quakes in both areas resulted in no recorded injuries and little or no recorded damage.
Wednesday’s findings do not address the Cleburne quakes. Cleburne City Manager Chester Nolen said he expects SMU officials to complete a Cleburne study in the next few months.
Several earthquakes were reported in Cleburne last year, the first occurring on June 2. None have been reported for several months.
Nolen said he will wait for the Cleburne study to be released before making any comment, but he did comment on the DFW report.
“From the info, as it relates to the DFW quakes, some of the earthquakes appear to be induced, that is caused by some physical activity by human beings,” Nolen said. “But the report doesn’t specifically identify the quakes in Cleburne, so it remains to be seen whether ours have any relation.”
The DFW study ran from November 2008 to January 2009 and recorded 11 quakes. Before October 2008, no earthquakes occurred near DFW, including earthquakes too small to be located by the U.S. Geological Survey, according to the report.
The 11 events recorded have a preferred focal depth of 4.4 kilometers and lie along a 1.1 kilometer line. The epicenter of the events lies less than half a kilometer from a 4.2 kilometer deep saltwater disposal well, according to the report.
“On the basis of time and spatial correlations, we conclude the DFW sequence may be the result of fluid injection at the saltwater disposal well, but we are puzzled as to why earthquakes occur at this particular location but not near other SWD wells in the region,” the report reads.
Gas exploration in the Barnett Shale involve four activities that could conceivably affect ground stress levels or induce seismic activity, the report concludes. Those activities include drilling wells, hydraulic fracturing, the removal of gas and other fluids during production and saltwater disposal wells.
Few drilled natural gas wells existed in the DFW area before 2001, according to the report.
“This trend changed partly because of favorable gas prices but also because multi staged hydraulic fracture procedures in horizontal wells made urban production feasible,” the report reads. “Between January 2001 and February 2009, 2,200 gas wells were completed in Tarrant and Dallas counties.”
Injection into the SWD well in question began September 2008, seven weeks before the first DFW earthquakes occurred.
“The state tectonic map prepared by the Texas Bureau of Economic Geology shows a northeast-trending normal fault in the subsurface that intersects the Dallas-Tarrant county line approximately at the location of the DFW focus,” the report reads.
The authors of the report said that natural earthquakes do occur in central and northeast Texas, but also note the occurrence of earthquakes “associated with, and possibly related to petroleum production” and other man-made activity in Texas and other areas.
The report concludes by asking whether the recent DFW quakes were natural or triggered by gas exploration activity, most likely the injection of saltwater into disposal wells.
“The spatial and time correlations are consistent with an induced or triggered source,” the report reads. “Prior to Oct. 31. 2008, there had been no local felt earthquakes known in Dallas and Tarrant Counties, which have been settled since about 1850.
“About seven weeks before the DFW quakes began, injection commenced in a SWD well only a few hundred meters from DFW epicenters and with an injection depth of 3.1 to 4.1 kilometers, this approaches the 4.4 to 4.8 kilometer [depth] we determined for the 11 earthquakes recorded by the SMU temporary local network. These earthquakes and the SWD well are within about one kilometer of a mapped subsurface fault, which trends in nearly the same direction as the alignment of epicenters.
“Fluid injections between 200,000 and 300,000 b/month into other faulted areas have triggered small earthquakes and are reported in literature, recently in Paradox Valley, Colo. It is plausible that the fluid injection in the southwest SWD well could have affected the in-situ tectonic stress regime on the fault, reactivating it and generating the DFW earthquakes.”
The authors note that the historic record reports no large, having a magnitude of more than 4.6, induced earthquakes have ever been reported in Texas, and that those quakes that have occurred are small and cause little or no damage. The most extreme example the authors cite is a 1978 quake in Snyder that broke several windows and caused pictures and mirrors to fall off several walls.
“There are thousands of injection wells in Texas, the vast majority of which produce no felt or instrumentally recorded seismicity,” the report reads.
More than 12,000 wells have been completed in the Barnett Shale in the past decade, all of which received hydraulic fracture treatments, the authors note.
“More than 200 SWD wells are active in the area of Barnett production,” the report reads. “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.”
The authors note that fracing and saltwater disposal are critical components of Barnett Shale exploration, which produces a large, relatively clean, source of energy.
To read the SMU report, visit www.smu.edu/newsinfo/pdf-files/earthquake-study-10march2010.pdf