EarthquakesCombination of gas field fluid injection and removal likely cause of 2013-14 Texas quake
Seismologists found that high volumes of wastewater injection combined with saltwater (brine) extraction from natural gas wells is the most likely cause of earthquakes occurring near Azle, Texas, from late 2013 through spring 2014. SMU seismologists have been studying earthquakes in North Texas since 2008, when the first series of felt tremors hit near DFW International Airport between 30 October 2008 and 16 May 2009. Next came a series of quakes in Cleburne between June 2009 and June 2010, and this third series in the Azle-Reno area northwest of Fort Worth occurred between November 2013 and January 2014. The SMU team also is studying an ongoing series of earthquakes in the Irving-Dallas area that began in April 2014.
An SMU-led seismology team found that high volumes of wastewater injection combined with saltwater (brine) extraction from natural gas wells is the most likely cause of earthquakes occurring near Azle, Texas, from late 2013 through spring 2014.
In an area where the seismology team identified two intersecting faults, they developed a sophisticated 3D model to assess the changing fluid pressure within a rock formation in the affected area. They used the model to estimate stress changes induced in the area by two wastewater injection wells and the more than seventy production wells that remove both natural gas and significant volumes of salty water known as brine.
An SMU release reports that conclusions from the modeling study integrate a broad-range of estimates for uncertain subsurface conditions. Ultimately, better information on fluid volumes, flow parameters, and subsurface pressures in the region will provide more accurate estimates of the fluid pressure along this fault.
“The model shows that a pressure differential develops along one of the faults as a combined result of high fluid injection rates to the west and high water removal rates to the east,” said Matthew Hornbach, SMU associate professor of geophysics. “When we ran the model over a 10-year period through a wide range of parameters, it predicted pressure changes significant enough to trigger earthquakes on faults that are already stressed.”
Model-predicted stress changes on the fault were typically tens to thousands of times larger than stress changes associated with water level fluctuations caused by the recent Texas drought.
“What we refer to as induced seismicity — earthquakes caused by something other than strictly natural forces — is often associated with subsurface pressure changes,” said Heather DeShon, SMU associate professor of geophysics. “We can rule out stress changes induced by local water table changes. While some uncertainties remain, it is unlikely that natural increases to tectonic stresses led to these events.”
DeShon explained that some ancient faults in the region are more susceptible to movement — “near critically stressed” — due to their orientation and direction. “In other words, surprisingly small changes in stress can reactivate certain faults in the region and cause earthquakes,” DeShon said.
