Alex Mills
By Alex Mills
A study of seismic activity in North Texas was released on Tuesday, April 21, that concluded that water extracted during oil and gas operations and re-injected “is the most likely cause of earthquakes” in the area during a three-month period from November 2013 to January 2014.
Water is a key component during the drilling and completion process. Water also is recovered as crude oil and natural gas is produced and it must be disposed according to state regulatory standards. The recovered water generally has a high salt content, and it cannot be reused. Disposal well operators must seek a permit from the Texas Railroad Commission that states the zone where the water will be re-injected, the amount of water injected, and pressure limits.
There are some 13,000 permitted commercial injection wells throughout Texas.
The study was released by SMU and it stated that scientists from SMU, the University of Texas at Austin, and the U.S. Geological Survey (USGS) participated in the study.
The study was the first release of detailed information about the seismic events, and it did not recommend any solutions.
The study raised more questions about the events.
For example, it states that the team identified two intersecting faults and assessed the changing fluid pressure through a 3D model. The model estimates stress changes induced by the injection wells, and the conclusions from the modeling study integrate a broad-range of estimates for uncertain subsurface conditions. The estimates are just that: estimates. The SMU report even acknowledges that: “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 first series of felt tremors hit near DFW International Airport between Oct. 30, 2008, and May 16, 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.
In both the DFW sequence and the Cleburne sequence, the operation of injection wells used in the disposal of natural gas production fluids was listed as a “possible” cause of the seismicity. The introduction of fluid pressure modeling of both industry activity and water table fluctuations in the Azle study represents the first of its kind, and has allowed the SMU team to move beyond assessment of “possible” causes to “the most likely” cause identified in this report, according to a news release from SMU.
Prior to the DFW Airport earthquakes in 2008, an earthquake large enough to be felt had not been reported in the Fort Worth Basin since 1950. The North Texas earthquakes of the last seven years have all occurred in areas developed for natural gas extraction from a geologic formation known as the Barnett Shale. The Texas Railroad Commission reports that production in the Barnett Shale grew exponentially from 216 million cubic feet a day in 2000, to 4.4 billion cubic feet a day in 2008, to a peak of 5.74 billion cubic feet of gas a day in 2012.
The study notes that there are “many thousands of injection and/or production wells that are not associated with earthquakes.”
It is important to note that the Railroad Commission adopted changes to its disposal well rules in 2014, and no earthquakes have been reported in the Azle area since then. RRC regulations require a seismic risk assessment of the injection well location as well as ensuring that the operator will provide any additional information the RRC deems necessary as part of the permitting process.
This is an issue based on site-specific geologic characteristics, and it should be regulated on the state level.
“This report points to the need for even more study in connection with earthquakes in North Texas,” said Brian Stump, SMU’s Albritton Chair in Earth Sciences. “Industry is an important source for key data, and the scope of the research needed to understand these earthquakes requires government support at multiple levels.”
Alex Mills is President of the Texas Alliance of Energy Producers. The opinions expressed are solely of the author.
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