Five Questions about SMU’s New Azle Earthquake Study

Tuesday, April 21, 2015

Today, scientists from Southern Methodist University, the University of Texas, and the U.S. Geological Survey released a new research paper in the journal Nature Communications, entitled “Causal factors for seismicity near Azle, Texas.” Through a process of elimination, the researchers concluded that “brine production combined with wastewater disposal represent the most likely cause of recent seismicity near Azle.” However, several issues in the paper raise questions about its conclusion, including potentially major flaws related to subsurface pressure.

Raising the Bar

While the paper’s methods do raise concerns, it is worth emphasizing that SMU, UT, and USGS deserve credit for developing a model that provides greater understanding of the conditions that can ultimately lead to induced seismicity.

More specifically, the SMU team should be commended for recognizing a basic weakness in much of the existing scientific research on induced seismicity. Many previous assessments that have “linked” oil and gas activities to small earthquakes have done so through mere correlation in time and space. In other words, because an injection well was located in proximity to an epicenter, and the seismic events began after the well went into operation, researchers suggest a “plausible” connection between the two. As this latest paper notes, however, many of these studies “do not evaluate other possible anthropogenic causes of seismicity or do not utilize physical models to quantify stress changes.”

As the SMU team acknowledges, in reference to the fact that simple correlation is insufficient for understanding induced seismicity:

“Although there is an increase in injection volumes in the mid-2013 before the recent events, even higher volumes and pressures are reported in prior years at both injectors, when no felt earthquakes occurred.” (p. 5)

By developing an actual subsurface model that assesses what actually happens underground – rather than making a simple correlative assessment – the SMU team should be praised for raising the scientific bar.

This study also gives further credence to the need for ongoing site specific assessments. The potential for seismic activity must be addressed based on downhole pressure, injected volumes, and location, including the orientation of certain faults. Peer-reviewed studies have consistently identified these variables as necessary to understand induced seismicity, and not to convey a blanket, one-size-fits-all approach that suggests geological or pressure conditions in any given area are analogous to operations in other parts of the country.

As the study notes, “tens of thousands of currently active injection wells apparently do not induce earthquakes or at least not earthquakes large enough to be felt or recorded by seismic networks.” This corroborates research from Energy In Depth, released last month, that found 99.9 percent of injection wells in the Barnett Shale region of North Texas have not been associated with felt seismicity.

Notably, the SMU paper is an example of ongoing earthquake research made possible in part by active participation by the oil and gas industry. Through partnerships with research institutions like SMU, as well as Stanford University and the University of Oklahoma (among others), the industry has been actively sharing subsurface data with scientists and state geologists to advance public understanding of induced seismicity – and, more importantly, to help researchers arrive at science-based conclusions that can inform solutions.

But as any scientist will likely admit, a model is only as good as its inputs, and it will never provide a perfect explanation. While the SMU report certainly is a great starting point and an excellent example of the collaborative efforts by industry and researchers to find the causes of recent seismic events, there are aspects of the research that deserve a closer look.

Let’s explore a few questions that the report raises, through its conclusions and its methods.

Question 1: Can Football Air Pressure Trigger Earthquakes?

The researchers note that the Azle events originated from a “critically stressed fault,” which the SMU team suggests was triggered to slip by a pressure of about 5 pounds per square inch (psi). That’s roughly half of what a properly inflated NFL football requires – even if you’re the New England Patriots. The depths at which fluids are injected have natural pressures that can exceed several thousand psi, raising questions about whether such a small pressure difference could actually trigger a series of earthquakes large enough to be felt on the surface.

If so, that means even in the absence of waste water injection activities, the area would not likely have remained earthquake free for very long. The fact that the fault was already “critically stressed” suggests that any number of potential variables could have triggered the event — be they natural causes or man-made.

Read the original article here: