A magnitude 3.4 earthquake occurred near the Dallas Fort Worth airport in 2008.¹ More followed in the next weeks. Oil and gas companies had recently hydraulically fractured wells on the airport property. Coincidence? News reports made the connection, and headlines stated that hydraulic fracturing had caused the earthquakes.
But hydraulic fracturing operations typically last only a few hours or days and don’t inject enough liquid into the surrounding rock to cause earthquakes that anyone might feel. Instead, wastewater disposal injections, which last longer and pump more water into the ground, are a more likely cause of the seismic activity.
The wastewater generated by the process of hydraulic fracturing, often called flowback, and the produced water from ongoing oil and gas production are both injected into a disposal well. These waste fluids are injected at a depth to ensure that the very high-salinity fluid is stored far beneath any freshwater aquifers so as not to contaminate them. The deeper the injection, the further from any underground sources of drinking water. However, the downside to this approach is that at deeper depths, the overburden is greater and more faults typically exist. Injection of waste fluids at high rates and volumes near faults has the ability to create movement along the fault, and thus increase the likelihood of induced seismicity.
In the DFW case, the earthquake occurred six weeks after wastewater injection began in a suspect well. The earthquake’s hypocenter was about one kilometer from the injection well and at about the same depth as the injection. This gives us some hearty correlation information, but does not definitively prove causation. In order to get closer to certainty that any given well causes any given earthquake, we also need to know the history of earthquakes in the area, the volume of fluids injected, and the location of all existing faults. Below, one of our experts explores all the characteristics of the DFW incident and renders his conclusion on the probable cause of the earthquake.
Case Study: Earthquakes in the Dallas-Forth Worth Area – Cliff Frohlich – The University of Texas at Austin
On October 31, 2008, the area surrounding the Dallas/Ft. Worth Airport experienced a magnitude 3.4 earthquake. More earthquakes followed. There were wells on the airport property that had been hydraulically fractured and it was widely reported that, “The earthquakes were caused by fracking.” So, was there a link between hydraulic fracturing and the earthquakes? To evaluate what happened, we need to understand the difference between the process of hydraulic fracturing and wastewater injection, but first, some historical background.
Earthquake scientists realized that human activity could influence seismic activity in the 1930s after the construction of Hoover Dam. Shortly after Lake Mead, behind the dam, began filling, the area was hit with a sequence of earthquakes. Monitoring with seismographs showed that the seismicity rate of the surrounding area corresponded to the water level in the lake. One plausible explanation is that the sheer weight of the water in Lake Mead changed the stress in the rock formations below and this induced seismic activity.
The pumping of liquids into porous underground rock formations can also induce seismic events. We learned this in the 1960s when the US Army maintained a toxic waste disposal well at the Rocky Mountain Arsenal near Denver. A month after injection began, residents felt the first earthquakes. Monitoring over the next several years showed a clear correlation between injection rate and seismic activity. Normally, even when a fault is stressed and primed to move, the weight of the overlying rock presses the two sides of the fault together and friction keeps the fault from slipping. In the Denver case, injected wastewater pushed the two sides of the fault apart. This decreased the frictional resistance allowing the fault to slip and release the stress that might or might not have caused an earthquake at some later time.
Wastewater affects friction on a fault in much the same way as compressed air effects friction on an air hockey table. When the air is off, friction between the table and the puck is too high for the puck to slide easily. When the air is on, the puck slips, just like a fault during the earthquake. So did hydraulic fracturing cause the earthquakes at the Dallas/Ft. Worth Airport? In fact, hydraulic fracturing almost never causes earthquakes large enough for people at the surface to feel. Hydraulic fracturing operations last for a few hours or at most a couple of days. In this amount of time, the volume of water injected isn’t very large, the pressure does not spread out very far from the well.
One thing hydraulic fracturing does is generate wastewater. These waters are brines, very salty water, so you can’t just dump it somewhere. Usually wastewater is collected from numerous wells and injected into other wells for disposal. It’s the process of disposing of the wastewater that most often induces seismic events. So wastewater disposal is most likely the cause of the earthquakes at the Dallas/Ft. Worth Airport. In the Dallas/Ft. Worth area, companies inject wastewater into the Ellenberger Formation because the rock is highly permeable and located deep below the surface. The Ellenberger is capped by impermeable rock so wastewater pumped into it becomes trapped and won’t flow back up to the surface. This keeps the waste contained, but it also increases pressure which can effect subsurface faults.
So why do we think the 2008 Dallas/Ft. Worth earthquakes were caused by wastewater injection? First of all, in the 150 years before 2008, there had been no earthquakes reported in the Dallas/Ft. Worth area. Then, earthquakes began six weeks after wastewater injection began. A subsequent scientific investigation showed the earthquakes were less than a kilometer from the injection well and at about the same depth as the injection. Finally, the earthquakes occurred along a straight line and this line coincided with a known mapped fault. Thus, it is plausible that wastewater reached the fault and decreased the friction, inducing the earthquakes.
It’s important to note that the earthquakes were not near any of the hydraulic fracturing sites on the airport property, so it was incorrect to say that they were caused by fracking. When we consider wastewater injection wells as a group, the vast majority have no earthquakes nearby. That is, most injection wells do not trigger earthquakes, but when earthquakes do occur near injection wells, we need to evaluate whether causation is plausible. To do this, we consider historical seismicity, timing of injection, the earthquake’s location and depth with respect to the location and depth of the injection well, the volume of injected wastewater and the presence of known faults nearby. In many cases, information about all these variables isn’t available, but when we have all or most of it as we did for the Dallas/Ft. Worth earthquakes, we can reach a reasonable conclusion about whether human actions induced the seismic activity.
1. Cliff Frohlich et al., 2011, “The Dallas–Fort Worth Earthquake Sequence: October 2008 through May 2009,” Bulletin of the Seismological Society of America, https://scits.stanford.edu/sites/default/files/327.full__1.pdf (accessed March 9, 2017).
Images: “Dallas Airport” by Dzung Tran licensed under CC BY ND 2.0