Shaky Ground: How the Disposal of Wastewater from Oil Production Increases the Risk of Earthquakes in Oklahoma
Since the early 2000s, there has been an increase in the number of earthquakes observed near oil and gas exploration and production sites in the US, particularly in states such as Oklahoma, Arkansas, and Texas. Until recently, details of the relationship between exploration techniques and the observed increase in seismic activities have been unclear.
A study by researchers at Stanford University sheds light on the issue: by taking a closer look at seismic events in Oklahoma, they found that hydraulic fracturing (fracking) and enhanced oil recovery techniques (EOR), which entail injecting water into underground oil and gas reservoirs, do not trigger earthquakes. Instead, their study indicates that the resulting large amounts of wastewater, injected deep underground in dedicated disposal areas, increase pressure on existing faults lines, thus intensifying the risk of earthquakes.
Clearly, there is cause for concern: From 1974 (the onset of modern seismic recording) until 2008, Oklahoma experienced about one magnitude 3 earthquake per year. In 2013 and 2014, the state registered 100 events of this magnitude or higher.
The researchers analyzed three areas of Oklahoma with the highest seismic activity rates over the past few years, which are located around the towns of Cherokee, Perry, and Jones. For comparison, they observed three adjacent areas that experienced no increase in earthquakes during the same time period. Detailed data was collected for all areas, including the location and magnitude of seismic activities, the type of oil production activities in the area, as well as the location and amount of wastewater injection.
The study revealed that in the areas around Cherokee, Perry, and Jones, there was a five to ten-fold increase of the amount of disposed wastewater over the past five years, along with a significant increase in seismic activity. In contrast, in the adjacent areas, which experienced no change in earthquake frequency, the wastewater disposal rates remained comparatively small, as in previous years.
First, the results clearly suggest that the disposal of large amounts of salty wastewater from drilling activities triggers seismic activity. The majority of this wastewater (95 percent) results from drilling into an oil reservoir during the production process, where the highly saline water gets separated from the oil and becomes what is known as “produced water.” The remaining five percent of wastewater is flow-back water from fracking, a technique that is viewed with skepticism, where a mixture of water, sand, and chemicals gets injected into a horizontal well to free oil (or gas) captured in shale. Because such a small amount of wastewater is a result of fracking, the evidence suggests that fracking itself does not trigger earthquakes.
Furthermore, earthquakes mainly increase in areas where produced water is injected into dedicated saltwater disposals (SWD), whereas when water is injected back into an existing extraction area, no significant increase in earthquake events is found. Following the increase in drilling activities in Oklahoma over the past several decades, more and more dedicated disposal areas were needed.
Determining whether wastewater disposal ultimately leads to an earthquake depends on the geology surrounding the disposal site as well as the amount of wastewater injected. The more wastewater injected, the more the pore pressure increases in the disposal zone, which can lead to an earthquake. Oklahoma’s disposal area, the Arbuckle formation, is located directly along fault lines that experience stress caused by wastewater disposal over a period of time. This stress eventually leads to a slip that releases the energy that causes an earthquake. Small pressure perturbation caused by wastewater injection can spread within the disposal zone for 10 kilometers (6.2 miles) or more, reaching critically stressed faults far away from the injection area. This explains both the observed delay in time between injection and earthquake events, as well as the distance between an injection well and the seismic epicenter.
The fact that the increase in earthquakes in Oklahoma does not result from fracking, but rather from the injection of saline wastewater in underground disposal areas is an important discovery. However, given the amount of wastewater already injected in recent years, seismic activity in Oklahoma is only expected to decrease slowly, even if future quantities are reduced. In addition, further research is still needed to fully understand the environmental consequences of fracking and enhanced oil recovery activities.
Article source: Walsh, F. Rall, and Mark Zoback. “Oklahoma’s Recent Earthquakes and Saltwater Disposal.” Science Advances, 2015.
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