Interactions with Pre-existing Faults

In late 2016, a 5.8-magnitude earthquake shook the town of Pawnee, Oklahoma.¹ It was the largest in the state’s history and geologists in both industry and the government ultimately linked the incident to wastewater injection disposal by oil companies. However, such a strong earthquake resulting from human activity is extremely rare, so it came as a surprise. Typically, induced seismicity does not rise above a magnitude of 3, so induced earthquakes are barely felt by humans and cause no structural damage.

So what happened in Oklahoma?

Mapping the aftershocks indicated that the earthquake occurred along a fault geologists were not previously aware of, and this raised the question: Would the earthquake, in fact, have happened without the wastewater injection?

Oil and gas operations, as well as wastewater disposal operations, are carefully planned to avoid faults. Additionally, these injection wells typically delve far, far beneath the surface to avoid any possibility of contaminating freshwater aquifers. However, deeper disposal carries the tradeoff of disposing water in rock under greater stress and possibly more densely populated with faults.

In Oklahoma, the Arbuckle Formation receives the overwhelming majority of wastewater injection from local oil and gas production. This is the deepest sedimentary layer in the area, about which far less is known than the hydrocarbon-producing layers above it. The formation stands out geologically as a prime destination for disposal fluids, as the formation itself is under-pressured and more readily accepts fluids than other layers with higher natural pressures. However, the Arbuckle is located just above the crystalline basement rock, the layer of the crust in which no sedimentary rock exists. This deep basement rock is under greater vertical stress than more shallow formations and contains many older, unknown faults, some of which may be in contact with the overlying Arbuckle. Fluid injections into the Arbuckle do affect the stress states of the formation itself, and may cause incremental changes to the stress states of other formations, which can trigger activation of previously inactive faults.

Some groups argue that the best way to mitigate the risk of induced seismicity is to stop injecting into the Arbuckle. As part of a traffic light system approach, the Oklahoma Corporation Commission, in a series of directives between 2013 and late 2016, has shut down some wells, has prevented other new-well startups, and has required reduced fluid injection volume in still other wells to mitigate fluid pressure increases in the Arbuckle.²

Some possible solutions for Oklahoma oil and gas operations, should the Arbuckle no longer provide a viable injection zone, are for companies to inject wastewater very far from production sites, to inject into shallower zones or to send the water to a treatment facility. All these alternatives to the Arbuckle have consequences. These restrictions themselves come at a cost, of course, to the energy industry and, some would argue, to Oklahoma as a whole since, according to one estimate, the industry is responsible for one in five jobs statewide.³

The U.S. Geological Survey, in response to increased seismic activity in places like Oklahoma thought to be related to human activity, has changed its approach to publicizing its periodical seismic risk forecast. The document used to be released every few years, but is now slated for annual release and to include risk not just from natural earthquakes but from induced earthquakes.

Citations

1. TW, September 8, 2016, “Oklahoma’s M5.8 Earthquake Activated an Unknown Fault,” The Watchers, https://watchers.news/2016/09/08/oklahoma-s-m5-8-earthquake-activated-an-unknown-fault/ (accessed March 21, 2017).

2. Oklahoma Corporation Commission, 2016, Earthquake Response Summary http://www.occeweb.com/News/2016/11-23-16EARTHQUAKE%20ACTION%20SUMMARY.pdf (accessed March 21, 2017).

3. Richard A. Oppel, Jr. and Michael Wines, April 3, 2015, “As Quakes Rattle Oklahoma, Fingers Point to Oil and Gas Industry,” The New York Times, https://www.nytimes.com/2015/04/04/us/as-quakes-rattle-oklahoma-fingers-point-to-oil-and-gas-industry.html?_r=2 (accessed March 21, 2017).

Images: “Faultfinder_48” by Doc Searls licensed under CC BY SA 2.0