Induced seismicity in Oklahoma

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Induced seismicity in Oklahoma is a recent political, economic, and scientific issue that has come to light in the past decade. Earthquakes are not typical of Oklahoma but in the past 10 years, Oklahoma has surpassed even California in earthquake frequency. Increase in seismicity rates has been hypothesized to be triggered by both hydraulic fracturing and wastewater disposal. Research is still ongoing and no definite conclusion has been reached as of now. Earthquakes typically naturally occur in a specific sequence of foreshock, mainshock, and aftershock. The earthquakes in Oklahoma have been occurring in swarms specifically in regions with increases in hydraulic fracturing and wastewater injection.

Induced Seismicity

Induced seismicity is typically defined as smaller magnitude earthquakes (3.0 or below) that are caused by changes of the stress and strain in the earth due to human activity. Although most induced earthquakes are smaller magnitude, there are several induced earthquakes that are a magnitude 4.0 or greater. Most induced seismicity can be contributed to processes such as hydraulic fracturing or wastewater disposal. Induced seismicity can also be caused by hydrocarbon storage,[1] nuclear explosions,[2] geothermal energy production,[3] mining,[4] and groundwater extraction.[5] Induced seismicity is specifically causing the natural stresses of the earth to change. When the stresses of the earth's crust are increased or decreased, or the pore fluid pressure increases, faults are more likely to slip. When a fault slips, that creates an earthquake. Manmade processes that inject fluids deep into the crust such as wastewater injection, hydraulic fracturing, and hydrocarbon storage increase the pore fluid pressure. This can lead to fault slip and lubrication of the fault causing a decrease in fault friction. The reason faults aren't always constantly slipping is due to the force of friction, so when humans are injecting fluids down into a fault it can help overcome that friction force. [6]

Seismicity in Oklahoma

Oklahoma has become the center of seismic activity in the continental US in the past decade. This is unusual because Oklahoma does not have a large history of earthquakes before the 21st century, and Oklahoma is not in the tectonically active region of the United States, unlike the West Coast. Many possible reasons have been brought to the table for why Oklahoma has started having so much seismic activity, but no single hypothesis has been universally accepted yet. There have been theories of hydraulic fracturing and wastewater disposal being a contributing factor to increases in seismicity, but due to pressure from high profile oil companies, it was not acknowledged as a potential cause until 2015 by the Oklahoma Geological Survey (OGS). The OGS reportedly knew about the potential correlation between human activities and seismicity since 2010 but were pressured into keeping silent.[7] A joint statement between the United States Geological Survey (USGS) and OGS was made in 2013 stating that activities such as wastewater injection could be contributing to the increase in seismicity,[8] and it was later confirmed in April 2015 by OGS that the level of seismicity is unlikely due to natural causes.[9] Previously, some proposed theories on why seismicity rates were increasing were ideas such as water levels in lakes rising[10] or simply due to completely natural causes and had nothing to do with human activity.

Figure 1: A map of all earthquakes, hydraulic fracturing wells, wastewater disposal wells, and earthquakes associated with hydraulic fracturing wells in Oklahoma from 2009-2017.

Prior to 2008

Prior to 2008, the largest magnitude earthquake recorded in Oklahoma was a 5.5 in El Reno in 1952.[11] Between 1978 and 2008 the average earthquake with a magnitude of 3.0 or higher (displayed in figure 2 below) was 1.6.[12] This low number of earthquakes is due to the fact that Oklahoma is not a tectonically active area and it is not along a plate boundary. Most seismicity in the United States is expected to be along the West Coast, where the San Andreas Fault is. There are only 852 recorded M3.0 or higher earthquakes from 1973 to 2009, compared to almost 3,500 from 2009 to 2018.[13]

Figure 2: Earthquakes in Oklahoma Per Year

2008 to Present

The largest recorded Oklahoma earthquake was the M5.8 in Pawnee on September 3rd, 2016.[14] The increase of seismicity from 2008 to 2018 is dramatic. Oklahoma went from an average of 1.6 M3.0 earthquakes a year to over 3000 M3.0 earthquakes recorded between 2009 and August of 2018 (clearly depicted in Figure 1 above).[15] Nationwide, Oklahoma has contributed 45% of all magnitude 3.0 or higher earthquakes between 2008 and 2013 alone.[16] Figure 3 displayed below shows that by 2014, Oklahoma was the most seismically active state in the contiguous United States, even beating out California.[17] In 2014, Oklahoma had 585 M3.0 or greater earthquakes, while California had less than 200.[18] The seismicity rates were 70 times greater than background seismicity rates in 2013, and in 2015 were over 600 times greater than background rates.[19]

Figure 3: Map of earthquakes in the central US

Influence of Oilfield Activities

Induced seismicity caused by oil field injection such as wastewater disposal or hydraulic fracturing is the current most popular proposal for why these earthquakes are happening. There are clear positive trends between increases in wastewater injection volumes and injection durations and increases in seismicity.[20] Injection volume and depth are also proposed to be factors of induced seismicity as well. There has been a fair amount of debate about whether or not hydraulic fracturing can cause induced seismicity, and it is still currently being researched by OGS and students and faculty at the University of Oklahoma.

Hydraulic Fracturing

Hydraulic Fracturing is the process of drilling a well and then injecting fluids (usually a combination of water, chemical proppants, and sand) into the source rock to artificially fracture it in order to allow oil, water, and hydrocarbons to escape. Injecting fluid down into rock formations increases the pore pressure, which reduces the normal load on faults which then allows the fault to slip and earthquakes to occur.[21] The process also increases the permeability of the rock and allows companies to extract oil from unconventional reservoirs. The use of this technique can reactivate well-oriented pre-existing faults which in turn can cause earthquakes. Hydraulic fracturing is extremely common in the SCOOP/STACK play in Oklahoma, where there have also been clear increases in seismicity.[22] This region is particularly notable because there is very little wastewater disposal occurring, so all seismicity increases are more than likely contributed to hydraulic fracturing, although research on the subject is still ongoing.

Wastewater Disposal

Wastewater disposal is the process of disposing of wastewater by injecting it deep into the crust. Wastewater can be frac fluid but is often simply salt water that comes up during the drilling process. Wastewater disposal is accredited as the primary cause of induced seismicity in the United States according to the USGS.[23] Not all disposal wells cause seismicity, but seismicity can be induced by wells who do not have added pressure at the wellhead and wells can induce seismicity up to 10 miles away and at extremely great depths.[24] Wastewater disposal rates nearly doubled in Oklahoma[25] and have increased even more between 2008 and 2018. Seismicity induced by wastewater disposal has been directly related to how deep the injection was,[26] as well as injection rates per month. In Oklahoma, most wastewater disposal injection sites inject into the Arbuckle Formation.[27] Figure 4 below depicts the monthly injection rates into the Arbuckle formation and the monthly seismicity rates. It is obvious that as injection rates increase, seismicity rates increase as well, although there is a lag time involved between the peak injection volumes and peak seismicity.

Figure 4: Injection Volume Compared with Earthquakes per Month

Mitigation of Hydraulic Fracturing and Wastewater Disposal Impact in Oklahoma

The current seismicity mitigation being employed in Oklahoma is referred to as the traffic light system.[28] Certain injection wells throughout the state are being issued "yellow light" permits, which require them to shut down all activities at the well site if seismicity increases. The well is moved up to "red light" and permanently shut down if the seismicity rates surrounding the site increase. Yellow light permits have been issued to wells within 6 miles of an M4.0 earthquake or higher, or within 2 miles of a fault the OGS has deemed capable of slipping.[29] So far this method has worked well, with over 350 wells[30] meeting yellow light criteria. It is important that induced seismicity is managed appropriately because a large magnitude (M6.0) or above could cause catastrophic damage to the homes, businesses, and industries of Oklahoma alike. Unlike California, Oklahoma is not prepared or equipped for large magnitude earthquakes, and the damages could become more and more severe over time if induced seismicity is not controlled. Figure 5 shown below depicts a hazard map for the state of Oklahoma for 2017, detailing what parts of Oklahoma are at most risk of damage from shaking. Damages have already been reported after several M5.0[31] and higher earthquakes in Oklahoma, and it is critical that the Oklahoma Corporation Commission and the OGS work to mitigate the effects of induced seismicity.

FIgure 5: Earthquakes from 1973 to 2017 and hazard map.[32]

Ongoing Research

The Oklahoma Geological Survey, as well as students and faculty at the University of Oklahoma, are currently conducting research on the spatiotemporal correlation of hydraulic fracturing and induced seismicity in Oklahoma. According to preliminary findings, hydraulic fracturing can be accounted as responsible for around 15% of earthquakes in Oklahoma between 2008 and 2018.[33] Researchers are attempting to locate specific variables from hydraulic fracturing that can be direct causes of seismicity. There are also lower levels of wastewater disposal in the SCOOP/STACK regions of Oklahoma, but high levels of both hydraulic fracturing and seismicity. This area is currently being studied in greater detail. A paper on the subject is expected to be published within the next year, and current findings will be presented at the Fall 2018 AGU Meeting. Many other researchers are also studying the subject, with their contributions listed below under "Important Papers".

See also

Important Papers

References

The references will be updated and properly cited in APA format by December 1st, 2018. This section is currently under revision and the author is aware the references are currently not properly cited. [34]
[35]
[36]

Additional help

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