Proper Context on a Recent Study on Wastewater Disposal and Seismicity

A recent study by researchers from the University of California-Santa Cruz finds that wastewater disposal from oil and natural gas activity can cause earthquakes as far as 10 kilometers away. The researchers also concluded that injecting wastewater into shallower sedimentary rock rather than the basement rock “often generates larger and more distant earthquakes.” From the study,

“Fluid injection above basement may produce distant, larger earthquakes consistent with elastic stress decay, while injection into basement favors closer, smaller events consistent with pressure diffusion.”

Though the report has generated some headlines – and no doubt stoked some fears regarding induced seismicity – it is important to understand that the study really offers nothing new to justify increased public concern. Regulators already had an understanding that wastewater injection can potentially trigger earthquakes from significant distances from injection zones prior to the study. More importantly, mitigation measures have already been implemented in top oil and gas producing regions that not only address the topline findings of this paper, but have also proven effective in further mitigating the already very small risk of induced seismicity from wastewater injection. Furthermore, one of the study’s co-authors admits that the study’s topline conclusion that basement rock injection may be safer than wastewater disposal into sedimentary rock “needs to be taken with a rather large grain of salt” for numerous practical reasons.

U.S. Induced Seismicity Risk Small and Declining

The United States Geological Survey (USGS) has stated that, of the 40,000 disposal wells used for oil and natural gas wastewater, “only a small fraction of these disposal wells have induced earthquakes that are large enough to be of concern to the public.”

For further perspective, a recent peer-reviewed University of Alberta study finds that major oil and gas producing states such as Pennsylvania, North Dakota and West Virginia saw no trend of increased seismicity whatsoever from 2008 to 2014, despite hydrocarbon production increasing dramatically in each area during that time. That study states:

“Contrary to Oklahoma, analysis of oil and gas production versus seismicity rates in six other States in the USA and three provinces in Canada finds no State/Province-wide correlation between increased seismicity and hydrocarbon production…”

North Dakota is a particularly interesting case study, considering the prevalence of wastewater disposal wells in a state that has become the second-largest oil producer in the United States.

Essentially, the study concludes that what has occurred in Oklahoma in recent years is anomalous. And, as a recent USGS report notes, earthquakes linked to wastewater injection have declined “rapidly” in Oklahoma and portions of southern Kansas since 2015,

“Rates of earthquakes across the United States M≥3.0 grew rapidly between 2008 and 2015 but have steadily declined over the past 3 years, especially in areas of Oklahoma and southern Kansas where fluid injection has decreased.”

Oklahoma regulators have acknowledged that, “Seismicity can be induced at distances of 10 miles or more away from the injection point,” and have taken several regulatory measures to reduce wastewater injection in earthquake-prone areas.

Those measures have produced tangible results over the past three years, as USGS data also show Oklahoma’s monthly average of M-2.8 or greater earthquakes at the midway point of 2018 was 76 percent below the monthly average in 2015. These declines have come even as oil production has surged to record levels in the Sooner State.

Regulators in Texas – by far the United States’ top oil and gas producing state – have also already implemented regulations that largely address the 10-kilometer area of concern identified in this study. The Texas Railroad Commission debuted rules in 2014 that require operators to provide data on a roughly nine-kilometer radius from any proposed injection well in an area of historical seismicity.

Collectively, the overall risk of induced seismic activity caused by wastewater injection is very low in the United States, while mitigation efforts in areas where suspected induced seismicity has been most concentrated – Oklahoma –have proven effective.

“Conclusion needs to be taken with a rather large grain of salt”

Study co-author Emily Brodsky writes in an op-ed discussing her study’s findings that, “Sedimentary rock injection is not a safer alternative to basement injection,” but also emphasizes that the “conclusion needs to be taken with a rather large grain of salt.”

Here’s why.

As Oklahoma Geological Survey (OGS) director Jeremy Boak told Scientific American, highly permeable sedimentary rock takes in water “like a vacuum,” while basement rock lacks permeability, which explains why Boak explains “right now, probably no one is injecting into the basement” for both safety and storage capacity reasons.

As Brodsky discusses in her op-ed, basement rocks are only permeable if a fault is present – which obviously doesn’t make for an ideal injection zone.

“… In order to efficiently dispose of wastewater, wells must be in permeable rock where the water can flow away from the well. Basement rocks are generally low permeability and therefore are not very efficient areas in which to dispose of wastewater.”

“One of the few ways that basement rocks can have high permeability is when there are faults that fracture the rock. But, of course, if these high permeability faults are used for injection, the chances of having an earthquake skyrocket…”

Scientific American also takes some hyperbolic liberties with the study’s topline conclusion, reporting that, “When oil and gas companies drill wells on land, the technique they use to avoid unleashing earthquakes can actually create powerful tremors, according to a new study.” But, again, some context and further clarification is in order. First of all, Scientific American is referring to wastewater disposal, not drilling or fracking, which are completely different processes. As the USGS explains in its list of myths and misconceptions regarding induced seismicity notes that, “Fracking is NOT causing most of the induced earthquakes,” further clarifying that: “Wastewater disposal is the primary cause of the recent increase in earthquakes in the central United States.”

And as Boak recently noted, the seismic energy of all of Oklahoma’s earthquakes since 2009 adds up to the equivalent of just one M-6.1 earthquake; California has had two such 6.1-or-greater quakes since 2014 – with no correlation to oil and gas activities.


Though this study is useful in adding to a deeper understanding of induced seismicity, it’s policy implications should be muted by two simple facts: wastewater injection into basement rock is rarely a viable option, and the already small induced seismic risk of wastewater disposal into sedimentary rock is declining due to mitigation measures that address this study’s finding that seismic activity can be triggered at significant distances from injection zones.

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