Think about the quake consequences of hydraulic fracturing and steam injection (SAGD) and the consequences of the massive volumes of mystery chemicals injected that remain underground and “migrate.”
Oilfield wastewater may trigger earthquakes for ‘decades’ by Patrick Galey, July 16, 209, phys.org
Wastewater from oil and gas production injected deep into wells could cause earthquakes strong enough to be felt on the surface for years to come, according to new research published Tuesday.
The United States is undergoing a boom in oil and gas production as well as fracking, the process of shooting water mixed with sand and chemicals deep into the earth to bring up hydrocarbons trapped inside rock.
Wastewater from fossil fuel production has long been associated with tremors, as producers dispose of it by injecting jets into separate wells dug below ground.
The United States Geological Survey says that wastewater disposal from oil and gas production is the number one cause of human-induced earthquakes in the central and eastern US.
A team of experts from Virginia Polytechnic and State University now believe that the wastewater, due to its higher density, can pose an earthquake risk for years to come, since it displaces existing groundwater stocks that keep the ground stable.
They developed a model based on the wastewater flows in two fracking-heavy states, Kansas and Oklahoma.
The team found that the wastewater altered the subterranean fluid pressure to such an extent that it posed a quake risk for decades.
“That has some very interesting and I think important consequences for how we understand the hazard posed by oilfield wastewater disposal,” said Ryan Pollyea, lead author of the study, published in Nature Communications. [What about the hazards posed by frac’ing?]
Tremors of magnitude 3 or greater used to be relatively rare in the central United States.
But in the wake of vast fossil fuel exploration, their numbers have skyrocketed from around 20 a year in 2008 to more than 400 annually.
One particularly strong quake struck Oklahoma in September 2016, measuring 5.6 magnitude—large enough to be felt in seven states, from Texas to Iowa.
A peer-reviewed study a few months later suggested that four of the most five powerful Los Angeles Basin quakes of the early 20th-century oil boom may have been caused by oil and gas production.
Pollyea and the team found that the earthquakes were also getting stronger: in the two states analysed the number of magnitude 4 quakes increased 150 percent since 2016, while the number of 2.5-magnitude tremors went down by over a third.
They are also getting deeper.
“We have found a new mechanism to explain how fluid pressure causes and increases earthquakes deep under ground,” Pollyea said.
“Our study can be used to improve hazard models for injection-induced earthquakes by accounting for fluid pressure variations that occur after injection operations are reduced or stopped,” he told AFP.
Stronger earthquakes can be induced by wastewater injected deep underground by Science Daily, July 16, 2019
Virginia Tech scientists have found that in regions where oilfield wastewater disposal is widespread — and where injected water has a higher density than deep naturally occurring fluids — earthquakes are getting deeper at the same rate as the wastewater sinks.
Perhaps more critically, the research team of geoscientists found that the percentage of high-magnitude earthquakes increases with depth, and may create — although fewer in number — greater magnitude earthquakes years after injection rates decline or stop altogether.
The study, led by Ryan M. Pollyea in the Virginia Tech College of Science’s Department of Geosciences, was published July 16 in Nature Communications. It shows that in areas such as Oklahoma and southern Kansas there is evidence that oilfield wastewater injected underground into the Arbuckle formation has a much higher density than natural fluids occurring within the deeper seismogenic zone faults.
The problem: The wastewater sinks and increases fluid pressure deep underground when it has a higher density than fluids already there naturally. Pressure changes so deep — at depths up to 5 miles or greater — can cause more high-magnitude earthquakes even though the overall number of earthquakes is decreasing.
“Earthquakes are now common in the central United States where the number of magnitude-3 or greater earthquakes increased from about 19 per year before 2008 to more than 400 per year since,” said Pollyea, an assistant professor of geosciences and director of the Computational Geofluids Laboratory at Virginia Tech. (Pollyea adds that the overall earthquake rate per year has been declining since 2016.)
“In many cases, these earthquakes occur when oilfield wastewater is disposed of by pumping it into deep geologic formations,” Pollyea added. “As wastewater is injected deep underground, fluid pressure builds up and migrates away from injection wells. This destabilizes faults and causes ‘injection-induced’ earthquakes, such as the damaging 5.8-magnitude earthquake that struck Pawnee, Oklahoma, in 2016.”
“This was a surprising result,” Chapman said. “It suggests that sinking wastewater increases fluid pressure at greater depths and may cause larger earthquakes.”
By analyzing earthquake data, the researchers found that the number of earthquakes greater than a magnitude 4 increased more than 150 percent from 2017 to 2018 while the number of earthquakes with magnitude 2.5 or greater decreased 35 percent during the same period.
More bluntly, the overall number of earthquakes is starting to decrease, but the percentage of higher-magnitude earthquakes is increasing.
“Our models show that high-density wastewater may continue sinking and increasing fluid pressure at depths of 5 or more miles for 10 or more years after injections stop,” Pollyea said. “There is a larger proportion of high-magnitude earthquakes at depths greater than 5 miles in north-central Oklahoma and southern Kansas, but there are fewer total earthquakes at these depths. This implies that the rate of high-magnitude earthquakes is decreasing more slowly than the overall earthquake rate.”
The study also found that fluid pressure caused by sinking wastewater remains in the environment much longer than previously considered.
“Our models show that high-density wastewater continues sinking and increasing fluid pressure for 10 to 15 years after injections stop, and this may prolong the earthquake hazard in regions like Oklahoma and Kansas,” Pollyea said.
It’s important to note that Pollyea and his colleagues are not saying that all oilfield wastewater disposal operations cause earthquakes, nor are they predicting a large and potentially damaging earthquake in the Midwest region. Nor does the study indicate that density-driven pressure build-up occurs everywhere that oilfield wastewater operations occur.
Researchers have known since the 1960s that pumping fluids deep underground can trigger earthquakes, Pollyea said, but this study is the first to show that the density of the wastewater itself plays a role in earthquake occurrence. The heavier the fluid, the greater the effect of displacement of natural fluids and the greater the fluid pressure change. To wit: Take a cup of salty ocean water heavy with dissolved particulates and dump it into a glass of regular tap water. Before the two eventually mix, the heavier ocean water will sink to the bottom, displacing the “lighter” tap water upward.
“Past pore-pressure models have assumed the density of injected fluids are the same as that in host rocks, but the real world begs to differ, and injected fluids are often heavier,” said Shemin Ge, a professor and chair of the Department of Geological Sciences at the University of Colorado, who was not involved with the study. “This study looks into the effect of heavier fluids on pore pressure and consequently on inducing earthquakes. Heavier injected fluids have the tendency to migrate downward, which, interestingly, tracks the occurrence of earthquakes.”
Funding for this study came from the United States Geological Survey, Earthquake Hazards Program.
Materials provided by Virginia Tech. Note: Content may be edited for style and length.
- Ryan M. Pollyea, Martin C. Chapman, Richard S. Jayne & Hao Wu. High density oilfield wastewater disposal causes deeper, stronger, and more persistent earthquakes. Nature Communications, 2019 DOI: 10.1038/s41467-019-11029-8
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