Scientists have linked earthquakes in Oklahoma with wastewater injection wells associated with the oil and gas industry, in a new paper published in the journal Science.
Four injection wells with a "high rate" of injection, meaning they accepted a large quantity of barrels of wastewater per month, likely caused 20 percent of earthquakes in that area from 2008-2013, the researchers said.
In Weld County, research is ongoing on whether recent small earthquakes are linked to an injection well near the Greeley/Weld County airport.
On June 25, the Colorado Oil and Gas Conservation Commission ordered a 20-day halt to wastewater injection at the well in question. This was after scientists measured a 2.6 magnitude event. That event followed a 3.4 magnitude earthquake that happened May 31. Following the initial quake, researchers from the University of Colorado had installed a series of seismic stations to monitor for more earthquakes.
One main difference the Oklahoma situation and the one in Northern Colorado involves the amount of water injected. In Oklahoma, the quantity was significantly larger, said Matthew Weingarten, a University of Colorado Boulder doctoral candidate who co-authored the Science paper.
"Most of these wells were injecting at or above a million barrels a month. And the Greeley well, by comparison, is only one well and operating at 350,000 barrels a month," Weingarten said.
The four high-rate injection wells in the Oklahoma study linked to a flurry of earthquakes called the Jones Swarm were very near each other, essentially adding up to 4 million barrels of wastewater being injected in the same location per month. A barrel contains 42 gallons.
The Greeley well is still considered a high-volume well, said Shemin Ge, a University of Colorado hydrogeologist and another co-author on the paper. (The monthly injection rate for February, the last time online data was available, is 306,433 barrels. It has been as high as 363,888 barrels, in October 2013).
Another difference, said Ge, is that the wells in Oklahoma were linked with earthquakes as far as 20 miles away. Typically, wells are associated with earthquakes nearby, perhaps a few miles away. But the model the researchers created to study how fluids from the Oklahoma wells were affecting the faults connected earthquakes occurring in more distant locations.
Ge said this is probably because the wells have been injecting for a long time, so a lot of fluid was able to fill into the rock below, creating pressure and allowing faults to slip.
"What we found out in Oklahoma is the earthquakes migrate away from injection well location. So at early times they are closer to the injection well, and then later on, the earthquakes migrate away from the injection," she said.
While scientists have not come to a consensus on why some injection wells cause earthquakes and others do not, Ge said this paper points to the rate of injection, how many barrels per month, as an important clue.
"The major conclusion or finding from this study is that the rate of injection is a pretty key, important parameter. And so again, high rate is something we should keep a very close eye on."
Eventually, said Ge, she hopes this research will inform policymakers and injection well drillers so they can avoid causing earthquakes.
In Greeley, the new seismic network put out by researchers is detecting small earthquakes, which provides one set of data for research on the links between an injection well and earthquakes. Ge said she is also collecting data from the Colorado Oil and Gas Commission about injection rates and pressure, and hopes to get more.
"So with these data we would like to put together a similar modeling study like the one we did for Oklahoma," she said.
CU doctoral student Weingarten cautioned that while there are tens of thousands of injection wells in the United States, only a few have been actually linked with earthquakes. In the study in Oklahoma, the researchers looked at 89 wells, but focused in on the high-volume ones as causes of the earthquakes.
"What we're trying to figure out now in the scientific community is whether or not there are any specific ways that injection wells are operating that make the hazard of induced seismicity go up," Weingarten said.
