New research by the University of Colorado Boulder's Department of Geological Sciences connects a rash of small earthquakes in the Raton Basin, a resource-rich area in southern Colorado and northern New Mexico, to deep underground wastewater disposal by oil and gas companies.
Wastewater, pumped back into the ground through injection wells, was found to have increased pore pressure in lower rock beds, likely causing more than 1,800 earthquakes up to magnitude-4.3 during a period of time studied from 2008 to 2010.
Six of the earthquakes were reported to ShakeMap, a U.S. Geological Survey earthquake detection app. None were known to cause any damages.
The research confirms and builds upon the findings of two previous studies linking earthquakes to the wastewater injections in Raton Basin.
"What makes our research different is by having established a causative mechanism for the seismic activity — which is the increase in pore pressure from the area's wells," said CU doctoral student Jenny Nakai, the lead author of the study.
Nakai said the research team did not look at the relationship between the Raton Basin earthquakes and hydraulic fracturing, or fracking.
Wastewater disposal wells are used to pump water deep back into the ground after it is extracted during the collection of methane from underground coal deposits.
By looking at well-water injection data provided for the public by the Colorado Oil & Gas Conservation Commission, the CU team of geologists was able to draw together a correlation between injection volumes and seismic activity.
The Raton Basin's earthquakes went mostly undetected by the U.S. Geological Survey, which gathers seismic data around the country for public use.
As seismometers were placed in Colorado and New Mexico from the EarthScope USArray Transportable Array — a program intending to measure earthquakes and map Earth's interior across the country in two-year periods — the seismic activity was picked up and recorded from 2008 to 2010.
Using this data, they were able to map out fault lines and model pore pressure in the "basement rock" of Raton Basin — rock several miles deep in the Earth's crust.
"Basement rock is typically more brittle and fractured than the rock layers above it," said Professor Anne Sheehan, co-author of the study and a fellow at CU's Cooperative Institute for Research in Environmental Sciences. "When pore pressure increases in basement rock, it can cause earthquakes."
According to Nakai, pressure thresholds between .01 and .1 Megapascals — a metric pressure unit equal to 1,000,000 newtons per square meter — is enough to cause an earthquake. Pressure measurements from the wastewater wells surpass these numbers regularly.
There are 28 wastewater disposal wells in the basin, and at least 200 million barrels of wastewater have been injected by the oil and gas industry since 1994.
In a statement responding to the study's findings, Dan Haley, the president and CEO of Colorado Oil and Gas Association, said: "In Colorado, according to the U.S. Geological Survey, 98.8 percent of injection wells operate without seismicity. ... While we are still reviewing the study conducted by CU, Colorado has a one of the best permitting programs for underground injection control in the country, and requires operators to submit detailed documentation on the geology where they plan to drill or inject water prior to receiving a permit."
The study was published in the Journal of Geophysical Research: Solid Earth. The co-authors included CU Professor Shemin Ge, of geological sciences; former CU doctoral student Matthew Weingarten, now a postdoctoral fellow at Stanford University; and Professor Susan Bilek, of the New Mexico Institute of Mining and Technology in Socorro.
