Originally Posted: Feb. 13, 2018
High rates of injection and large volumes can perturb critically stressed faults, triggering earthquakes years after wastewater wells are shut in
Efforts to stop human-caused earthquakes by shutting down wastewater injection wells that serve adjacent oil and gas fields may oversimplify the challenge, according to a new study from seismologists at Southern Methodist University, Dallas.
The seismologists analyzed a sequence of earthquakes at Dallas Fort Worth International Airport and found that even though wastewater injection was halted after a year, the earthquakes continued.
The sequence of quakes began in 2008, and wastewater injection was halted in 2009. But earthquakes continued for at least seven more years.
“This tells us that high-volume injection, even if it’s just for a short time, when it’s near a critically stressed fault, can induce long-lasting seismicity,” said SMU seismologist Paul O. Ogwari, who developed a unique method of data analysis that yielded the study results.
The earthquakes may be continuing even now, said Ogwari, whose analysis extended through 2015.
The study’s findings indicate that shutting down injection wells in reaction to earthquakes, as some states such as Oklahoma and Arkansas are doing, may not have the desired effect of immediately stopping further earthquakes, said seismologist Heather DeShon, a co-author on the study and an associate professor in the SMU Earth Sciences Department.
“The DFW earthquake sequence began on Halloween in 2008 — before Oklahoma seismicity rates had notably increased,” said DeShon. “This study revisits what was technically the very first modern induced earthquake sequence in this region and shows that even though the wastewater injector in this case had been shut off very quickly, the injection activity still perturbed the fault, so that generated earthquakes even seven years later.”
That phenomenon is not unheard of. Seismologists saw that type of earthquake response from a rash of human-induced earthquakes in Colorado after wastewater injection during the 1960s at the Rocky Mountain Arsenal near Denver. Similarly in that case, injection was started and stopped, but earthquakes continued.
Such a possibility has not been well understood outside scientific circles, said DeShon. She is a member of the SMU seismology team that has studied and published extensively on their scientific findings related to the unusual spate of human-induced earthquakes in North Texas.
“The perception is that if the oil and gas wastewater injectors are leading to this, then you should just shut the injection wells down,” DeShon said. “But Paul’s study shows that there’s a lot to be learned about the physics of the process, and by monitoring continuously for years.”
Ogwari, DeShon and fellow SMU seismologist Matthew J. Hornbach reported the findings in the peer-reviewed Journal of Geophysical Research in the article “The Dallas-Fort Worth Airport Earthquake Sequence: Seismicity Beyond Injection Period.”
Known DFW Airport quakes number more than 400
The DFW Airport’s unprecedented earthquake clusters were the first ever documented in the history of the North Texas region’s oil-rich geological system known as the Fort Worth Basin. The quakes are also the first of multiple sequences in the basin tied to large-scale subsurface disposal of waste fluids from oil and gas operations.
The DFW Airport earthquakes began in 2008, as did high-volume wastewater injection of brine. Most of the seismic activity occurred in the first two months after injection began, primarily within .62 miles, or 1 kilometer, from the well. Other clusters then migrated further to the northeast of the well over the next seven years. The quakes were triggered on a pre-existing regional fault that trends 3.7 miles, or 6 kilometers, northeast to southwest.
Ogwari, a post-doctoral researcher in the SMU Roy M. Huffington Earth Sciences Department, analyzed years of existing seismic data from the region to take a deeper look at the DFW Airport sequence, which totaled 412 earthquakes through 2015.
Looking at the data for those quakes, Ogwari discovered that they had continued for at least seven years into 2015 along 80% of the fault, even though injection was stopped after only 11 months in August of 2009. READ MORE