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Impact of Wastewater Injection on Seismic Activity in Oklahoma
The period between 2009 and 2015 saw a significant surge in seismic events in Oklahoma, largely attributed to wastewater injection practices associated with oil and gas production. Recent research published in The Seismic Record indicates that regulatory measures aimed at altering injection practices have been successful in reducing the rate of induced earthquakes in the region.
This study, conducted by Robert Skoumal from the U.S. Geological Survey and his team, reinforces the hypothesis that manipulating the depth of wastewater injection wells can significantly influence seismic activity. This insight has valuable implications for managing induced seismic events in neighboring states such as Texas and New Mexico.
“The characteristics of each basin vary; however, we consistently observe a correlation between the depth of injection and the occurrence of earthquakes nationwide,” Skoumal stated.
In response to rising seismicity, the Oklahoma Corporation Commission (OCC) enacted regulations in 2015 requiring that specific wells injecting water into the Arbuckle Group’s lower strata be backfilled with cement. This initiative was designed to restrict injection to shallower levels, thereby reducing seismic risks.
The findings suggest that if only half of the volumes injected into the lower Arbuckle had undergone this backfilling, the anticipated seismicity rate for Oklahoma in 2024 could have been approximately 2.5 times higher than what is currently observed. In a scenario without any plug-backs, the rate could soar to nearly 4.4 times greater.
“This research marks a pivotal moment, as it allows for a comprehensive analysis of how reducing injection depths impacts induced earthquakes across a broad region,” Skoumal noted.
He also highlighted the significance of the OCC’s efforts, mentioning, “The science indicated that proximity to the basement rock is critical, and the OCC’s measures effectively minimized that proximity. This study validates that the plug-back initiatives have indeed curtailed earthquake occurrences.”
The correlation between hydrocarbon production and increased seismic activity in Oklahoma first became apparent around 2009, reaching its zenith in 2015, when the state recorded 888 earthquakes of magnitude 3 or greater. This included five earthquakes of magnitude 5 or more, with the notable Prague and Pawnee events in 2011 and 2016, respectively, inflicting considerable damage.
Numerous investigations, including those conducted by the study’s authors, have established a direct link between the upsurge in seismicity and the disposal of wastewater into the Arbuckle formation in central Oklahoma. Skoumal elaborated on this, explaining, “The Arbuckle formation is particularly suitable for disposal because of its high permeability, which allows fluids to disperse swiftly, creating capacity for additional influx. It is also underpressured, enabling easy injection without pumping, and is conveniently located throughout the region.” He pointed out that for every gallon of oil extracted, approximately ten gallons of water emerged, necessitating an efficient disposal method, thus making the Arbuckle an appealing choice.
Since the peak of seismic activity in 2015, Oklahoma has observed a decline in earthquake occurrences, partly due to economic shifts that have led to reduced oil production and subsequently, lower volumes of wastewater disposal. To assess the effectiveness of the OCC’s plug-back policies and lesser injection volumes on reduced seismicity, Skoumal and his colleagues developed numerous earthquake models. These models integrated data on plug-backs, total wastewater injection volumes, and observed seismic events.
The researchers concluded that a model focusing exclusively on the wastewater volume injected into the lower Arbuckle, while considering fully effective plug-backs, provided the most accurate explanation for the trends in the recorded seismicity.
While seismic rates have decreased in Oklahoma, there is growing concern as the Permian Basin in western Texas and southeastern New Mexico experiences increasing seismic incidents, including six earthquakes of magnitude 5 or higher since 2020. “The strategies of minimizing injection volumes and implementing plug-back practices were successful in Oklahoma and could potentially be adapted to address similar challenges in other regions,” Skoumal advised.
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