June 18, 2015 by Canadian Underwriter
High-rate injection wells – those pumping out more than 300,000 barrels per month – are much more likely to be associated with earthquakes than low-rate wells, says a new study published this week in the journal Science.
The study on induced seismicity, High-rate injection is associated with the increase in U.S. mid-continent seismicity, examined the relationship between wastewater injection and U.S. mid-continent seismicity using a newly assembled injection well database for the central and eastern United States (CEUS).
The database contained 187,570 wells as of December 2014, with 56% actively injecting fluid and the remaining 44% being inactive or abandoned. The researchers then obtained earthquake location and magnitude data from the Advanced National Seismic System’s earthquake catalog. After filtering, about 10% of all wells – or 18,757 injection wells – were associated with earthquakes, mostly in the states of Oklahoma and Texas, the study said.
“We find that the entire increase in earthquake rate is associated with fluid injection wells,” wrote the authors, from the University of Colorado’s Department of Geological Sciences and the U.S. Geological Survey, noting that many of these earthquakes have been documented as induced by wastewater injection. “Managing injection rates may be a useful tool to minimize the likelihood of induced earthquakes,” the authors advised.
“We identify far more injection wells that are potentially related to earthquakes than those indicated by published cases,” the researchers added, noting that over the past four decades, more than 60% of all CEUS seismicity (earthquakes greater than a magnitude of 3) was associated with injection wells using the filtering criteria. Furthermore, varying the spatial distance of association by several kilometres only changes the percentage by 5%.
“Before the year 2000, an average of ~20% of all CEUS seismicity was associated with injection wells. The yearly percentage of associated earthquakes has risen sharply to ~87% from 2011 to 2014,” the study said. “The percentage increase of associated earthquakes, combined with the rising CEUS earthquake rate, implies that recent seismicity in the CEUS is preferentially occurring near injection wells.”
The researchers concluded by saying that “our analysis shows that injection rate is the most important well operational parameter affecting the likelihood of an induced seismic event in regions and basins potentially prone to induced seismicity. High-rate [salt water disposal] wells are nearly twice as likely as low-rate wells to be near an earthquake. These high-rate wells perturb the ambient reservoir pressure by a larger magnitude and over a larger area than low-rate wells, thus increasing the likelihood that pressure changes will reach an optimally oriented, critically stressed fault.”