THE RELATIONSHIP OF WASTEWATER INJECTION TO THE INCREASE IN CENTRAL AND EASTERN U. S. SEISMICITY

An unprecedented increase in central US seismicity began in 2009 with the number of M3.0+ earthquakes increasing from an average of 24 per year in 1973-2009 to 193 per year in 2009-2013 to 688 in 2014. Almost the entire seismicity increase occurred in states with significant oil and gas production: Oklahoma, Texas, Arkansas, and Colorado. To address this new potential hazard, a multidisciplinary scientific team came together at the USGS's John Wesley Powell Center in 2013 with expertise in seismology, hydrogeology, geomechanics and public policy. The team published several papers linking the seismicity increase to the disposal of co-produced brines in wastewater injection wells. Here, we discuss the results of two of those studies, at both a broad- and case-study scale, which highlight the injection well operations and geologic conditions that govern the likelihood of induced seismicity.

On a broad-scale, we compiled a database of 188,570 oil and gas injection wells in the central and eastern U.S. to analyze whether injection rate, pressure, depth, proximity to crystalline basement and cumulative volume were associated with nearby earthquakes. High-rate injection (>300,000 barrels per month) is the only operational parameter which showed a statistically significant correlation to the occurrence of earthquakes within 15 km of a well. The importance of injection rate is also supported by our work at the case-study scale. In central Oklahoma, we linked a swarm of earthquakes to wastewater injection wells using a model of reservoir pore pressure changes from injection. Results show that 4 high-rate injection wells in central Oklahoma, disposing of ~4 million barrels per month cumulatively (~200 kg/s), have a greater effect on reservoir pore pressure than 69 lower-rate wells, despite the lower-rate wells operating for more than a decade prior to the onset of high-rate injection. The simulated pressure perturbation from these 4 wells in the high-permeability injection reservoir tracks hypocenter migration of the central Oklahoma earthquake swarm from December 2009 to December 2012. While these results suggest the importance of injection rate, other factors such as reservoir properties and faults also need to be considered to further understand induced seismicity.