The U.S. Environmental Protection Agency (EPA) has determined that although conclusive proof linking oil and natural gas disposal wells and earthquakes at injection sites across the country is difficult to demonstrate, a relationship is undeniable.

In its final report on the topic, the agency settled on a list of the common characteristics of induced seismicity, culled from case studies in four states. It advised state regulators to manage Class II underground injection wells in a proactive and cautious manner.

“Conclusive proof of induced seismicity is difficult to demonstrate but is not a prerequisite for taking early prudent action to address the possibility of induced seismicity,” the EPA said in its 415-page report.

The agency first studied induced seismicity 25 years ago. It said in its most recent report that Class II injection wells — which dispose of brines and other fluids associated with the production of oil and gas — are not the only causes of human-induced earthquakes. But as oil and gas production has increased markedly with the nation’s shale boom in recent years, the EPA sought to address concerns that earthquakes triggered by the waste wells could possibly contaminate drinking water sources. There are currently 30,000 Class II injection wells across the country, and the agency said in its report that it was unaware of any such contamination.

The EPA has handed regulatory primacy to 33 states over the years that are now responsible for managing their own disposal wells. The agency first completed its draft report in 2012 and released it in mid-2013 before its final report was issued after peer-review and public comment (see Shale Daily, July 29, 2013).

The report makes no regulatory recommendations and instead suggests how best to manage induced seismicity when it occurs. State representatives from Ohio, Colorado, Texas, West Virginia and Arkansas, which have all either reported or concluded in recent years that Class II wells triggered minor earthquakes, participated in the study.

EPA said a sufficient pressure buildup from disposal activities, unknown faults or faults of concern, and pathways allowing increased pressure to communicate with faults are the likely components of injection well-induced seismicity. The agency’s working group “noted that no single recommendation addresses all of the complexities related to injection-induced seismicity, which is dependent on a combination of site geology, geophysical and reservoir characteristics.” The agency also said the absence of historical seismic events in the vicinity of disposal wells “does not provide complete assurance that induced seismicity will not occur.”

Through its case studies, EPA also found a series of common characteristics existed among injection wells linked to earthquakes. These included petroleum engineering data that indicated “correspondence” between disposal well behavior and seismicity; earthquakes that increased in magnitude over time and injection into fractured disposal zones near deep basement rock.

In dealing with such activity, the agency advised state and federal regulators to conduct thorough site assessments to better understand an area’s geologic characteristics. It also said a proactive approach is necessary when seismicity is recorded near disposal wells that could include more frequent monitoring, modification of individual well permits or even shuttering a well if all recommended approaches fail.

The final report focuses solely on Class II disposal operations and not enhanced recovery wells or high-volume horizontal fracturing (fracking). A separate draft report the agency has been at work on since 2010 about fracking’s impact on drinking water resources is expected to be released in March (see Shale Daily, Oct. 21, 2014).

EPA did say in its latest report, however, that fracking “has a low likelihood of inducing significant seismicity,” noting that stimulation occurs over shorter periods of time and has a lower pressure footprint than injection wells where rates are sustained over longer periods of time.