Risks, though, are higher for wastewater injection wells inducing earthquakes that can be felt by people, according to National Research Council report.
To determine the risk for energy-related technologies causing earthquakes, the National Research Council (NRC) studied a variety of potential sources for earthquakes, including geothermal energy production, hydraulic fracturing, wastewater injection and carbon sequestration.
Not surprisingly, the highest number of earthquakes was associated with geothermal energy. The next highest number was from underground injection of wastewater produced by hydraulic fracturing, noted the NRC.
However, according to the report, Induced Seismicity Potential in Energy Technologies, hydraulic fracturing has a low risk for inducing earthquakes that can be felt by people. Out of about 35,000 hydraulically fractured shale gas wells, only one in Oklahoma produced a seismic event that was felt by the public.
Although induced seismic events associated with these energy technologies have not resulted in loss of life or significant damage in the United States, some effects have been felt by local residents and have raised concern about additional seismic activity and its consequences in areas where energy development is ongoing or planned.
While scientists understand the general mechanisms that induce seismic events, they are unable to accurately predict the magnitude or occurrence of these earthquakes due to insufficient information about the natural rock systems and a lack of validated predictive models at specific energy development sites, according to NRC.
The Department of Energy sponsored the study through the NRC “to examine the scale, scope, and consequences of seismicity induced during the injection of fluids related to energy production; identify gaps in knowledge and research needed to advance the understanding of induced seismicity; identify gaps in induced seismic hazard assessment methodologies and the research needed to close those gaps; and assess options for interim steps toward best practices with regard to energy development and induced seismicity potential.”
Three major findings emerged from the study: (1) the process of hydraulic fracturing a well as presently implemented for shale gas recovery does not pose a high risk for inducing felt seismic events; (2) injection for disposal of wastewater derived from energy technologies into the subsurface does pose some risk for induced seismicity, but very few events have been documented over the past several decades relative to the large number of disposal wells in operation; and (3) CCS, due to the large net volumes of injected fluids, may have potential for inducing larger seismic events.
Although CCS may have the potential for inducing seismic events because significant volumes of fluids are injected underground over long periods of time, insufficient information exists to understand the potential to cause earthquakes since no large-scale projects are yet in operation.
The factor most directly correlated with induced earthquakes is the total balance of fluid introduced or removed underground, the report said. Because oil and gas development, CCS, and geothermal energy production each involve net fluid injection or withdrawal, all have at least the potential to induce earthquakes that could be felt by people.
As noted in the executive summary of the report, “Tens of thousands of wastewater disposal wells are currently active in the United States; water injection for disposal has been suspected or determined a likely cause for induced seismicity at approximately eight sites in the past several decades.
“However, the long-term effects of increasing the number of wastewater disposal wells on the potential for induced seismicity are unknown. In addition, wells used only for wastewater disposal usually do not undergo detailed geologic review prior to injection, in contrast to wells for enhanced oil recovery and secondary recovery,” according to the summary.
However, technologies designed to maintain a balance between the amounts of fluid being injected and withdrawn, such as most geothermal and conventional oil and gas development, appear to produce fewer induced seismic events than technologies that do not maintain fluid balance, the researchers noted.
Several federal and state agencies have regulatory oversight related to different aspects of underground injection activities associated with energy technologies. Responses from these agencies to energy development-related seismic events have been successful thus far, the report said, but interagency cooperation is warranted since the number of earthquakes could increase due to expanding energy development.
The report recommended further research in five specific areas to address gaps in the present understanding of induced seismicity. First, collect field and laboratory data on active seismic events possibly caused by energy development and on specific aspects of the rock system at energy development sites (for example, on fault and fracture properties and orientations, injection rates, fluid volumes).
Second, develop instrumentation to measure rock and fluid properties before and during energy development projects. Third, provide hazard and risk assessment for individual energy projects.
Fourth, develop models, including codes that link geomechanical models with models for reservoir fluid flow and earthquake simulation.
Finally, conduct research on CCS, incorporating data from existing sites where carbon dioxide is injected for enhanced oil recovery, and developing models to estimate the potential magnitude of seismic events induced by the large-scale injection of carbon dioxide for storage.
More information is available from the National Academies website.
Contact the author, Scott Weeden, at sweeden@hartenergy.com.
