NRC report concludes hydraulic fracturing poses low risk for causing earthquakes, but risks are higher for wastewater injection wells; CCS impact undetermined

17 June 2012

Schematic diagram of a shale gas well following hydraulic fracture treatment, with the relative depths of local water wells shown for scale. Formation depths and horizontal well length varies; numbers shown are approximate length and depth averages in North America. The upper right inset shows the fractures (yellow) created during hydraulic fracture treatment in stages. Source: NAS, adapted from Southwestern Energy Click to enlarge.

Earthquakes attributable to human activities are called “induced seismic events” or “induced earthquakes.” Hydraulic fracturing has a low risk for inducing earthquakes that can be felt by people, but underground injection of wastewater produced by hydraulic fracturing and other energy technologies has a higher risk of causing such earthquakes, according to a new report from the National Research Council.

In addition, carbon capture and storage may have the potential for inducing seismic events, because significant volumes of fluids are injected underground over long periods of time. However, insufficient information exists to understand the potential of carbon capture and storage to cause earthquakes, because no large-scale projects are as yet in operation. The committee that wrote the report said continued research will be needed to examine the potential for induced seismicity in large-scale carbon capture and storage projects.

In the past several years induced seismic events related to energy development projects have drawn heightened public attention. Although only a very small fraction of injection and extraction activities at hundreds of thousands of energy development sites in the United States have induced seismicity at levels that are noticeable to the public, seismic events caused by or likely related to energy development have been measured and felt in Alabama, Arkansas, California, Colorado, Illinois, Louisiana, Mississippi, Nebraska, Nevada, New Mexico, Ohio, Oklahoma, and Texas.

Anticipating public concern about the potential for energy development projects to induce seismicity, the US Congress directed the US Department of Energy to request that the National Research Council examine the scale, scope, and consequences of seismicity induced during fluid injection and withdrawal activities related to geothermal energy development, oil and gas development including shale gas recovery, and carbon capture and storage (CCS). The study was also to 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 to close those gaps; and assess options for steps toward best practices with regard to energy development and induced seismicity potential.

—“Induced Seismicity Potential in Energy Technologies”

Three major findings emerged from the study, as outlined above:

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 waste water 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.

Hydraulic fracturing (“fracking”) extracts natural gas by injecting a mixture of water, sand, and chemicals in short bursts at high pressure into deep underground wells. The process cracks the shale rock formation and allows natural gas to escape and flow up the well, along with some wastewater. The wastewater can be discarded in several ways, including injection underground at a separate site.

Carbon capture and storage, also known as carbon capture and sequestration, involves collecting carbon dioxide from power plants, liquefying it, and pumping it at high rates into deep underground geologic formations for permanent disposal. Geothermal energy harnesses natural heat from within the Earth by capturing steam or hot water from underground.

Induced seismicity associated with fluid injection or withdrawal is caused in most cases by change in pore fluid pressure and/or change in stress in the subsurface in the presence of faults with specific properties and orientations and a critical state of stress in the rocks, the report notes.

Stress Induced by Fluid Injection or Withdrawal. Injection or extraction of fluid into or from a permeable rock induces a pore pressure change in the reservoir and a perturbation in the stress field in the reservoir and in the surrounding rock. The physical mechanism responsible for this hydraulically induced stress perturbation can be illustrated by considering the injection of a finite volume of fluid inside a porous elastic sphere surrounded by a large impermeable elastic body (above). The magnitude of the induced pore pressure (Δp), once equilibrated, is proportional to the volume of fluid injected. a) Injection of a finite volume of fluid inside the porous elastic sphere embedded in a large impermeable elastic body induces a pore pressure increase Δp inside the sphere as well as a stress perturbation Δσ inside and outside the sphere, caused by the expansion ΔV of the sphere. (b) If the sphere is freed from its elastic surrounding, it will expand by the amount ΔV* due to the pore pressure increase Δp. (c) A confining stress Δσ* needs to be applied on the free sphere to prevent the expansion ΔV* caused by Δp. If the material in the surrounding medium is much softer than the material in the sphere, then ΔV ≃ ΔV* and Δσ ≃ 0; if the medium is much stiffer, then ΔV ≃ 0 and Δσ ≃ Δσ*. Δσ refers only to the radial stress in the exterior region. Assuming that the sphere is removed from the surrounding body, the pore pressure increase (Δp) induces a free expansion of the sphere (ΔV*), similar in principle to the familiar thermal expansion experienced by a solid subject to a temperature increase. To force the expanded sphere back to its earlier size requires the application of an external confining stress (Δσ*), which is then relaxed. The final state corresponds to a constrained expansion of the sphere (ΔV), which is less than the free expansion; this state can be associated to a stress perturbation (Δσ) that is isotropic and uniform inside the sphere, but non-isotropic and non-uniform outside the sphere. The magnitude of the stress perturbation decays away from the sphere, becoming negligible at distance about twice the sphere radius. The stress induced inside the sphere is compressive when the pore pressure increases (fluid injection) but tensile if the pore pressure decreases from its ambient value (fluid withdrawal). This example illustrates the fundamental mechanism by which the stress field in the rock is modified by injection or withdrawal of fluid. The complexities associated with geological settings—in particular, the actual shape of the reservoir, its size, as well as the non-uniformity of the pore pressure field—affect the nature of the stress perturbation. The horizontal and vertical stress variations within most geological reservoirs are rarely identical; inside a tabular reservoir of large lateral extent compared to its thickness, only the horizontal stress is affected by the pore pressure change. Source: “Induced Seismicity Potential in Energy Technologies”

The factor that appears to have the most direct consequence in regard to induced seismicity is the net fluid balance (total balance of fluid introduced into or removed from the subsurface), although additional factors may influence the way fluids affect the subsurface. While the general mechanisms that create induced seismic events are well understood, we are currently unable to accurately predict the magnitude or occurrence of such events due to the lack of comprehensive data on complex natural rock systems and the lack of validated predictive models.

—“Induced Seismicity Potential in Energy Technologies”

Because oil and gas development, carbon capture and storage, 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. Other findings of the report include:

• Energy technology projects that are designed to maintain a balance between the amount of fluid being injected and withdrawn, such as most oil and gas development projects, appear to produce fewer seismic events than projects that do not maintain fluid balance. Hydraulic fracturing in a well for shale gas development, which involves injection of fluids to fracture the shale and release the gas up the well, has been confirmed as the cause for small felt seismic events at one location in the world.

• Waste water disposal from oil and gas production, including shale gas recovery, typically involves injection at relatively low pressures into large porous aquifers that are specifically targeted to accommodate large volumes of fluid. The majority of waste water disposal wells do not pose a hazard for induced seismicity though there have been induced seismic events with a very limited number of wells. The long-term effects of a significant increase in the number of waste water disposal wells for induced seismicity are unknown.

• Projects that inject or extract large net volumes of fluids over long periods of time such as CCS may have potential for larger induced seismic events, though insufficient information exists to understand this potential because no large-scale CCS projects are yet in operation. Continued research is needed on the potential for induced seismicity in large-scale CCS projects.

• Induced seismicity in geothermal projects appears to be related to both net fluid balance considerations and temperature changes produced in the subsurface. Different forms of geothermal resource development appear to have differing potential for producing felt seismic events. High-pressure hydraulic fracturing undertaken in some geothermal projects has caused seismic events that are large enough to be felt. Temperature changes associated with geothermal development of hydrothermal resources has also induced felt seismicity.

A number of 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, the report says, but interagency cooperation is warranted as the number of earthquakes could increase due to expanding energy development.

Although induced seismic events have not resulted in loss of life or major damage in the United States, their effects have been felt locally, and they raise some concern about additional seismic activity and its consequences in areas where energy development is ongoing or planned. Further research is required to better understand and address the potential risks associated with induced seismicity.

—“Induced Seismicity Potential in Energy Technologies”

The study was sponsored by the US Department of Energy. The National Academy of Sciences, National Academy of Engineering, Institute of Medicine, and National Research Council make up the National Academies. They are independent, nonprofit institutions that provide science, technology, and health policy advice under an 1863 congressional charter. Panel members, who serve pro bono as volunteers, are chosen by the Academies for each study based on their expertise and experience and must satisfy the Academies’ conflict-of-interest standards. The resulting consensus reports undergo external peer review before completion.

Resources

• National Research Council. Induced Seismicity Potential in Energy Technologies. Washington, DC: The National Academies Press, 2012.