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Induced Seismicity Potential in Energy Technologies Induced Seismicity Potential in Energy Technologies In the past several years, some energy technologies that inject or extract fluid from the Earth, such as oil and gas development and geothermal energy development, have been found or suspected to cause seismic events, drawing heightened public attention. Although only a very small fraction of injection and extraction activities among the hundreds of thousands of energy development sites in the United States have induced seismicity at levels noticeable to the public, understanding the potential for inducing felt seismic events and for limiting their occurrence and impacts is desirable for state and federal agencies, industry, and the public at large. To better understand, limit, and respond to induced seismic events, work is needed to build robust prediction models, to assess potential hazards, and to help relevant agencies coordinate to address them. o meet the challenge of ensuring a reliable energy supply for the United TStates, several energy technologies are being developed and used. Among them are technologies that involve the injection or withdrawal of fluids from Earth’s subsurface. These technologies produce energy in the form of oil, natural gas, and geothermal energy, and some also produce wastes that may be managed through disposal or storage by injection deep into the ground. Waste water from oil and gas production and carbon dioxide from a variety of industrial processes may be managed through underground injection. Since the 1920s, scientists have known Figure 1. Sites in the United States and Canada with documented that pumping fluids in or out of the Earth reports of seismicity caused by or likely related to energy has the potential to cause seismic events. development from various energy technologies. The size of the Seismic events attributable to human circle indicates the range of magnitude of the seismic event. The reporting of small induced seismic events is limited by the activities are called “induced seismic detection and location thresholds of local surface-based seismic events.” For example, earthquakes in monitoring networks. Basel, Switzerland between 2006 and 2008 were related to geothermal energy develop- four energy technologies that involve fluid injec- ment, and a string of small seismic events in tion or withdrawal from Earth’s subsurface: Arkansas, Ohio, and Texas in the past several geothermal energy; conventional oil and gas years has been related to waste water disposal development including enhanced oil recovery; associated with oil and gas production. These shale gas recovery; and carbon capture and storage. events have not resulted in loss of life and generally have not caused significant structural Understanding the Mechanisms of Induced damage, but their effects were felt by local Seismicity residents, and have drawn the issue of induced Research at several energy development sites has seismicity into public view. provided a better understanding of the factors that This report examines the scale, scope, and induce seismicity, which include the presence, consequences of induced seismicity related to orientation, and physical properties of nearby faults, the volumes, rates, pressures, and temperatures potential for induced seismicity. However, there are of fluids being injected or withdrawn, and the rock site-specific characteristics that can make a difference. properties of Earth’s subsurface in that location. For example, the high-pressure hydraulic fracturing In general, existing faults and fractures are stable, undertaken to produce geothermal energy from hot, but a change in subsurface pore pressure—the pres- dry rocks has caused seismic events that are large sure of fluid in the pores and fractures of rock—for enough to be felt. In The Geysers geothermal steam example due to the injection or extraction of fluid field in northern California, a vapor-dominated from Earth’s subsurface, may change the crustal geothermal system, the large temperature difference stresses acting on a nearby fault, creating a seismic between the injected fluid and the geothermal reser- event. Net fluid balance (the total balance of fluid voir results in significant cooling of the hot subsurface introduced into or removed from the subsurface) rocks. This causes the rocks to contract and allows the appears to have the most direct correlation to the release of local stresses that result in significant magnitude of induced seismic events. Thus, energy induced seismicity that is not directly related to technology projects that maintain a balance between change in pore pressure. the amount of fluid injected and the amount with- drawn may induce fewer felt seismic events than Oil and gas development technologies that do not maintain balance. Conventional oil and gas development extracts oil, While the general mechanisms that create induced gas, and water from pore spaces in rocks in subsur- seismic events are well understood, scientists are face reservoirs. Well bores penetrate rock that is under currently unable to accurately predict the magnitude significant natural pressure, allowing oil and water or occurrence of such events due to the lack of within the reservoir to flow to the surface, usually comprehensive data on the complex natural rock aided by pumping. Once the pressure in the oil and systems at particular energy development sites. gas reservoirs declines, additional techniques such as Predictions of induced seismicity at specific energy secondary recovery and tertiary recovery (the latter is development sites will continue to rely on both theo- often called enhanced oil recovery) can be used to retical modeling, and data and observations from extract some of the remaining oil and gas. More than measurements made in the field. 100,000 wells are presently used for secondary recovery; over the past several decades approximately Energy Technologies 18 sites have been linked to incidences where the Of all the energy-related injection and extraction injection was suspected or determined a likely cause activities conducted in the United States, only for induced seismicity. Among the tens of thousands a very small fraction have induced seismicity Drilling Rig & at levels noticeable to the public (that is, above Domestic Treatment Well Microseismic magnitude 2.0). Different energy technologies Well Monitoring Well typically use different injection rates and pressures, fluid volumes, and injection dura- tion—factors that affect the likelihood and magnitude of an induced earthquake. Geothermal energy extraction Geothermal energy is the use of heat from the Earth as an energy source. There are three different types of geothermal energy resources: vapor-dominated, where steam is contained in pores or fractures of hot rock; liquid dominated, where hot water is contained in the rock; and enhanced geothermal systems, where hot, dry rock is fractured and a fluid is injected to circulate Figure 2. Schematic diagram of a shale gas well following hydraulic fracture and heat. All three processes usually attempt treatment, with the relative depths of local water wells shown for scale. to maintain a balance between fluid volumes Formation depths and horizontal well length varies; numbers shown are extracted for energy production and those approximate length and depth averages in North America. The upper right replaced by injection, which helps keep inset shows the fractures (yellow) created during hydraulic fracture treatment in stages. reservoir pressure constant and reduces the SOURCE: Adapted from Southwestern Energy. of wells used for enhanced oil recovery in the United felt seismic events due to increases in pore pressure States, incidences of felt induced seismicity appear to over time; potential effects of large-scale carbon be very rare. capture storage projects require further research. Shale formations may contain oil, gas, and/or liquids. Shales have very low permeability that prevent Hazards and Risk Assessment these fluids from easily flowing into a well bore, and so Understanding what is meant by hazard and risk wells may be drilled horizontally and hydraulically related to induced seismicity is critical to any discus- fractured to allow hydrocarbons to flow up the well sion of the options. The hazard of induced seismicity bore. Hydraulic fracturing to date has been confirmed considers the earthquakes and other physical effects as the cause for small, felt seismic events at one loca- that could be generated by human activities associated tion in the world. with energy production or carbon sequestration. The risk of induced seismicity considers how induced Wastewater Disposal Wells earthquakes might cause damage to structures and Injection wells can be drilled to dispose of the water human injuries or deaths. If seismic events occur in generated by geothermal and oil and gas production areas where there are no structures or humans present, operations, including shale gas production. Tens of there is no risk. thousands of waste water disposal wells are currently Currently, there are no standard methods to imple- active in the United States; water injection for disposal ment risk assessments for induced seismicity. The has been suspected or determined a likely cause for types of information and data required to provide a induced seismicity at approximately eight sites in the robust risk assessment include net pore pressures and past several decades. However, the long-term effects of stresses; information on faults; data on background increasing the number
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