Source: Dan Hawkes
Earth scientists throughout the world have for decades been looking at ways of accessing difficult-to-get-at sources of energy, as well as developing alternative energy sources such as geothermal. In addition they have also been developing methods to mitigate adverse effects of conventional energy production, i.e., by testing underground sites for sequestering large amounts of carbon dioxide to retard carbon emissions to the atmosphere and thereby combat global warming. Both energy recovery and geologic carbon sequestration efforts, however, run the risk of creating induced seismicity (small earthquakes created by such activities as high-rate, high-volume fluid injection or fluid withdrawal that are sometimes large enough to be felt by nearby populations). Berkeley Lab Earth Sciences Division’s personnel are seeking means to not only mitigate induced seismicity to ensure the safety and acceptance of the neighboring public, but to understand and use the induced seismicity as a useful tool to optimize the productivity and effectiveness of such sites.
The techniques and procedures involved in energy recovery and carbon sequestration are similar. In the case of enhanced energy recovery systems—enhanced oil and gas recovery (EOR) and enhanced geothermal systems (EGS)—site permeability is increased by inducing slippage along preexisting fractures within reservoirs (often called hydroshearing) and/or by hydraulic fracturing, using high-rate water injections and/or chemical stimulation to create new fractures that will increase permeability by connecting existing fractures. In the geothermal case, production can be sustained by injecting water into injection wells and circulating that water through the newly created permeability, where it is heated as it travels to the production wells. As the circulating water cools, the newly created fractures and chemical dissolution of minerals may also create new permeability, continually expanding the reservoir and exposing more heat to be mined. Similarly, underground (geologic) carbon sequestration (GCS) involves underground injection, but of CO2 (in liquid form) rather than water—and massive amounts of it. To make a significant impact on CO2 accumulation, billions of tons of CO2 per year need to be injected and sequestered in the subsurface. The pressures in CO2 sequestration will not be high enough to cause hydrofracturing, but small scale seismicity may be a public perception risk and/or a risk to the integrity of the CO2 “reservoir”.
With respect to oil and gas recovery, induced seismicity has been observed for many years, ever since large-scale extraction of fluids began. The most famous early instance was in Wilmington, California, in the mid-20th century, where oil production triggered a series of earthquakes. In this instance, the cause of the seismicity was traced to subsidence due to rapid extraction of oil without replacement of fluids. Once this was realized, the oil extraction was balanced with water injection to mitigate the seismicity. Ever since then, the oil and gas industry has adopted these practices not only to mitigate seismicity, but also to mitigate damage to the oil wells in the producing field. (Wells would be sheared off in the subsurface as subsidence occurred.)
Induced seismicity in geothermal systems has been observed for over thirty years in a variety of sites in the U.S. and internationally. In California, The Geysers and Coso geothermal sites have a long history of geothermal production; thousands of earthquakes are induced annually. These are predominantly microearthquakes (MEQs) not felt by people, but there have also been earthquakes up to magnitude 4.5. In the many operating hydrothermal fields around the world, there is no evidence whatsoever of any induced seismicity causing significant damage to the surrounding community. Recent sites in the news lately, such as Basel, Switzerland, Landau, Germany and Soultz, France, experienced moderate seismicity due to enhanced geothermal system (EGS) activities. Although this seismicity was short lived, it attracted the attention of the public due to it close proximity to nearby communities.
Induced seismicity has been anticipated to be an issue with carbon sequestration. To date, however, there have been few large-scale injections of CO2 for sequestration purposes. Note that CO2 has been injected safely underground for years in many oil and gas reservoirs to extract more oil and or gas, with little seismic impact. But these formations are relatively shallow, had low regional seismicity to begin with, and pressures were balanced to extract fluids as wells inject CO2. Some initial pilot projects meant to sequester CO2 in oil and gas environments are, however, currently under way—for example the Weyburn project in Canada, The Sleipner project in the North Sea, and the In Salah Project in Algeria. To date, all have had low seismicity and are not in populated areas.
Although history tells us there is little to fear from induced seismicity, the scientific community is alert to public concern. As a part of their work, ESD's Ernie Majer and his team have created a website to inform the public and provide a resource on the latest information, including real-time maps of induced seismicity in areas being researched.