Introduction to the special issue on site characterization for geological storage of CO 2

Concentrations of carbon dioxide (CO2) in the atmosphere have been rising rapidly as a result of human activities since the Industrial Revolution. From about 280 ppm before the start of this period in history, these values have increased to about 379 ppm in 2005, which is by far higher than the natural range of CO2 determined from ice cores over the last 650,000 years (IPCC 2007). The primary source of this increase is the use of carbon-rich fossil fuels such as coal, oil, and natural gas for energy production. It is widely accepted today that these changes in atmospheric CO2 concentrations are a major contributor to global warming by trapping heat radiating from the earth’s surface, by the so-called greenhouse effect (NAP 2001; IPCC 2007). Several technological options have been proposed to stabilize atmospheric concentrations of CO2 (Pacala and Socolow 2004). One suggested remedy is to separate and capture CO2 from power plants burning fossil fuels and from other stationary industrial sources and to inject the produced CO2 into deep subsurface formations for longterm storage and sequestration (IPCC 2005). The most promising targets for geologic sequestration of the CO2 would be deep saline formations as well as depleted or near-depleted oil and gas reservoirs. Saline formations offer the largest CO2 storage capacity and are widely distributed worldwide. Hydrocarbon reservoirs, on the other hand, are not so common, but have the advantage of having caprocks with proven ‘‘sealing’’ capacity (buoyant fluids remained trapped in place over geologic times), and the injected CO2 may enhance oil or gas production. The scale at which CO2 would have to be sequestered is large. Considering Norway’s North Sea Sleipner project, which is the world’s first industrial-scale CO2 geologic storage operation with an annual injection of about one million tonnes of CO2 since 1996 (e.g., Torp and Gale 2003), Pacala and Socolow (2004) estimate that 3,500 Sleipner-sized capture and storage (CCS) projects, would be needed worldwide over the next 50 years to help stabilize atmospheric CO2 concentrations. While geologic sequestration is a promising method for CO2 mitigation, continued research and development efforts are important to advance the state of the art in many areas so that CCS can be effectively and safely deployed on a global scale (e.g., Litynsky et al. 2006). One challenging research area is the optimization of the methodology to assess the suitability of potential geologic sequestration sites. Careful site characterization is necessary to demonstrate that storage in geologic formations is both feasible, i.e., that the formations have a large enough storage capacity and good injectivity, and effective, i.e., that the formations provide safe long-term containment, without adverse impacts to human health or the environment (Bachu 2000). The importance and objective of site characterization are reiterated in the special report on carbon dioxide capture and storage issued by the intergovernmental panel on climate change (IPCC 2005), which stated that ‘‘site characterization, selection and performance prediction are crucial for successful geological storage. Before selecting a site, the geological setting must be characterized to determine if the overlying cap rock will provide an J. Birkholzer (&) C.-F. Tsang Earth Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, MS 90-1116, Berkeley, CA 94720, USA e-mail: jtbirkholzer@lbl.gov