Environmental and geochemical aspects of geologic carbon sequestration: a special issue.

A Special Issue T need to reduce atmospheric CO2 emissions from combustion of fossil fuels becomes more clear each day with the availability of new supporting information on, for example, global temperature, ocean levels, ice cover, extreme weather events, and ecological change. According to estimates from the International Energy Agency, total global emissions of CO2 were approximately 31.6 Gt in 2011, up from 23.7 Gt in 2001 and 21.1 Gt in 1991. Because of the long residence time of CO2 in the atmosphere, it will be necessary not only to stabilize CO2 emissions, but to decrease them significantlyin the order of 80% by 2050to avoid scenarios of warming, coastal inundation, drought, flooding, and other impacts with negative social and economic consequences that will be very difficult to manage. Due to the enormous amount of CO2 produced each year, the Intergovernmental Panel on Climate Change of the United Nations, as well as many other international bodies, has recognized that geologic carbon sequestration (GCS) will be an important part of any group of strategies to reduce CO2 emissions. Approximately 60% of global emissions of CO2 are from point sources such as electric power plants and cement kilns for which CO2 capture and geologic sequestration technology could be implemented. Since the late 1990s, a large amount of research and development on GCS has been undertaken to develop the technology and reduce its costs, and to assess technology performance and risk. While the individual components of CO2 capture, transport, and subsurface injection technology have existed for many years, they have not been integrated and implemented at anything approaching the scale needed for significant reductions in CO2 emissions, except for a small number of initial demonstration projects. Research is being conducted in Australia, Europe, the U.S., and elsewhere to enable large-scale field testing of carbon capture and geologic sequestration systems across a wide range of capture, transport, injection, and storage scenarios. Complementary research is being performed to develop improved scientific understanding of the fundamental physical and chemical processes involved, and to build models to simulate GCS technology performance and associated risks. Understanding of geochemical processes is critical for optimizing and controlling geologic carbon sequestration, and for assessing its risks. The ability to maintain injection rates into a porous formation over decades, and to maintain porosity in formations receiving fluids never before present is entirely dependent on geochemical processes. Interactions of the injected CO2 (and impurities remaining after capture from flue gases) with brines and hydrocarbons present in the receiving formations under high pressures and temperatures will yield solutions and mixed phases with unusual properties that will determine the performance of GCS. The interactions of these fluids and mixtures with the receiving formation and with overlying caprock also will determine the potential for fluid migration and leakage. The effectiveness of several important trapping mechanisms for CO2 migrating from storage zones − especially capillary trapping, solubility trapping, and mineral trapping (discussed in the feature article) will also be governed by geochemical processes. This special issue of Environmental Science & Technology provides a panorama of current research on environmental and geochemical aspects of geologic carbon sequestration. It includes a feature article by the guest editors, and three critical reviews. The state of knowledge of caprock sealing mechanisms is examined in the first review, the second review evaluates how leakage may impact the geochemistry of aquifers and the vadose zone, and the third review explores the partitioning behavior of organic compounds in carbon sequestration environments. The reviews are followed by six papers on CO2 dissolution in brines and adsorption on minerals relevant to storage formations under geologic sequestration conditions. The focus then turns, through 14 papers, to CO2 interactions with three classes of minerals (carbonates, silicates, and aluminosilicates) when CO2 is injected and present in saturated brines. Next, five papers investigate physical-chemical processes involved with wetting phenomena, ganglion dynamics, viscous fingering, and CO2 trapping. Finally, ten contributions examine various processes relevant to the potential for leakage and transport out of storage zones, and to the monitoring of leakage. The coupling of chemical and physical processes is critical to the integrated assessment of GCS. The geochemical process studies presented in the special issue provide base-level science to inform the development and application of integrated performance assessment models for geologic carbon sequestration. In the U.S., the Department of Energy (DOE) has been developing one such modeling framework, CO2-PENS (Predicting Engineered Natural Systems), a comprehensive system-level computational modeling framework for performance assessment of geologic sequestration of CO2. CO2-PENS encompasses a (nonstatic) group of models that collectively enable probabilistic simulations of CO2 capture, transport, and injection in geologic reservoirs (Stauffer et al.,  2009, 43(3):565−570). Integrated performance assessment is the organizing framework for research being conducted by the U.S. DOE and other organizations around the world. To advance prospects for implementation of geologic carbon sequestration, it is important for research to be aligned with an integrated framework for analysis of performance and risk. Such focused research will yield an improved understanding of the technology in ways that matter to the public and to companies that will be charged with implementing a very much needed technology. Shin Jun*,† Daniel E. Giammar† Charles J. Werth‡ David A. Dzombak

Field of study: Environmental science