Natural CO2 Accumulations in Europe: Understanding Long-Term Geological Processes in CO2 Sequestration

Approximately one-third of anthropogenic CO2 emissions arises from transport, one-third from industrial and domestic sources, and one-third from power generation. While achieving substantial reductions in emissions from either of the first two will be a long-term process, the technology to capture CO2 from power plants is available and could lead quickly to significant reductions in emissions—provided mechanisms are available to dispose of the CO2 thus captured. The capture and underground storage of industrial quantities of carbon dioxide is currently being demonstrated at the Sleipner West gas field in the Norwegian sector of the North Sea. Natural CO2 accumulations offer the potential to understand the long-term geological processes involved in CO2 sequestration. By identifying the effects of CO2 on rock properties, such as changes in permeability and porosity or rock strength, models can be corroborated against empirical data. This can build confidence in their ability to predict likely responses of reservoirs and cap-rocks to geological sequestration. In addition, where CO2 is actively leaking to the surface, the effects of CO2 on groundwaters and ecosystems can be identified, and migration mechanisms can be described. The interactions between CO2-charged porewaters and both reservoirs and their caprocks through petrographic characterization, porewater and gas geochemistry, geomechanical testing, and gas migration studies in low permeability caprocks have been described. Leakage pathways are identified through soil gas surveys for CO2 and associated tracer gases. Geochemical analyses of carbonated waters are assessing the effects of CO2 on groundwaters. An understanding of these processes will be subsequently gained through geochemical and geomechanical modeling.