Natural CO2 accumulations in Europe: Understanding long-term geological processes in CO2 sequestration

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. CO2 gas accumulations are common throughout the world. The “Natural Analogues for the Storage of CO2 in the Geological Environment” (NASCENT: www.bgs.ac.uk/nascent) project is studying several of these accumulations in Europe. The sites include both areas where CO2 gas pools have been trapped for geological timescales and areas where CO2 is leaking at the ground surface. We are determining the interactions between CO2-charged porewaters and both reservoirs and their caprocks through petrographic characterisation, porewater and gas geochemistry, geomechanical testing and gas migration studies in low permeability caprocks. 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 modelling. This paper reviews the European sites being studied to address these issues and the data obtained so far.