The Importance of Secondary Traps and Sinks in Offshore CO2 Sequestration

Abstract

Abstract Geologic carbon sequestration currently focuses on injection of supercritical CO2 into reservoirs, often in a depleted hydrocarbon field. Seal integrity above these reservoirs is therefore critical for safe long-term storage of CO2. It is generally assumed that if a reservoir was able to contain hydrocarbons for millions of years, it should be suitable for CO2 storage. However, many hydrocarbon reservoirs leak, probably best documented by the ubiquitous natural gas and oil seeps observed e.g., in the Gulf of Mexico. With integration of seismic and geochemical data and fate-transport modeling there is capability to select efficient storage locations in the deep coastal sediment and provide accurate assessment of CO2 residence time. We propose that even if reservoirs leak CO2, abiotic and biogeochemical reactions from the injection point to the ocean floor surface provide diverse traps that result in efficient sediment CO2 sequestration. First, during the migration concentration in porewater becoming supersaturated can result in a carbonate formation. This carbonate formation may cause a trap and with the CO2 build up there can be pressurization that results in horizontal migration to a new vertical path or pressurization that results in fractures of carbonate or clay layers. This transport can occur at multiple intervals during migration to the surface sediment and this step-by-step trapping slows down long-term vertical migration.