(289f) Offshore Stored CO2 Residence Time: Accurate Assessment with Geochemical, Seismic, and Modelling Integration

Coastal geologic carbon sequestration focuses on injection of supercritical CO2 into deep sediment coastal reservoirs. Seal integrity above these reservoirs is 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 are leaking, probably best documented by the ubiquitous natural oil slicks observed e.g., in the Gulf of Mexico. Despite realization that many reservoirs will leak CO2, we propose, abiotic and biogeochemical reactions during CO2 migration 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 creates long term vertical migration. With final transport to the shallow sediment there is a complex series of biogeochemical cycles that will sequester deep sediment sourced CO2 that are controlled by the presence of electron acceptors. Moving down the biogeochemical eH gradient to the absence of sulfate, CO2 is a near final electron acceptor and can be cycled by bacteria into methane or organic material.

This potential effectiveness of the seafloor to act as a buffer depends strongly on localized flux rates of CO2 and organic carbon loading that controls microbial metabolism rates. Highly concentrated CO2 flux, at or approaching advection, would overwhelm the biogeochemical systems and result in a direct flow of CO2 to the overlying water column. In contrast, with lower vertical migration rates due to trapping and low permeability through clay seals would slow down any biogeochemical reactions as well as move these reaction fronts closer to the seafloor. These parameters need to be understood and constrained above individual reservoirs for an evaluation of the seafloor as potential secondary seal for any leakage of CO2 from deeper reservoirs. We predict that careful selection of deep sediment CO2 sequestration sites will provide long term trapping with a thorough seismic data review of fractures and vertical fluid and gas migration focus points integrated with assessment of shallow sediment biogeochemical trapping potential. This presentation will review characteristics of sediment that result in vertical migration of injected CO2 and an overview of trapping events during the vertical migration. With a thorough 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.