Carbon Storage in Deep Sea Sediments: An Important Part of the CCS Portfolio
Authors: Schrag Daniel P. 
Published: October 2008

Abstract
Carbon dioxide injection into deep sea sediments below 2900 m water depth and a few hundred meters of sediments may provide permanent geologic storage by gravitational trapping. At high pressures and low temperatures common in deep sea sediments a few hundred meters below sea floor, CO2 will be in its liquid phase and will be denser than the overlying pore fluid. The lower density of the pore fluid provides a cap to the denser CO2 and ensures gravitational trapping in the short term. The overall storage capacity for CO2 in such deep sea formations below the ocean floor is primarily determined by the permeability, and will vary with seafloor depth, geothermal gradient, porosity, and pore water salinity. Furthermore, the dissemination of the injected CO2 in the sediments and potential chemical reactions between CO2, pore fluid and sediments will define its fate in the storage reservoir.

Introduction
The main objectives of our research was to evaluate the potential for sub-seabed CO2 storage in deep sea sediments using a range of approaches including experiments, permeability analysis, and modeling. Our analysis (House et al., 2006) has shown the feasibility of this type of storage, and also emphasizes that escape or leakage from such sites would be negligible. The most difficult challenge is to overcome the low permeability of typical deep-sea sediments, and a variety of approaches are suggested for future research.
Due to the high compressibility of CO2(l) relative to water, CO2(l) becomes denser than water at high pressures and low temperatures. These temperature-pressure regimes do not exist in terrestrial settings; they are, however, common in the deep ocean. When CO2(l) is injected into the ocean at a depth of ~3000 m, it sinks, forming a lake of CO2(l) on the seafloor. Ocean currents, however, can mix the injected CO2(l) causing a large fraction to eventually be released into the atmosphere. To ensure that deep ocean currents will not mix the CO2 into shallower regions, CO2 can be injected below the seafloor. Furthermore, if the seafloor depth of injection is greater than ~3000 m, then the injected CO2 will be denser than the ambient pore-fluid.
We refer to the sub-seafloor region with low enough temperatures and high enough pressures to compress CO2 to greater density than seawater as the Negative Buoyancy Zone (NBZ). When CO2 is injected beneath the NBZ, the lower density pore-fluid acts as a buoyancy-cap on the system and ensures gravitational stability. The gravitational stability of the system in deep-sea sediments is in contrast with terrestrial geologic storage where the high pressures and high temperatures cause the injected supercritical CO2 to be gravitationally unstable. The buoyancy-cap, provided by the pore water, serves the same purpose in deep-sea sediments as a cap rock serves in terrestrial geologic formations. The buoyancy-cap, however, is superior to a cap rock because conduits in a cap rock enable buoyant CO2 to escape. In contrast, the gravitational stability provided by the buoyancy-cap guarantees that fractures in the sediment column cannot serve as conduits for the CO2, and even large geomechanical perturbations—such as earthquakes—cannot cause the CO2(l) to be released. Additional work has also demonstrated that calcium carbonate dissolution is not an important factor in the process, and that karstification will not be an important process, nor can collapse of karst structures result in escape of CO2.