Permanent Carbon Dioxide Storage in Deep Sea Carbonate Sediments

Deep sea sediments provide a permanent storage option for captured anthropogenic CO2. The high pressures and low temperatures below ~3000 m of seawater and several hundred meters of marine sediment compress CO2(l) to greater density than the surrounding pore fluid. The lower density pore fluid is an effective cap on the system as the CO2 can only escape by diffusion. Furthermore, these pressure-temperature conditions allow hydrate formation, which reduces permeability near the seafloor enhancing the storage stability. Hydrate formation may also interfere with the injection process by reducing permeability near the well-head. Therefore, the injection well-head should be located below the hydrate formation zone (HFZ). For a geothermal gradient of 30 C/km and a water depth of 3500 m, the bottom of the HFZ is at 310 mbsf while the bottom of the neutral buoyancy zone (NBZ) is at 200 mbsf. Whether the well-head is below the NBZ or in the NBZ, the injected CO2(l) will flow towards the depth of neutral buoyancy. Further transport will only be accomplished by diffusion. From this point, it will take millions of years for diffusion to carry the CO2 out of the sediment and into the ocean, diluting the discharge to a negligible level. We present simple models of the transport and reaction of injected CO2 at these conditions. We show that carbonic acid dissolution of the host rock will slightly increase pore space, potentially resulting in large increases in permeability. Host rock dissolution, however, will not compromise geomechanical stability through large karst formation because the total dissolution is limited to only a few percent of the rock volume.