The immobility of CO(2) in marine sediments beneath 1500 meters of water.

Injecting liquid CO(2) into deep-sea sediments below ca. 3 km of seawater has been suggested for the permanent storage of anthropogenic CO(2). At the pressures and temperature found below 3 km of seawater, CO(2) becomes denser than seawater and so is likely to remain permanently sequestered in the sediment. Deepwater engineering, however, is expensive and seawater depths of greater than 3 km are often only reached far from shore. Here, we consider the less expensive alternative of injecting CO(2) into marine sediments at depths shallower than required for denser-than-seawater CO(2) storage. We compare the mobility of liquid CO(2) that has been injected into deep-sea reservoirs with the mobility of supercritical CO(2) that has been injected into geologically equivalent (i.e., identical porosity, permeability, and effective stress) reservoirs with terrestrial pressure and temperature conditions. We demonstrate that buoyant liquid CO(2) with a density of about 90 % that of seawater is sufficiently immobile that it can be considered trapped by gravity and capillarity. In contrast, supercritical CO(2) under typical terrestrial conditions is highly mobile and only trapped by the appropriate confining layer in either a structural or stratigraphic trap. As a result of its very high mobility under terrestrial conditions, CO(2) injected in an unconfined formation would spread beneath the confining layer to produce a large flat cylindrical-shaped plume of pure-phase CO(2). In contrast, the less mobile CO(2) in a typical deep-sea reservoir produces a spherical-shaped plume, resulting in a pure-phase-CO(2) footprint that is much smaller than the pure-phase-CO(2) footprint formed in the confined-terrestrial reservoir.