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Recent studies report rapid corrosion of metals and carbonation of minerals in contact with carbon dioxide containing trace amounts of dissolved water. One explanation for this behavior is that addition of small amounts of H 2 O to CO 2 leads to significant ionization within the fluid, thus promoting reactions at the fluid-solid interface analogous to corrosion associated with aqueous fluids. The extent of ionization in the bulk CO 2 fluid was determined using a flow-through conductivity cell capable of detecting very low conductivities. Experiments were conducted from 298 to 473 K and 7.39 to 20 MPa with H 2 O concentrations up to *1,600 ppmw (mole fraction of water, x H 2 O &3.9 9 10 -3 ), corresponding to the H 2 O solubility limit in liquid CO 2 at ambient temperature. All solutions showed conductivities \10 nSÁcm -1 , indicating that the bulk solutions were essentially ion-free. This observation suggests that the observed corrosion and carbonation reactions are not the result of ionization in CO 2 -rich bulk phase, but does not preclude ionization in the fluid at the fluid-solid interface.

Concern about the role of greenhouse gases in global climate change has generated interest in sequestering CO(2) from fossil-fuel combustion in deep saline formations. Pore space in these formations is initially filled with brine, and space to accommodate injected CO(2) must be generated by displacing brine, and to a lesser extent by compression of brine and rock. The formation volume required to store a given mass of CO(2) depends on the storage mechanism. We compare the equilibrium volumetric requirements of three end-member processes: CO(2) stored as a supercritical fluid (structural or stratigraphic trapping); CO(2) dissolved in pre-existing brine (solubility trapping); and CO(2) solubility enhanced by dissolution of calcite. For typical storage conditions, storing CO(2) by solubility trapping reduces the volume required to store the same amount of CO(2) by structural or stratigraphic trapping by about 50%. Accessibility of CO(2) to brine determines which storage mechanism (structural/stratigraphic versus solubility) dominates at a given time, which is a critical factor in evaluating CO(2) volumetric requirements and long-term storage security.

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