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Carbonation of ultramafic rocks in geological reservoirs is, in theory, the most efficient way to trap CO2 irreversibly; however, possible feedback effects between carbonation reactions and changes in the reservoir permeability must be considered to realistically assess the efficiency and sustainability of this process. We investigated changes in the hydrodynamic properties of sintered dunite samples by means of percolation experiments, under conditions analogous to that of in situ carbonation. Our results show that carbonation efficiency is controlled by the local renewal of the reactants and the heterogeneity of the pore structure. Preferential flow zones are characterized by the formation of magnetite and of a silica-rich layer at the olivine surfaces, which eventually inhibits olivine dissolution. Conversely, sustainable olivine dissolution together with coprecipitation of magnesite, siderite, and minor Mg-TOT-phyllosilicates, occur in reduced-flow zones. Thus carbonate precipitation only decreases porosity in zones where diffusion-controlled transport is dominant. Consequently, while high flow rates will decrease the carbonation efficiency of the reservoir and low flow rates may reduce the permeability irreversibly close to the injection point, moderate injection rates will ensure a partial carbonation of the rock and maintain the reservoir permeability.

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Limestone dissolution by CO2 -rich brine induces critical changes of the pore network geometrical parameters such as the pore size distribution, the connectivity, and the tortuosity which govern the macroscopic transport properties (permeability and dispersivity) that are required to parameterize the models, simulating the injection and the fate of CO2 . A set of four reactive core-flood experiments reproducing underground conditions ( T=100∘C and P=12 MPa) has been conducted for different CO2 partial pressures (0.03

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[1] Claystone caprocks are often the ultimate seal for CO 2 underground storage when residual CO 2 gas reaches the reservoir top due to buoyancy. Permeability changes of a fractured claystone due to seepage of CO 2 -enriched brine and water vapor-saturated CO 2 gas are investigated. Results show that brine flow induces a large porosity increase (up to 50%) in the vicinity of the fracture due to dissolution of calcite and quartz, while permeability remains unchanged. Conversely, cyclic flows of CO 2 -brine and CO 2 -gas increase the fracture aperture abruptly after each gas flow period, producing a progressive decrease of the caprock seal capacity. Aperture increase is controlled by decohesion of the clay framework within a micrometer-scale-thick layer induced by CO 2 -gas acidification. Results show that hydraulic aperture increases linearly with duration of the preceding CO 2 -brine flow period, emphasizing the kinetic control of the quartz grains dissolution during the brine flow periods. Citation: Andreani, M., P. Gouze, L. Luquot, and P. Jouanna (2008), Changes in seal capacity of fractured claystone caprocks induced by dissolved and gaseous CO 2 seepage, Geophys.