Geological CO2 sequestration at pilot-plant scale will be developed at Hontomin (Spain). CO2 will be injected into a limestone reservoir that contains a NaCl- and sulfate-rich groundwater in equilibrium with calcite and gypsum. The caprock site is composed of marl. The present study seeks to evaluate the interaction between the Hontomin marl and CO2-rich sulfate solutions under supercritical CO2 conditions (P-Total = 150 bar, pCO(2) = 61 bar and T = 60 degrees C).; Flow-through percolation experiments were performed using artificially fractured cores to elucidate (i) the role of the composition of the injected solutions (S-free and S-rich solutions) and (ii) the effect of the flow rate (0.2, 1 and 60 mLh(-1)) on fracture permeability. Major dissolution of calcite (S-free and S-rich solutions) and precipitation of gypsum (S-rich solution) together with minor dissolution of the silicate minerals contributed to the formation of an altered skeleton-like zone (mainly made up of unreacted clays) along the fracture walls. Dissolution patterns changed from face dissolution to wormhole formation and uniform dissolution with increasing Peclet numbers.; In S-free experiments, fracture permeability did not significantly change regardless of the flow rate despite the fact that a large amount of calcite dissolved. In S-rich solution experiments, fracture permeability decreased under slow flow rates (0.2 and 1 mLh(-1)) because of gypsum precipitation that sealed the fracture. At the highest flow rate (60 mLh(-1)), fracture permeability increased because calcite dissolution predominated over gypsum precipitation.

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