Interaction between a fractured marl caprock and CO2-rich sulfate solution under supercritical CO2 conditions

Abstract

Geological CO2 sequestration at pilot-plant scale will be developed at Hontomín (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 Hontomín marl and CO2-rich sulfate solutions under supercritical CO2 conditions (PTotal=150bar, pCO2=61bar and T=60°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 60mLh−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 1mLh−1) because of gypsum precipitation that sealed the fracture. At the highest flow rate (60mLh−1), fracture permeability increased because calcite dissolution predominated over gypsum precipitation.