Experimental simulation of the impact of a thermal gradient during geological sequestration of CO2: The COTAGES experiment

We investigated the reactivity of crushed oolithic limestone that is exposed to high-pressure CO2 and a high thermal gradient (100-35 degrees C in a 78 cm long tubular reactor) for 30 days. The original COTAGES experiment described in this paper was carried out in the presence of saline aqueous solution (4 g L-1 NaCl) at equilibrium with CaCO3 at 100 degrees C. Mass transfer was observed from the cold area of the reactor (35-55 degrees C) to the warmest area (100 degrees C) and constituted approximately 10% of the initial mass of the most reactive sample, suggesting the presence of calcite dissolution and precipitation. Petrographical analyses using electronic and cathodoluminescence microscopes and X-ray microtomography showed that the dissolution mainly affected the cortices of ooliths located in the periphery of the grains and created more than 700 pore mm(-2) in the most reactive sample. However, because of their small sizes (mostly less than 100 mu m(2)), the pores did not significantly increase the porosity (2% on average). The crystallisation of a microcalcite fringe in the warmed area covered the entire surface of the grains and isolated the porous network of grains in a sample. This study demonstrates the strong reactivity of oolithic limestone in the presence of a thermal gradient and pressure similar to realistic CO2 geological sequestration conditions with mass transport from the injection well towards the reservoir rock. (C) 2012 Elsevier Ltd. All rights reserved.