Experimental Simulation of the Impact of a Thermal Gradient on a Clayey Caprock Submitted to the Combined Action of Water and CO2

Abstract

This paper aims at studying the effect of a thermal gradient on a clayey caprock in the framework of a coupled system implying CO2 injection (as supercritical or dissolved in a brine) and heat recovery. We investigated the reactivity of a typical caprock represented by a Callovo-Oxfordian clay submitted to a high CO2 pressure (100 bar) and a thermal gradient (100 to 35°C in a 78 cm long tubular reactor) for 30 days. The original COTAGES experiment used in this paper was carried out with a saline aqueous solution (4 g.l-1 NaCl) initially in equilibrium with calcite at 100°C. A mass transfer was observed from the cold area of the reactor (35-55°C) to the warmest area (100°C). Dissolution and precipitation processes are responsible for a mass transfer representing approximately 6% of the initial mass of the most reactive sample. The petrographical analyses showed that the dissolution has mainly affected the carbonated minerals because the clay minerals didn’t show any important mineralogical transformation. However the dissolution of the carbonate fraction didn’t lead to a loss of mechanical cohesion of the rock while the samples from the hot zone are completely disaggregated despite the precipitation of secondary carbonate minerals. The analyses of major and trace elements confirmed the important mass transfers during the experiment. As expected, carbonate precipitation was linked to strontium trapping. However transition elements and especially Co, Ni, Cu and Zn showed a significant enrichment in the hot zone implying that mobilization of potential toxic elements has to be taken into account when the caprock undergoes the combined action of water, CO2 and a thermal gradient.