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The MIRAGES experiment mimics an injection well at the lab scale (1/20). This experiment allows the injection of supercritical CO2 under geological conditions of pressure and temperature. The injection flow rate, confinement and injection pressures and temperatures are recorded during the 30 days of the experiment. Chemical parameters (pH, major element contents) are also monitored. The reservoir is represented by a core sampled in the formation of Lavoux limestone. The core is drilled to form an injection well in which an injection tube (made of stainless steel) is sealed with class G Portland cement together with two discs of Callovo‐Oxfordian clay representing the caprock. After the experiment, the core sample is studied to follow the petrophysical changes of the well materials and rocks. The interfaces between the reservoir, caprock, cement, and steel are investigated using scanning electron microscopy, cathodoluminescence, and Raman spectrometry. The main results suggest (i) good cohesion of the different interfaces despite the carbonation of the cement, (ii) the precipitation of different carbonate phases relating the changes in the chemistry of solution as a function of time, (iii) the enrichment in silica of the cement phase subjected to the action of CO2 providing evidence of new mechanisms of in situ silica re‐condensation, and (iv) the very good mechanical and chemical behavior of the caprock clay despite the alkaline flux from the cement and the acidic attack from the dissolved CO2.

Abstract:In the framework of CO 2 geological storage, one of the critical points leading to possible important CO 2 leakage is the behaviour of the different interfaces between the rocks and the injection wells. This paper discussed the results from an experimental modelling of the evolution of a caprock/cement interface under high pressure and temperature conditions. Batch experiments were performed with a caprock (Callovo-Oxfordian claystone of the Paris Basin) in contact with a cement (Portland class G) in the presence of supercritical CO 2 under dry or wet conditions. The mineralogical and mechanical evolution of the caprock, the Portland cement, and their interface submitted to the attack of carbonic acid either supercritical or dissolved in a saline water under geological conditions of pressure and temperature. This model should help to better understand the behaviour of interfaces in the proximal zone at the injection site and to prevent risks of leakage from this critical part of injection wells. After one month of ageing at 80 • C under 100 bar of CO 2 pressure, the caprock, the cement, and the interface between the caprock and cement are investigated with Scanning Electron Microscopy (SEM) and cathodoluminescence (CL). The main results reveal (i) the influence of the alteration conditions: with dry CO 2 , the carbonation of the cement is more extended than under wet conditions; (ii) successive phases of carbonate precipitation (calcite and aragonite) responsible for the loss of mechanical cohesion of the interfaces; (iii) the mineralogical and chemical evolution of the cement which undergoes successive phases of carbonation and leaching; (iv) the limited reactivity of the clayey caprock despite the acidic attack of CO 2 ; and (v) the influence of water on the transport mechanisms of dissolved species and thus on the location of mineral precipitations.