Experimental Modeling of the injection of CO₂-rich solution in a geothermal doublet. Study of the geochemical impacts in the near-well region

This work was conducted in the framework of the technical assessment of a novel Carbon Capture and Storage (CCS) concept integrating aqueous dissolution of CO2 and injection via a geothermal doublet. This study focuses on i) the quantification and modelling of the hydrodynamic and geochemical impact induced by the injection of a CO2-laden solution in a reservoir rock and ii) the evaluation of long term integrity of the well materials (cement, steel casing) in order to ensure a safe injection protocol. A dedicated experimental device named MIRAGES.2 was developed to mimic, at the lab scale, the continuous radial injection of a CO2-enriched solution under realistic conditions of a geological reservoir. The miniature well consists in a steel tube that is cemented to the core plug with a class G Portland cement. The test bench is divided in two parts: the first one is devoted to the CO2-solution mixing process, and the second one enables to perform the injection of the solution in the core-plug. In addition, the implementation of original in situ measurement techniques (in-situ HP/HT Raman and pH probes, flowmeter) was carried out in order to ensure optimal acquisition of physical and chemical data (pressure, temperature, pH, concentrations of different species in solution...) during the experiments. A method of image processing acquired on post-experimental samples by X-ray micro-tomography has been developed. This technique revealed the 3D architecture of the mesoscopic porous network. This experimental protocol revealed the physicochemical evolution of: the different interfaces between cement and steel, and between cement and reservoir ; the near-well region of the reservoir ; The injected fluid. A set of 7 experiments was performed. The injection duration (12 h, 24 h, 2.5 d, 10 d and 21 d), the fluid salinity and the core drilling inclination with respect to the bedding were investigated. The experiments demonstrate the non-uniform propagation of the acidic solution from the injection point in the form of preferential pathways called « wormholes ». Once a single wormhole breaks through the core-plug, all the other competing wormholes stop growing and their density tend to decrease as the solution is injected. Despite a predominant localized phenomenon, changes in petrophysical properties of the rock in regions far from the wormholes was observed. Following the continuous renewal of the acidic solution, a uniform dissolution in the upper part of the injection well was also highlighted. Roughness surface measurements coupled with microscopic observations have revealed the presence of calcite precipitation which induces the clogging of secondary wormholes. Cement ageing in contact with the reactive solution induces localized chemical imbalances. Changes in magnesium concentration, inhibitor of calcite precipitation, released during cement alteration, govern the local calcite saturation states of the interstitial solution. These phenomena could explain the observed precipitation in a medium mainly undersaturated with respect to the calcite. The experiments also demonstrated the important role of the salinity of the injected solution, which dissolves up to five times more host rock than a freshwater solution. Finally, a multi-scale structural study was carried out and established the close relationship between the distribution of structural defects generated by regional tectonics and the orientation of the dissolution networks observed in our experiments. These results refine the analysis and assessment of environmental impacts and risks in the context of the CO2 injection in a geothermal doublet. They demonstrate the discontinuities present in the rock control the dissolution paths at the reservoir scale.