Impacts of CO2 perturbation on well composite samples: experiments and numerical simulations
Résumé
Abandonned wells in the depleted reservoirs envisaged to host CO2 geological storages are the main possible leakage pathways of CO2 towards shallow aquifers. Then, for impact assesment, the alteration of well materials with CO2 in the reservoir conditions requires to be characterized. Here, the interaction of a composite well sample - formed of steel casing, surrounded by Portland cement, itself surrounded by sandstone - with wet CO2 and CO2-saturated brine under pressure and temperature controlled conditions was studied combining a set of batch experiments and reactive transport modeling. In the experiments, lasting up to 8 weeks, noticeable mineralogical changes were observed in the cement, at the interface with the sandstone, leading to a carbonation of the cement. Main mineralogical changes consisted in dissolution of portlandite, replacement of CSH phases rich in Si by Ca-rich CSH phases and precipitation of calcite, amorphous silica and zeolite. Interestingly, no re-dissolution of calcite was observed at the outer boundary of the cement, in relation with the penetration of the carbonation front, as observed in experiments involving only cement and CO2-saturated brine. For the two other components of the composite well samples, few changes were observed. The steel shown a moderate corrosion with some precipitations of Fe-oxides at its surface. No changes were observed in the sandstone. These changes in mineralogy were reproduced with the reactive transport model, which highlights the successive dissolution/precipitation reactions. A good agreement was also obtained with the brine composition evolution record during the experiment. It is worth noting that the model suggested slight mineralogical changes in the sandstone consisting in dissolution of carbonates at the boundary of the sample, in direct contact with the CO2-saturated brine. This observation, in link with the experimental observations in the cement, indicate a buffering effect of the rock on the CO2 perturbation. This possible buffering was observed by observations at a larger scale on industrial analogue well samples and suggests a preservation of the well integrity.