Experimental and numerical investigation of CO₂–brine–rock interactions in the early Palaeozoic mudstones from the Polish part of the Baltic Basin at simulatedin situ conditions

This study discusses experiment-induced alterations of shale rocks collected across the ordovician and silurian boundary at a potential site for carbon dioxide sequestration located in the Polish part of the Baltic Basin. Mudstone samples submerged in brine in custom-built reactors were subjected to CO₂ pressures of 30–35 MPa at a temperature of 80°C for 40 days. After the experiments were completed, the mineralogical composition of the studied rocks, their porosity characteristics, and chemistry of brines were analyzed and compared to the original compositions in order to detect experiment-induced changes. Comparison of mineral composition before and after the experiments demonstrate a set of subtle but systematic mineral changes. The most conspicuous was a presence of Fe in the form of Fe-oxyhydroxides that was related to the corrosion of the Fe-bearing steel pipes supplying the experimental setup with CO₂. This effect is important from the perspective of industrial carbon dioxide sequestration. However, the second source of Fe ions in experimental brines was related to the decomposition of chlorites, pyrite, and chloritized biotites. The suggested breakdown of pyrite is in line with the modification of the chemical composition of the experimental brines and observed transfer of S ions to the solution. The appearance of Mg and K ions in the experimental brines was related to the decomposition of chlorites and biotites. In turn, observed transfer of Sr cations to the solution could be attributed to the dissolution of biotites and K-feldspars. The effects of analogue experiments were confronted with the results of geochemical modeling performed using the PHREEQC program. The numerical modeling not only allowed to reproduce all the main mineral and chemical changes postulated on the basis of our analogue experiment, but also enabled to put constraints on the time scale of occurring geochemical processes.