| Popis: |
We investigated CO2-free and naturally CO2-flooded sandstone samples from a deep saline aquifers formation, which represents potential carbon storage reservoirs. A descriptive geochemical model is also coupled to the laboratory study for the better understanding of geochemical interaction between sandstone and CO2. The studied area is located in the, Western Hungary in the Little Hungarian Plain where one of the largest CO2-producing fields in Europe can be found. In this region, we have the opportunity to compare rocks of the same sandstone formation without CO2 (not affected by natural CO2 flooding) and naturally CO2 flooded sandstone, where the CO2 was trapped around 7-4 million years ago. As boreholes sampled not only the parts of the formation, which were flooded by CO2 (Mihályi-Répcelak), but also the parts which were not affected at all by this flooding (Ölbő).Besides petrographic observations, scanning electron microscopy and mineral chemistry analyses, X-ray diffraction and infrared spectroscopy were used to determine 7 CO2-free and 6 CO2-flooded samples textural features, mineral compositions and the presence of OH-bearing minerals. We carried out thermodynamic-batch modelling with PHREEQC geochemical modelling software and compared to the laboratory results.The sandstone samples from the CO2 bearing reservoirs contain quartz, mica, kaolinite, K-feldspar and carbonates such as dolomite, calcite, ankerite and siderite. The CO2-free samples also contain chlorite, plagioclase and pyrite and all mentioned above. In the CO2-flooded samples a carbonate phase, dawsonite (NaAlCO3(OH)2) could be also observed in significant amounts (3-16 w/w%). This is an indicator mineral of large amount of CO2 inflow in the CO2-water-rock system. In addition, chlorite is apparently missing in the CO2-flooded samples. According to the petrographic observations and X-ray diffraction (XRD) results, it is clear that the plagioclase content is higher (∼ 11 w/w%) in the CO2-free samples compared to the CO2-flooded ones (2 flooded reservoir rocks. The lower amount of K-feldspar and plagioclase in the CO2-flooded samples can be explained by precipitation of dawsonite. These minerals can dissolve as a result of CO2-flooding and serve Na+ and/or Al3+ ion for dawsonite formation. The amount of the carbonate minerals also reveal systematic differences between the CO2-free and CO2-flooded sandstone, the amount of ankerite is higher (from 6 to 12 w/w%) in the later ones implying that some parts of the ankerite formed after the CO2 flooding event. The investigation of this unique area provides opportunity to study sandstone before interaction with CO2 and after millions of years being in contact with CO2.Acknowledgements:This research was financed by Hungarian Scientific Research Fund (K131353).Dóra Cseresznyés’ work is supported by the Cooperative Doctoral Programme granted by The Ministry for Innovation and Technology (ITM), National Research, Development and Innovation Office. |
