Petrographic and geochemical investigation of naturally CO2-free and CO2-flooded sandstones from the Central Pannonian Basin

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 (<1 w/w%). The modal amount of K-feldspar is also lower in the CO₂ 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.