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A temporary solution to massive anthropogenic CO2 emissions can be the capture of industrial CO2 from flue gas and sequestering it in geological formations. For safe and effective storage of CO2, interaction processes in the rock-pore fluid–CO2 system should be known. Investigation of natural CO2 accumulations provides valuable examples to what physical and chemical effects could be expected during CO2 influx at future CO2 storage sites. One of the key controlling factors of the processes occurring in natural CO2 reservoirs is the lithology of the storage rocks, which is primarily determined by the formation conditions of these rocks. In this respect, the lithologies of individual CO2 accumulation areas influence the processes between the host rock, the pore fluid, and the CO2 in different ways. In the current study, we focus on a well-studied natural CO2 storage reservoir, namely the Mihályi-Répcelak area, NW Hungary. We provide insight into the so far unstudied conglomerate reservoirs that represent a stratigraphically deeper reservoir unit with significantly different lithology and pore water compositions compared to the sandstone reservoirs. Our results indicate that dawsonite /NaAlCO3(OH)2/ formation also affected the conglomerate reservoirs, which indicates that at least part of the CO2 could be trapped in mineral form. An important role of salinity in reducing the CO2 mineral trapping capacity of the storage system is also demonstrated. Furthermore, H isotope analysis of diagenetic kaolinite was applied to trace the origin of the pore water that was present during the rock formation. Based on the data, dawsonite formation was induced by the flux of meteoric water that infiltrated during a warm and humid period and mixed with ascending CO2.

Natural CO2 reservoirs provide an opportunity to study long-term fluid-rock interactions, which are essential to reassure the safety of mineral storage of carbon-dioxide. The Mihalyi-Repcelak field (Pannonian Basin, Central Europe) is one of the largest natural CO2-bearing reservoirs in Europe (25 Mt). The CO2 was trapped in Neogene sandstones, which contain various carbonate minerals (dolomite, ankerite, siderite, dawsonite). To reveal the origin of the parent fluid, from which these minerals precipitated, dawsonite and siderite were separated by a new physical method to minimise the uncertainties in the analysis of their stable isotope composition. The δ13CDaw values range from +1.3‰ to +1.6‰ and the calculated δ13CCO2 values in equilibrium with dawsonite (−4.8‰ - –2.0‰) overlap with the carbon isotope compositions of the local CO2 and the European Subcontinental Lithospheric Mantle (−3.9‰ - –2.1‰). This indicates that the dawsonite-forming CO2 had a magmatic origin. The siderite data indicates that some formed from the magmatic CO2, possibly simultaneously with dawsonite (−6.0‰ - –3.9‰), whereas the rest (−8.4‰ - –6.1‰) formed either from a fractionated CO2 with magmatic origin or before the CO2 invasion. The hydrogen isotope composition of structural OH− of dawsonite (−57‰ to −74‰) was determined and was used to estimate the origin of the interacting porewater. The calculated porewater data (δD: −69‰ - –103‰ and δ18O: −1.4‰ - +4.7‰) indicate that the parent fluid was meteoric water modified by water-rock interaction. Our data allows estimation of the total amount of CO2 stored in the dawsonite-bearing sandstone reservoir to be 25 kg/m3, well in line with previous modelling works, which gives a total of 2.01 × 106 t of CO2, higher than previous estimates. We suggest that individual mineral analysis complemented by hydrogen isotope analysis is to be employed to effectively trace in-reservoir fluid-rock interactions in CO2 reservoirs and provide valuable input data for geochemical modelling for better predicting conditions for mineral storage of CO2.

When CO2 is injected into a subsurface reservoir, it undergoes structural-stratigraphic, residual, dissolution and mineral trapping mechanisms. Among these, the most permanent and preferable is the mineral trapping, in which carbonate minerals build the CO2 into their structures. In this study the amount of CO2 trapped in minerals was estimated from both analytical data of and thermodynamic geochemical models for a natural CO2 occurrence. Models underestimate the amount of trapped CO2 to be 0.004 tons/m3 reservoir in comparison with measured amount of dawsonite and other carbonates in rock samples indicating that 0.02-0.18 tons CO2 is trapped in one m3 reservoir.

Stable isotope systems provide exceptional insight into fluid-solid interactions occurring in the sub-surface. We provide detailed insight into the stable isotope distribution of carbonate minerals in a natural CO 2 reservoir system from Mihalyi-Repcelak area, NW-Hungary. Several types of measurements and calculations have been applied for the estimation of CO 2 and H 2 O isotopic composition in equilibrium with dawsonite. The estimated δ 13 C of CO 2 exhibit values around -4.5 to -2.6 ‰ overlapping with assumed magmatic values and the presently measured gas data confirming magmatic origin, whereas we may assume that the water present during carbonate formation likely had meteoric origin.

The caprock is the only physical barrier in the migration path of CO2 during CO2 geological storage. For this reason, caprock integrity is one of the most important factors regarding the long-term safe underground storage of CO2. As a result of geochemical reactions, among the caprock minerals and CO2 saturated pore water, the physical properties of caprocks such as porosity and permeability may change, which could affect their sealing capacity. Natural CO2 occurrences can help to understand these long-term reactions under storage conditions on geological timescale.

Caprock integrity is one of the most important factors regarding the long-term safe underground storage of CO2. As a result of geochemical reactions among the caprock mineralogy and CO2 saturated pore water, the physical properties of caprock such as porosity, permeability may change, which could affect its sealing capacity. Natural CO2 occurrences can help to understand these long term reactions under storage conditions on geological timescale. Our study area, the Mihalyi-Repcelak natural CO2 occurrence, is believed to be leak-proof system on geological timescale. To identify and understand the mineral reactions in the caprocks we applied XRD, FTIR-ATR and SEM analysis of drill cores derived from the study area. The petrophysical properties of the studied rock samples were determined from the interpretation of geophysical well-logs and grain size distribution. The effective porosity (~4 %), permeability (0.026 mD) and clay content (~80 %) of the drill cores imply that the studied clayey caprocks represent an adequate physical barrier to the CO2. Our analytical results show that dawsonite has formed within the caprocks. In most cases the dawsonite crystallized after albite dissolution. This implies that CO2 or CO2-saturated brine can penetrate into the caprock resulting in mineral reactions and most likely changing the porosity and permeability of the sealing lithology. On the other hand the caprock may react as a geochemical buffer for the CO2 and, at least part of it, can be stored within the caprock as solid phase, thereby increasing the storage capacity of the system.