Search

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.

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.

Eleven unique core samples from the National Radioactive Waste Repository of Hungary, Bataapati were studied in this work. The samples all cross the granite–concrete interface and have been drilled from around 275 m depth from the surface, 1–15 months after concrete injection. Phase analytical techniques, optical microscopy, SEM-EDS and Raman-spectroscopy were used for the analysis of interactions between granitic rock and cementitious building material. Newly formed phases, Ca-carbonates and titanite, were observed at the interface. Carbonation may reduce the porosity and permeability in the contact zone. The presence of titanite indicates the changing geochemical and thermodynamical constrains along the reaction front of granite–concrete, furthermore, it may help in the validation of future geochemical models. The cementitious material is seen to penetrate among the sheets of biotite mineral in granite which process is probable to cause the attachment of granite and concrete.

Laser diffraction grain size data have been widely used in paleoenvironmental reconstructions as physicochemical alteration-related proxies. Many studies are available on comparison of different laser diffraction devices, optical theories and optical settings. The ignorance of some uncertainty factors can lead to poorly comparable granulometric datasets. Other important factor leading to the aforementioned effect is the inadequate chemical pretreatment procedures which are often overlooked, but are capable to basically affect the results. In this study we examine a few past and recent sediment types from different geomorphological environments from the Carpathian Basin: lake and fluvial sediments, paleosols and loess. Our aim is to review and create a reliable methodology for laser diffraction particle size analysis and optical particle shape investigations. We compare widely used pretreatment methods -which can be found in the literature- with each other. We are also taking into account that different sediment types need different pretreatment methods. We can state that the duration of chemical pretreatment can affect the optical properties (color), the texture and the mineral composition of the sediments, as well as the size and shape of mineral particles in the samples. The changes in these significant parameters can mislead the researcher’s main objectives. The study is supported by the ÚNKP-19-3 New National Excellence Program of the Ministry for Innovation and Technology. Support of the National Research, Development and Innovation Office NKFIH K120620 is gratefully acknowledged.

In spite of its considerable size and the presence of mature oil source rocks, the Neogene Danube Basin is characterized by the absence of commercial oil accumulations. However, important [Formula: see text] fields have been discovered in the basin, which have been possibly explained recently by displacement of oil by later migration of [Formula: see text] into the reservoirs. Previous studies performed independently in the southern, Hungarian part of the basin (known as the Little Hungarian Plain) have attempted to model petroleum generation/migration history, to identify sources of the [Formula: see text], and to understand the tectonic evolution of the basin and its deep-water flow regime. We were attempting to combine the results of these studies to interpret the fluid-migration history and reveal the influencing processes. This work supports the hypothesis that [Formula: see text] could have played a key role in preventing the formation of oil accumulations. During the latest Miocene, early mature oil and saline water, the latter formed by dissolution of up to now unidentified halites, moved together toward the Mihályi High, a regional uplifted structure in the central part of the basin. Modest amounts of oil could have been trapped there while saline water mixed with the low-salinity water of the reservoirs, with the [Formula: see text] arriving later and displacing the oil.

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.

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.

The purpose of this study was to investigate the results of re-interpreted old well-logs and porosity and permeability measurements of core samples from a natural CO 2 occurrence in NW-Hungary. The reservoirs formed in Neogene sediments over a basement high. We compared the estimated physical properties based on well logging and the results of core sample analysis. The goal of our investigation was to give a distribution of hydrogeological properties in the in Mihalyi –Repcelak area. We outlined the paleoenvironment of the reservoirs and caprocks searching for the explanation of the formation of this natural CO 2 storage.

Prediction of the fate of the injected CO2 in geological formations strongly relies on geochemical modeling, laboratory and field testing. However, all of these approaches have their major drawbacks, either being much too simplistic and/or out of realistic timescales (1–100 days vs. 100–1000 years) for deep subsurface storage. Natural CO2 occurrences provide an exceptional opportunity to study long-term behavior of the scCO2-porewater-reservoir rock system. Despite their complex geologic evolution, these occurrences have been used lately to identify CO2-related reaction features as well as to test and fine-tune geochemical models. The Mihalyi-Repcelak occurrence in NW Hungary, with core material available, not only provides a general insight into subsurface reaction processes related to CO2, but represents one of the largest deep saline aquifers in the Pannonian Basin, it also behaves as a natural laboratory for this potential Late Miocene saline reservoir. The multi-layered structure of the occurrence with separated hydrodynamic units enables the simultaneous analysis and the identification of reaction features both in the reservoirs and their sealing cap rocks providing a further step in understanding the long-term performance of storage complexes.