Research work related to the domestic storage of carbon dioxide (CO2) in Hungary dates back several decades. Currently, as part of a project led by geologist Dr. György Falus, Hungarian researchers are conducting studies to understand the processes that take place under the influence of CO2 that in the long term may contribute to the development of domestic CO2 capture technology. In order to understand these processes, experts performed stabilizer isotope studies in the Mihály-Répcelak Reservoir, an area where CO2 accumulates naturally, in order to determine the origin of the minerals in the reservoir rock. Experts from the Hungarian Mining and Geological Survey, the Lithosphere Fluid Research Laboratory of Eötvös Loránd University, the Scottish Universities Environmental Research Centre and the ELKH Centre for Astronomy and Earth Sciences (CSFK) are participating. Studies on the latest research findings have recently appeared in the prestigious international journals Chemical Geology and Environmental Earth Sciences.

Due to the development of technology and the growing needs of society, the number of point sources of CO2 emissions in the world – such as factories, power plants and means of transport – has risen sharply since the Industrial Revolution. This has significantly increased the concentration of CO2 in the atmosphere, from 280 ppm to nearly 413 ppm (NOAA/ESRL, 2021). In order to reduce CO2 emissions from human activities, a number of technologies are now available to meet the targets set out in international conventions. One of the most efficient of these is CCUS (Carbon Capture, Utilization and Storage) technology, which involves capturing CO2 from industrial point sources and transporting it to various geological formations. Such formations include cultivated natural hydrocarbon deposits and deep saline reservoirs.

Perspectives for geological storage of CO2 in Hungary

One of the most critical factors in the geological storage of CO2 is the issue of safety, i.e. the selection of appropriate storage systems, as not all geological formations are suitable for this purpose. Knowledge of the subsurface processes resulting from the sequestration of CO2 and the predictability and accurate traceability of these processes are of paramount importance. Researchers can help understand these physicochemical processes by studying the complex occurrence of natural CO2. There are several natural CO2 occurrences in Hungary, where CO2 has been preserved on a geological time scale without leakage (Figure 1). One of the most significant of these is the Mihályi-Répcelak territory, which is also the most studied area.

Figure 1: Territorial distribution of formations suitable for CO2 storage in Hungary (Falus et al., 2011)

The process of trapping CO2 is influenced by a number of factors, including local variations in rock mineral composition, variability in porosity, and the dynamics of regional water flows. These factors and the processes they influence are best studied in the individual natural CO2 accumulation areas, otherwise known as carbon sinks. Such formations include the sandstone and conglomerate reservoirs in the Mihályi-Répcelak area, which are unique in the world. Their study shows that the lithostratigraphic characteristics of rock influence the spatial extent and extent of CO2-induced processes, which is why understanding these phenomena is highly significant.

The origin of fluids and the processes that take place during storage, the importance of stable isotopes

In addition to detailed petrographic studies, it was necessary to use geochemical methods to understand the origin of some of the mineral phases and fluids that can be identified in the reservoir rock and the processes that take place in the rock-pore fluid system due to the large amount of CO2 influx. In the studies presenting the results of the research, stabilizer isotope studies played a major role, as they were used by experts to determine which minerals may have been formed as a result of the CO2 influx and which were present before that.

Figure 2: Scattered electron image of sandstone containing dawsonite in the Mihályi-Répcelak area: Daw = dawsonite, Ank = anchorite, Q = quartz, Ms = muscovite, Dol = dolomite, Kln = kaolinite, Sd = siderite (Cseresznyés et al., 2021)

The carbon isotope composition of dawsonite [NaAlCO3 (OH) 2], a special mineral in the rock (Figure 2), suggests that the origin of the CO2-producing CO2 is the same as the free carbon dioxide in the reservoir flowing from the Earth’s mantle, so this mineral was clearly created by the influx of CO2. Based on the carbon isotope composition of another iron-containing carbonate mineral, siderite [FeCO3], the researchers were able to distinguish two different generations, one of which was formed by a large amount of CO2 similar to dawsonite, while the other may have been formed earlier (Fig. 3).

Figure 3: Carbon isotope composition of CO2 in equilibrium with carbonate minerals in the Mihályi-Répcelak reservoir (Cseresznyés et al., 2021)

In addition to the analysis of carbonaceous minerals, CSFK researchers were the first in the world to determine the stabilizer isotope composition of dawsonite, i.e. the 2H/1H ratio of bound hydrogen, supplemented by the clay mineral kaolinite formed with dawsonite. Based on the data, the researchers drew conclusions about the origin of the pore water and the processes that affect its composition. They found that the carbon dioxide flowing from the deep mixed with the rainwater leaching from the surface present in the rock system, and that this solution then reacted with the rock. It was from this rock-solution reaction that the dawsonite was produced. The process is a geological analogy for the capture of artificially sequestered carbon dioxide, thus providing important information for the development of domestic carbon capture technology.