Caprock analysis from the Mihályi-Répcelak natural CO2 occurrence, Western Hungary

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 Miha´lyi-Re´pcelak 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