Geological and geophysical properties of cap rock in a natural CO2 occurrence, Mihályi-Répcelak area, Western Hungary

The physical and geochemical consistency of the cap rock is primarily important for safe geological storage of CO2. As a consequence of CO2 injection reactions took place between the minerals of the reservoir, the cap rock and CO2 saturated pore water. These reactions may change the mineral composition and petrophysical properties of the storage reservoir as well as the cap rock that provides the only physical barrier that retains carbon dioxide in the target reservoir formation. Study of the natural CO2 occurrences delivers information to understand which properties of a cap rock provide the sustainable closure and retainment. Knowledge of the long term effect of CO2 on the behavior of the cap rock is an important input in the selection procedure of a potential CO2 injection site. Yet, very few data exist on geochemical properties and reactivity of the cap rocks. During normal commercial operations the reservoir is typically cored, but not the cap rock. This study may enhance our knowledge about possible mineralogical reactions, which can occur in clayey-aleuritic cap rocks. The Mihályi-Répcelak natural CO2 occurrence is believed to be leakage safe. There is no known seepage on the surface. It is suggested that the aleuritic clay rich cap rock occurring at the natural reservoir can stop CO2 migration into other reservoirs or to the surface. The most important characteristics of cap rocks that they have low permeability (<0.1 mD) and porosity (eff.por. = 4%) and high clayeyness (approx. 80%). However, we demonstrate that in addition to these parameters the geochemical properties of cap rock is also important. In order to characterize the natural CO2 occurrence, we applied the following analysis, like XRD, FTIR, SEM. The petrophysical properties are determined from the interpretation of geophysical well-logs and grain size distribution. The most important result of this study that adequate petrophysical properties do not completely define the suitability of a cap rock. The effective porosity (~4 %), permeability (0.026 mD) and clayeyness (~80%) data imply that the studied aleurolites are good cap rocks. The mineral composition of cap rock is similar to that of reservoir rock, however, the ratio of components is different. The mineralogical analysis and petrography yield to the reaction between CO2 and the cap rocks. The most visible effect of CO2 presence is the dawsonite precipitation after albite dissolution within the cap rocks. Therefore, the CO2 may migrate through the cap rocks in geological time scale, however the total system could be leakage safe

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

The relevance of dawsonite precipitation in CO2 sequestration in the Mihályi-Répcelak area, NW Hungary

A natural CO2 reservoir system with sandstone lithology has been studied in NW Hungary due to similarities with a large saline reservoir formation widespread in the whole of the Pannonian Basin (Central Europe), suggested to be one of the best candidates for industrial CO2 storage. A range of analytical techniques has been used on core samples from the CO2-containing sandstone layers representing a wide pressure (90-155 bar), temperature (79-95°C) and pore fluid composition range (TDS between 18 000 – 50 700 mg/l) to identify the mineralogy and textural characteristics of the natural reservoir. The only clear CO2-related feature in the studied lithology was the occurrence of dawsonite (NaAlCO3(OH)2) always in close textural relation with albite. This is in clear agreement with our geochemical modelling results, which also underline the presence of albite as a precondition for the crystallization of dawsonite at the given p-T-X conditions. Our results suggest that, at least in the Pannonian Basin, dawsonite may be an important mineral to safely sequester industrial CO2 in the subsurface

The relevance of dawsonite precipitation in CO2 sequestration in the Mihályi-Répcelak area, NW Hungary

A natural CO2 reservoir system with sandstone lithology has been studied in NW Hungary due to similarities with a large saline reservoir formation widespread in the whole of the Pannonian Basin (Central Europe), suggested to be one of the best candidates for industrial CO2 storage. A range of analytical techniques has been used on core samples from the CO2-containing sandstone layers representing a wide pressure (90-155 bar), temperature (79-95°C) and pore fluid composition range (TDS between 18 000 – 50 700 mg/l) to identify the mineralogy and textural characteristics of the natural reservoir. The only clear CO2-related feature in the studied lithology was the occurrence of dawsonite (NaAlCO3(OH)2) always in close textural relation with albite. This is in clear agreement with our geochemical modelling results, which also underline the presence of albite as a precondition for the crystallization of dawsonite at the given p-T-X conditions. Our results suggest that, at least in the Pannonian Basin, dawsonite may be an important mineral to safely sequester industrial CO2 in the subsurface