Long-term impact of CO2 on the stability of mineral assemblages in porous reservoir sandstones - Analogue study in natural CO2 reservoirs from Central Europe.

DFG (AD 315/1-1) project , since 10/2005

Dr. Dirk Adelmann / Prof. Dr. Reinhard Gaupp

Natural occurrences of CO2 in reservoirs and deep aquifers suggest that storage underground is a viable long-term option to reduce emission of greenhouse gas in the atmosphere. Questions arise as to what mineralogical and physical effects elevated concentrations of CO2 will have on the reservoir rocks. A number of large scale natural systems of CO2 accumulation occur in Central Europe, which provide an ideal laboratory to investigate the long-term impact of CO2 on siliciclastic reservoir sandstones. As part of this project, three of them are chosen to be studied in order to understand what water-rock-CO2 interaction reactions take place over extended periods of time, what accounts for the stability of these CO2 accumulations and the relevance of these natural analogues to risk assessment. The test areas representing different P-T conditions, lithologies and CO2 contents are Southwestern Lower Saxony Basin (Upper Carboniferous sediments), the Western Thuringian Basin (hydrocarbon-bearing Middle Buntsandstein) and the Eifel mountain belt (Devonian sediments).

To identify the interactions of porous reservoir sandstones and associated mudrocks with CO2, emphasis is placed on the identification of traces of CO2 reactivity during the diagenesis. To achieve these goals we will quantify the mineral assemblages by using e.g. thin section, SEM, TEM, XRD, XRF, cathodoluminescence and electron microprobe techniques. Detrital composition and authigenic mineralogy of CO2-bearing sediments will be compared to CO2-unaffected sediments of similar geological formations and lithofacies. The comparison will give relevant information about diagnostic petrographic criteria for CO2 interaction in clastic reservoirs. Carbon- and oxygen isotope studies of authigenic carbonates and fluid inclusion analyses will give us additional estimates on the exposure time of CO2 and the temperature/fluid conditions of diagenetic mineral formation. Moreover, we will investigate the effect of mineral corrosion and changes in reactive surfaces at mm- to nm-scales. Our aim is to determine leaching morphologies of instable detrital components and cements as well as small-scale corrosion effects on grain surfaces, producing new reactive surfaces, microporosity and leaking paths. Finally, geochemical modelling will be used to (semi-)quantitatively predict the gas/liquid/rock interactions in the natural CO2 accumulations. The modelling should consider redox processes, organic matter, kinetics of chemical interactions and the CO2 solubility dependence on pressure, temperature and salinity. The long-term calibration for the models will then allow predictions of how CO2 injected into geological reservoirs is likely to behave and what risks of leakage are possible. It will determine the optimum metal availability conditions for mineral trapping of CO2.

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