Geological and mineralogical investigation of Rotliegend gas reservoirs in the Netherlands and their potential for CO2 storage
Underground storage of CO2 is receiving growing attention by the industry as an option to reduce CO2 emissions. Rotliegend sediments of Permian age exhibit under certain conditions good reservoir quality and therefore may offer a suitable candidate for underground storage. The current study aims at analysing Rotliegend reservoir rocks in terms of mineralogical features, diagenetic mineral types, the determination of the availability of minerals within the rock and mineral (reactive) surface areas which are exposed to open pores and the integration of these results in CO2-water-rock simulations. Target areas are Rotliegend sandstones from the northeast Netherlands which were deposited within the North German Basin and were buried to about 2000-3000 m depth. These sandstones are heterogeneously built up by different detrital and authigenic minerals. Amongst other things, the size, distribution and accessibility of these minerals within a sandstone body regulate the availability of chemical species for CO2-water-rock interactions in the pore space and was analysed by petrographic investigations and image analysis on thin sections. The results show that the volumetric composition of the samples is dominated by the mineralogy of detrital grains (e.g. quartz). On the other hand, the effective mineralogy is defined as the percentage amount of minerals which are exposed to open pores and is a function of the presence of grain covering minerals (e.g. Fe-oxide) and pore-filling cements (e.g. Carbonates). These data were integrated in simulations of CO2-water-rock interactions with the software code PHREEQC. The results show that the initial amounts of K-feldspar, carbonate and kaolinite regulate the availability of species within the formation water and the final amount of trapped CO2 in carbonates. Furthermore, reactive transport modelling coupled with geochemical modelling indicates only minor mineral reactions within the first 150 years.