Mineral changes in CO2 experiments — Examples from Danish onshore saline aquifers.

Abstract

The onshore parts of the Norwegian-Danish Basin and the northern rim of the North German Basin contain extensive saline aquifers of potential interest for CO2 storage. The Triassic Bunter Sandstone and Skagerrak formations comprise arkoses, subarkoses and lithic arkoses, with feldspar being dominated by K-feldspar. The authigenic phases are dominated by carbonates (mainly calcite and dolomite), anhydrite, clay minerals. The Upper Triassic–Lower Jurassic shallow marine and paralic sandstones (Gassum Formation) comprises mainly subarkoses and sublitharenites. The authigenic phases are typically carbonates (typically siderite and ferroan dolomite), clay minerals and pyrite. The Middle Jurassic fluvial, estuarine and shallow marine sandstones (Haldager Sand Formation) is characterised by quartz arenites and subarkoses. The authigenic phases comprise clay minerals, pyrite, carbonate, quartz and feldspar. Experiments in the laboratory were designed and performed in order to investigate the response of these mineralogical different reservoir sandstones to CO2 storage. In the batch experiments, the sandstone plugs were exposed to CO2 saturated synthetic fluid at present day reservoir temperatures and pressures (70 °C at 20 MPa) for a period of 7–14 months. Parallel experiments with N2 gas show minor adjustments of the chemistry of the pore fluids until equilibrium is reached prior to first sampling (∼30 days). After 7 months of experiment the fluids show increased values of HCO3−, Ca2+ and Mg2+ in the Skagerrak and Bunter Sandstone formations. Whereas the Gassum formation (and to a lesser extend the Haldager Sand Formation) show increased values of HCO3−, total Fe and Mn+ in the fluid. These changes are probably related to the dissolution of the typical carbonates in the different formations.