Changes of petrophysical, petrological, mineralogical, mechanical and chemical parameters were studied on sandstones from the Hessian Depression and sandstones from Neidenbach (Eifel) before and after alteration with CO2. The experiments were performed in a wide pressure and temperature range (10 <P <20 MPa; 100 <T <200 °C) to cover the conditions of most promising deep saline aquifers, with the consequence that CO2 is above the critical point and thus in supercritical state (scCO2). The experiments lasted from 10 days up to more than 6 months. In repeated experiments samples were analysed after every 2 to 6 weeks reaction time. Experiments were performed in two different setups: (1) the pore volume was fully saturated with a CO2-saturated artificial brine (3M NaCl-solution), and (2) the pore volume was partially saturated with brine and in direct contact with wet CO2. Petrophysical properties, mineralogical composition, heterogeneity and anisotropy of the samples were measured before and after the experiments with scCO2. Initial values of the untreated samples vary as follows: quartz from < 50 to > 85 wt%, density from 2.62–2.70 g/cm3, porosity from < 10 up to > 25 %, and permeability from <10−17 up to 10−12 m2. In both experimental setups, porosity increased by less than 2 vol%. The increase in permeability was less than one order in magnitude for setup (1), and more than 1.5 orders of magnitude for setup (2). The mineralogical composition was unchanged within the detection limit of X-ray powder diffraction (XRD), whereas X-ray fluorescence analysis (XRF) indicated mobilisation of Ca, Mg, Al, and K. In addition, dissolution was confirmed by the chemical analysis of recovered artificial brines using inductively-coupled plasma optical-emission-spectroscopy (ICP-OES), which showed an increase in the concentration of Ca, Mg, Al, and K after the scCO2-experiments. Partial solution of feldspar and clay was detected by optical inspection and SEM analysis. Low frequency electrical conductivity experiments (spectral induced polarization, SIP) exhibited both a significant increase in conductivity, which could be explained by dissolution at narrow pore throats thus causing a higher degree of interconnection of the pore system, and a shift of the phase angle, which indicates changes of the geometry of the pore-surface area. The uniaxial compressive strength was measured before and after scCO2-treatment on a set of homogeneous sandstones from Neidenbach (Eifel, Germany). These data were compared with natural analogues, e.g. bleached and unbleached sandstones from the Hessian depression. The uniaxial compressive strengths of untreated and scCO2-treated samples were found to fit the range reported for sandstones.

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