Determination of fluid flow changes and mineral alterations in caprocks due to CO2 treatment

Category Geochemistry
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Wollenweber, Jens; Alles, Sascha; Busch, Andreas; Kronimus, Alexander; Stanjek, Helge; Krooss, Bernhard M.
Holding Date 06 October 2008

One major concern in any geological CO2 storage scenario is the sealing efficiency of low-permeable sequences overlying potential storage reservoirs. Interaction of CO2 with these caprocks may lead to alterations of the mineralogical composition and changes in fluid transport properties. Using a combination of experimental (petrophysical and mineralogical) methods, transport processes of CO2 in pelitic caprocks (shales and marls) and its interactions with the mineral phase have been studied. Single and multiphase permeability tests, gas breakthrough and diffusion experiments were conducted under in-situ p/T conditions on cylindrical plugs (28.5 and 38.5 mm diameter, 10 to 20 mm thickness). Single-phase (water) flow tests were performed to assess permeability coefficients and to ensure complete water-saturation before gas breakthrough and diffusion experiments. The tests were repeated after each diffusion and gas breakthrough experiment to detect permeability changes resulting from CO2/mineral reactions. Repetitive capillary gas breakthrough experiments were conducted with helium and CO2 to test for reproducibility and to detect petrophysical changes of the rock samples resulting from CO2/water/rock interactions. The CO2 gas breakthrough and diffusion experiments resulted in enhanced water permeability. The evaluation of the gas breakthrough tests revealed an increase in effective gas permeability while gas capillary entry pressures decreased, indicating a lower capillary sealing efficiency as a result of repetitive exposition to CO2. Mass balance calculations performed for an experiment with a clay-rich marl sample showed that significant amounts of CO2 were stored in the sample during the first breakthrough tests. The quantities significantly exceeded the amounts of CO2 that could be possibly dissolved in the pore water under these conditions. High irreversible CO2 storage capacities in shales and marls were also evidenced by the diffusion tests. In order to substantiate these findings, high-pressure CO2 sorption experiments were performed on powdered samples. Comparison of X-ray diffraction data of the original and the treated (after CO2 diffusion) samples, showed a complete loss of anorthite associated with the formation of mainly calcite. In order to further elucidate the mineralogical alterations resulting from exposition to CO2, comprehensive microscopic investigations (microprobe, electron microscopy, and cathodoluminescence) are presently being performed.