This study reveals the first analyses of the composition and activity of the microbial community of a saline CO 2 storage aquifer. Microbial monitoring during CO 2 injection has been reported. By using Fluorescence in situ Hybridisation (FISH), we have shown that the microbial community was strongly influenced by the CO 2 injection. Before CO 2 arrival, up to 6x10 6 cells ml -1 were detected by DAPI-staining at a depth of 647 m below the surface. The microbial community was dominated by the domain Bacteria that represented approximately 60 to 90 % of the total cell number, with Proteobacteria and Firmicutes as the most abundant phyla comprising up to 47 % and 45 % of the entire population, respectively. Both the total cell counts as well as the counts of the specific physiological groups revealed quantitative and qualitative changes after CO 2 arrival. Our study revealed temporal shifts in the microbial community from chemoorganotrophic to chemolithotrophic populations, as evidenced by the outcompetition of sulphate-reducing bacteria by methanogenic archaea. In addition, an enhanced activity of the microbial population after five months CO 2 storage indicated that the bacterial community was able to adapt to the extreme conditions of the deep biosphere and to the extreme changes of these atypical conditions.

No abstract

The onshore CO 2 -storage site Ketzin consists of one CO 2-injection well and two observation wells. Hydraulic tests revealed permeabilities between 50 and 100 mD for the sandstone rock units. The designated injection well Ktzi 201 showed similar production permeability. After installation of the CO 2 -injection string, an injection test with water yielded a significantly lower injectivity of 0.002 m 3 /d*kPa, while the observation wells showed an injection permeability in the same range as the productivity. Several possible reasons for the severe decline in injectivity are discussed. Acidification of the reservoir interval, injection at high wellhead pressure, controlled mini-fractures and back-production of the well are discussed to remove the plugging material to re-establish the required injectivity of the well. It has been decided to perform a nitrogen lift and analyse the back-produced fluids. Initially during the lift, the backproduced fluids were dark-black. Chemical and XRD analyses proved that the black solids consisted mainly of iron sulphide. Sulphate-reducing bacteria (SRB) were detected in fluid samples with up to 10 6 cells/ml by fluorescent in situ hybridisation (FISH) indicating that the formation of iron sulphide was caused by bacterial activity. Organic compounds within the drilling mud and other technical fluids were likely left during the well completion process, thus providing the energy source for strong proliferation of bacteria. During the lift, the fraction of SRB in the whole bacterial community decreased from approximately 32 % in downhole samples to less than 5 %. The lift of Ktzi 201 succeeded in the full restoration of the well productivity and injectivity. Additionally, the likely energy source of the SRB was largely removed by the lifting, thus ensuring the long-term preservation of the injectivity.2

No abstract

No abstract