oa Microbial Aspects of Mineral Dissolution and Precipitation During CO2 Storage– First Results of Long-term CO2 Exposure

Microorganisms play a major role in mineral distribution and redistribution of elements within the earth’s crust. The storage of CO2 may affect the local composition of organic and inorganic components of reservoir systems, consequently influencing microbial activities. Biologically mediated reactions like mineral dissolution and precipitation are examples of how a reservoir could be affected. Within the frame of CO2SINK (Schilling et al., 2009) long-term experiments under in situ P-T-conditions are performed in order to investigate the impact of chemically and microbially induced dissolution and precipitation reactions during CO2 storage. Freshly drilled sandstone sections from the target reservoir at Ketzin from a depth of about 630 m were acquired and have been incubated together with synthetic brine in high pressure vessels at 5.5 MPa and 40 °C since September 2007. Compared to the initial composition, Ca2+, Mg2+ and K+ concentrations are increased and exceed those of the reservoir fluid. Observed effects may be caused by mineral dissolution in response to CO2 exposure. Since the total dissolved solids concentration (TDS) of the synthetic brine is about 20 % lower than that of the Ketzin reservoir fluid, additional information is still needed to only detect changes exclusively caused by CO2. In consistence with the inorganic declines, XRD, SEM and EMP analysis suggests feldspar dissolution. Organic acids are marker for the presence of active microorganisms. They are intermediate products of the bacterial metabolism, and are metabolised to gain energy. If excreted, organic acids can locally decrease the pH at the bacterial attachment site and may support mineral dissolution. Untreated sandstone samples showed an organic acid concentration of about 50 mg/kg, yielded by fresh water extraction. The concentration of organic acids in the vessel fluid was lower (19-87 µg / ml compared to about 250 µg / ml) than the expected concentration, indicating microbial degradation. The composition of the microbial community mainly consists of chemoheterotrophic bacteria (Methylophilales bacterium, Rhizobium radiobacter, Arthrobacter, Sphingomonas) and hydrogen oxidizing bacteria (Ralstonia, Hydrogenophaga), gaining their energy from the oxidation of organic molecules and hydrogen, respectively. During the long-term exposure experiment only minor changes of the microbial community composition were observed, reflecting the adaptation of the microorganisms to the modified conditions. The analysis of microbial metabolic activities and SEM/EDX studies will help to identify and quantify bacterial processes and to assess their long-term influence on storage efficiency.