<p>One of the major keys to the success of the carbon capture and storage (CCS) is understanding the geochemical effects that CO<sub>2</sub> has on the storage reservoir. The injection of CO<sub>2</sub> into the reservoir disturbs geochemical equilibrium as it induces acid-generation reactions with subsequent CO<sub>2</sub>-brine-mineral interactions, including dissolution of certain host minerals and precipitation of secondary minerals. The mineral precipitation, especially precipitation of carbon-bearing minerals in geological formations, is generally a favorable for CO<sub>2</sub> trapping mechanism that ensures long-term geologic CO<sub>2</sub> sequestration. These precipitates, however, may clog the wellbore and its surroundings, followed by loss of injectivity.</p><p>The current study is dedicated towards a better understanding of the geochemistry of the geological CO<sub>2</sub> storage based on the Ketzin CO<sub>2</sub> pilot site. The Ketzin CO<sub>2</sub> storage site, the first on-shore geological CO<sub>2</sub> storage site in the European mainland, is demonstrated a safe and reliable CO<sub>2</sub> storage operation after injection of about 67-kilo tons of CO<sub>2</sub> and offers the unique opportunity to work on data sets from all storage life-cycle (Martens et al., 2014). Through both field measurement and modeling studies, this contribution aims to explore the secondary mineral precipitation mechanisms and identify the major influential factors during the CO<sub>2</sub> sequestration. This approach supports the H2020 project SECURe establishing best practice in baseline investigations for subsurface geoenergy operations, underpinned by data of pilot and research-scale sites in Europe and internationally. The secondary minerals solubility was investigated as a function of the reservoir temperature, pressure, and CO<sub>2</sub> concentration, which occurred in the reservoir. Special focus is set to sulfate minerals, as field evidence exists that gypsum precipitates as a result of reservoir exposition to CO<sub>2</sub>. Batch modeling was performed using the PHREEQC code version 3 (Parkhurst and Appelo, 2013) with the Pitzer database (pitzer.dat). The coupling interface OGS#IPhreeqc (He et al., 2015) applied reactive transport modeling, and the coupled reactive-transport processes in the reservoir with complex chemistry can be modeled. Our results suggest that the gypsum precipitation was found to increase as CO<sub>2</sub> concentration ascends. However, no significant porosity and permeability alterations are observed since the gypsum precipitation acts as a Ca<sup>2+</sup> sink and leads to further carbonate dissolution. The results highlight the high reactivity of the near-well zone due to CO<sub>2</sub> injection and emphasize the need to be monitored in the injection well to avoid the potential formation of gypsum, which could lead to well clogging.</p><p>He, W., Beyer, C., Fleckenstein, J.H., Jang, E., Kolditz, O., Naumov, D., Kalbacher, T., 2015. A parallelization scheme to simulate reactive transport in the subsurface environment with OGS#IPhreeqc 5.5.7-3.1.2. Geosci. Model Dev. 8, 3333-3348.</p><p>Martens, S., M&#246;ller, F., Streibel, M., Liebscher, A., 2014. Completion of Five Years of Safe CO2 Injection and Transition to the Post-closure Phase at the Ketzin Pilot Site. Energy Procedia 59, 190-197.</p><p>Parkhurst, D.L., Appelo, C.A.J., 2013. Description of input and examples for PHREEQC version 3: a computer program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations, Techniques and Methods, Reston, Virginia, USA, p. 519.</p>
Wellbore integrity at the Krechba Carbon Storage Site, In Salah, Algeria: 2. Reactive transport modeling of geochemical interactions near the Cement–Formation interface
