Evaluation of secondary mineral precipitation by reactive transport modeling at the Ketzin CO2 storage site, Germany

One of the major keys to the success of the carbon capture and storage (CCS) is understanding the geochemical effects that CO₂ has on the storage reservoir. The injection of CO₂ into the reservoir disturbs geochemical equilibrium as it induces acid-generation reactions with subsequent CO₂-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 CO₂ trapping mechanism that ensures long-term geologic CO₂ sequestration. These precipitates, however, may clog the wellbore and its surroundings, followed by loss of injectivity.

The current study is dedicated towards a better understanding of the geochemistry of the geological CO₂ storage based on the Ketzin CO₂ pilot site. The Ketzin CO₂ storage site, the first on-shore geological CO₂ storage site in the European mainland, is demonstrated a safe and reliable CO₂ storage operation after injection of about 67-kilo tons of CO₂ 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 CO₂ 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 CO₂ 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 CO₂. 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 CO₂ concentration ascends. However, no significant porosity and permeability alterations are observed since the gypsum precipitation acts as a Ca²⁺ sink and leads to further carbonate dissolution. The results highlight the high reactivity of the near-well zone due to CO₂ 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.

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.

Martens, S., Möller, 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.

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.