Research Project CX2

Carbon Capture and Storage (CCS) will have to be utilized in the near future to mitigate the global climate change by reducing the emissions of CO2, which is thus not emitted, but rather injected in the subsurface. Also for an optimal use of renewable energies, gas will need to be stored underground to buffer the fluctuating supply and demand. Such gases may be either natural gas or synthetic methane (CH4) or hydrogen (H2) or simply compressed air. Optimal reservoirs for gas storage are located in variable depths, depending on geological conditions and the gas which is to be stored. They are separated from overlying aquifers by an impermeable cap rock. For a successfully operating storage, it is a necessity to avoid any leakage of stored gases. 

Thus, further research to improve sealing technologies used to mitigate potential leakage, like the injection of cement or biomineralization is necessary, which requires understanding of both the (bio-) and geochemical reactions and the transport of the different components in both gas and water phase. Microbially induced calcium carbonate precipitation (MICP) is currently becoming an established method of biomineralization and in previous projects, a numerical model was developed for this biomineralization method. However, precipitation may also be induced by injection of extracted or plant-based sources of urease or at elevated temperatures. Experimental investigations of our collaborators at Montana State University (MSU) in Bozeman, USA focus on ureolysis-based methods involving enzyme (EICP) or elevated temperatures (TICP) instead of microbes to incude calcium carbonate precipitation. The applicability of a certain method is largely determined by the depth below ground surface and the local geothermal gradient. MICP relies on the activity of living bacterial cells, which only thrive within a limited temperature range (<45°C). This temperature range may include the shallow subsurface, where CH4 or natural gas may be stored, but is lower than the temperatures commonly present at depths suitable for CO2 storage. As a consequence, other technologies such as EICP or TICP have to be developed and demonstrated in the field.

Within this project, a numerical model for enzymatically induced calcium carbonate precipitation (EICP) was successfully developed (see Hommel et al. 2020). The model was developed in close cooperation with the collaborators Robin Gerlach, Al Cunningham and Adrienne Phillips at Montana State University, who provided experimental data for the necessary kinetics as well as the column experiment data for model calibration and validation. 

Additionally, the project aims at improving the models for induced calcium carbonate precipitation (ICP) by accounting for the change in two-phase flow properties due to ICP. To this end, experimental investigations with the support of the SFB1313, especially Holger Steeb, have been and still are carried out, mineralizing glass bead columns using EICP to various degrees. After mineralization, the columns are scanned and the obtained images processed to generate pore networks.

These pore networks can then be used to determine capillary-pressure-saturation relations and relative-permeability-saturation relations by pore-network modelling. Comparing the results of the pore-network modelling for unmineralized column and columns mineralized to various degrees, the impact of calcium carbonate on the two-phase flow properties can be parameterized and then be included into the models for e.g. EICP (see Hommel et al. 2020).