Regional-scale brine migration along vertical pathways due to CO<sub>2</sub> injection – Part 2: a simulated case study in the North German Basin

Abstract

Abstract. Saltwater intrusion into potential drinking water aquifers due to the injection of CO2 into deep saline aquifers is one of the hazards associated with the geological storage of CO2. Thus, in a site-specific risk assessment, models for predicting the fate of the displaced brine are required. Practical simulation of brine displacement involves decisions regarding the complexity of the model. The choice of an appropriate level of model complexity depends on multiple criteria: the target variable of interest, the relevant physical processes, the computational demand, the availability of data, and their uncertainty. In this study, we set up a regional-scale geological model for a realistic (but not real) on-shore site in the North German Basin with characteristic geological features for that region. A major aim of this work is to identify the relevant parameters controlling saltwater intrusion in a complex structural setting and to test the applicability of different model simplifications. The model that is used to identify relevant parameters fully couples flow in shallow freshwater aquifers and deep saline aquifers. This model also includes variable-density transport of salt and realistically incorporates surface boundary conditions with groundwater recharge. The complexity of this model is then reduced in several steps, by neglecting physical processes (two-phase flow near the injection well, variable-density flow) and by simplifying the complex geometry of the geological model. The results indicate that the initial salt distribution prior to the injection of CO2 is one of the key parameters controlling shallow aquifer salinization. However, determining the initial salt distribution involves large uncertainties in the regional-scale hydrogeological parametrization and requires complex and computationally demanding models (regional-scale variable-density salt transport). In order to evaluate strategies for minimizing leakage into shallow aquifers, other target variables can be considered, such as the volumetric leakage rate into shallow aquifers or the pressure buildup in the injection horizon. Our results show that simplified models, which neglect variable-density salt transport, can reach an acceptable agreement with more complex models.

Exploring the Impacts of Source Condition Uncertainties on Far‐Field Brine Leakage Plume Predictions in Geologic Storage of CO2: Integrating Intermediate‐Scale Laboratory Testing With Numerical Modeling

“…; Birkholzer et al., 2009; Gupta & Yadav, 2020; Jones et al., 2015; Qafoku et al., 2017). Contaminating the USDWs can occur through two mechanisms; (a) the poor‐quality native brine leaks upward and mixes with the potable groundwater or (b) the leaking storage‐formation fluids trigger the migration of the overlying saline water affecting the shallow aquifer quality (Delfs et al., 2016; Kissinger et al., 2017; Tillner et al., 2013, 2016).…”

Regional-scale brine migration along vertical pathways due to CO<sub>2</sub> injection – Part 1: the participatory modeling approach

“…The results are based on the revised geological and numerical model that was designed within the PM process. A more extensive discussion of these results is provided in Kissinger et al (2017). The main findings can be summarized as follows: Hydrol.…”

“…The preliminary numerical modeling results were then critically discussed by stakeholders within the expert workshop. Subsequently, modeling results were finalized (Kissinger et al, 2017).…”