Abstract
Abstract. Injection of fluids into deep saline aquifers causes a pore pressure increase in the storage formation, and thus displacement of resident brine. Via hydraulically conductive faults, brine may migrate upwards into shallower aquifers and lead to unwanted salinisation of potable groundwater resources. In the present study, we investigated different scenarios for a potential storage site in the Northeast German Basin using a three-dimensional (3-D) regional-scale model that includes four major fault zones. The focus was on assessing the impact of fault length and the effect of a secondary reservoir above the storage formation, as well as model boundary conditions and initial salinity distribution on the potential salinisation of shallow groundwater resources. We employed numerical simulations of brine injection as a representative fluid. Our simulation results demonstrate that the lateral model boundary settings and the effective fault damage zone volume have the greatest influence on pressure build-up and development within the reservoir, and thus intensity and duration of fluid flow through the faults. Higher vertical pressure gradients for short fault segments or a small effective fault damage zone volume result in the highest salinisation potential due to a larger vertical fault height affected by fluid displacement. Consequently, it has a strong impact on the degree of shallow aquifer salinisation, whether a gradient in salinity exists or the saltwater–freshwater interface lies below the fluid displacement depth in the faults. A small effective fault damage zone volume or low fault permeability further extend the duration of fluid flow, which can persist for several tens to hundreds of years, if the reservoir is laterally confined. Laterally open reservoir boundaries, large effective fault damage zone volumes and intermediate reservoirs significantly reduce vertical brine migration and the potential of freshwater salinisation because the origin depth of displaced brine is located only a few decametres below the shallow aquifer in maximum. The present study demonstrates that the existence of hydraulically conductive faults is not necessarily an exclusion criterion for potential injection sites, because salinisation of shallower aquifers strongly depends on initial salinity distribution, location of hydraulically conductive faults and their effective damage zone volumes as well as geological boundary conditions.
Highlights
Carbon Capture and Storage (CCS) can contribute to the reduction of global anthropogenic carbon dioxide emissions
We demonstrate that pressure propagation in the reservoir determines the intensity and duration of fluid flow through the faults and shallow aquifer salinisation, mainly controlled by the lateral model boundary settings and the effective fault damage zone volume
The potential of freshwater salinisation is low for greater effective fault damage zone volumes or fault lengths, because the origin depth of the fluids displaced into the shallow aquifer is located a few decametres below the shallow aquifer in maximum due to relatively low vertical pressure gradients
Summary
Carbon Capture and Storage (CCS) can contribute to the reduction of global anthropogenic carbon dioxide emissions. Different geological underground formations have been suggested as target storage sites, such as deep saltwater-bearing aquifers (saline aquifers) providing the worldwide largest storage potential as part of the Earth’s widely distributed sedimentary basins (IPCC, 2005). Shallow aquifers in sedimentary basins can comprise considerable freshwater resources, which in turn are generally of great importance for regional water supplies. Brine displacement due to the elevated pore pressure in the storage formation is one potential risk of CO2 storage in deep saline aquifers. Saline fluids could reach shallower freshwater aquifers through different migration pathways, and significantly impair groundwater quality. Fault zones are of particular importance, as they might transect several cap rocks, and can provide large-scale permeable conduits between aquifers at different
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