Fault damage zone volume and initial salinity distribution determine intensity of shallow aquifer salinisation in subsurface storage

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.

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

“…The latter implies the need to have good knowledge of the a priori salt distribution. This is in good qualitative agreement with findings by (Tillner et al, , 2016, , and (Walter et al, 2013). The results also imply that it is unlikely to observe sudden and strong increases in the salt concentration due to CO 2 injection at locations where elevated concentrations have not been an issue before.…”

“…Kissinger et al: Part 2: A simulated case study in the North German Basin 2753 ary conditions and hydrogeological parameterization but also with respect to the initial salt concentration in the system. (Tillner et al, 2016) confirmed this finding, while additionally stressing the importance of the fault-damage-zone volume determining the intensity of the salinization of shallow freshwater aquifers.…”

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

“…Therefore, the calculation of salt transport into shallow aquifers is not only uncertain with respect to the boundary conditions and hydrogeological parametrization but also with respect to the initial salt concentration in the system. Tillner et al (2016) confirm this finding, while additionally stressing the importance of the fault-damage-zone volume determining the intensity of the salinization of shallow freshwater aquifers.…”

Fault Reactivation Can Generate Hydraulic Short Circuits in Underground Coal Gasification—New Insights from Regional-Scale Thermo-Mechanical 3D Modeling

“…The risks include gas losses, contaminant migration and uncontrolled water influx [3]. However, recent numerical simulations on fault reactivation are mainly associated to hydraulic fracturing [67], CO 2 storage [68,69] and geothermal heat recovery [70], whereas the process of fault reactivation was not yet adequately represented in the field of underground coal gasification [27].…”