Abstract. Intrusion of deep saline waters into freshwater aquifers does not only endanger the regional drinking water supply, but also rivers and stagnant waters and their fauna are threatened by salinisation. The upwelling of highly mineralised saline waters in large parts of the North German Basin is favoured by the presence of Elsterian glacial erosion windows in the Lower Oligocene Rupelian Clay, the most important hydraulic confining unit in this region. Lower precipitation rates and decreasing groundwater levels as a consequence of global climate change, but also anthropogenic interventions, such as increasing extraction rates or the use of the deep geologic subsurface as a reservoir, decrease the pressure potential in the freshwater column and may possibly accelerate this primarily geogenic salinisation process in the coming years. Density-driven flow and transport modelling was performed in the scope of the present study to investigate the upwelling mechanisms of deep saline waters across Quaternary window sediments in the Rupelian. Simulation results show that the interactions between the groundwater recharge rate and anthropogenic interventions such as extraction rates of drinking water wells or the utilisation of the deep subsurface, have a significant influence on the groundwater pressure potential in the freshwater aquifer and associated saltwater upwelling. In all scenarios, salinisation is most severe in the sediments of the erosion windows. Hydraulically conductive faults also intensify salinisation if located nearside erosion windows or induce a more distributed or localised salinisation in aquifers with drinking water relevance in areas that do not intersect with erosion windows. A decline in groundwater recharge thereby significantly favours upward saltwater migration. The simulation scenarios further show that a decrease in groundwater recharge also results in freshwater salinisation occurring up to 10 years earlier, which underlines the need for waterworks to initiate effective countermeasures quickly and in time.
Estimating the efficiency and sustainability of geological subsurface utilization, i.e., Carbon Capture and Storage (CCS) requires an integrated risk assessment approach, considering the occurring coupled processes, beside others, the potential reactivation of existing faults. In this context, hydraulic and mechanical parameter uncertainties as well as different injection rates have to be considered and quantified to elaborate reliable environmental impact assessments. Consequently, the required sensitivity analyses consume significant computational time due to the high number of realizations that have to be carried out. Due to the high computational costs of two-way coupled simulations in large-scale 3D multiphase fluid flow systems, these are not applicable for the purpose of uncertainty and risk assessments. Hence, an innovative semi-analytical hydromechanical coupling approach for hydraulic fault reactivation will be introduced. This approach determines the void ratio evolution in representative fault elements using one preliminary base simulation, considering one model geometry and one set of hydromechanical parameters. The void ratio development is then approximated and related to one reference pressure at the base of the fault. The parametrization of the resulting functions is then directly implemented into a multiphase fluid flow simulator to carry out the semi-analytical coupling for the simulation of hydromechanical processes. Hereby, the iterative parameter exchange between the multiphase and mechanical simulators is omitted, since the update of porosity and permeability is controlled by one reference pore pressure at the fault base. The suggested procedure is capable to reduce the computational time required by coupled hydromechanical simulations of a multitude of injection rates by a factor of up to 15. P is transferred to the geomechanical simulator. An additional coupling parameter for non-isothermal studies is the temperature T. If this data transfer is occurring only in one direction (one-way), the resulting volumetric strain v will not be fed back to the multiphase flow simulator for updating porosity and permeability (Fig. 1 (a)). In case of large volumetric strains V occurring during the simulation, a one-way coupling may lead to inaccurate results (Cappa and Rutqvist, 2011; Chabab
<p>Excess electricity produced from renewables can be converted into CH<sub>4</sub> by consuming CO<sub>2</sub> and H<sub>2</sub> by means of the Power-to-Gas (PtG) technology<sup> [1]</sup>. Previous work indicates that subsurface storage of CO<sub>2</sub> and CH<sub>4</sub> can meet the projected energy storage requirements <sup>[1] [2]</sup>. However, gas mixing occurs if both gases are stored in the same reservoir <sup>[3]</sup>, and energy is lost if CH<sub>4</sub> is used as cushion gas when both gases are separately stored in different reservoirs <sup>[2]</sup>. Therefore, an innovative approach to overcome the limitation of aforementioned storage schemes is introduced in this study. For that purpose, the focus is on a double reservoir setting in one anticline system as it is commonly found in, e.g., the Northern German Basin. Here, the confining layer and preexisting or artificial hydraulic connections between the two reservoirs enable the operator to reduce energy losses and avoid gas mixing. We have elaborated a numerical multiphase flow model including the wellbore systems and reservoirs to study the fluid flow and beneficiary effects of pressure interaction between both reservoirs. Based on the geological and operational data of our regional showcase in Germany <sup>[4] [5]</sup>, the energy storage efficiency is quantified, and the potential benefits of the proposed storage scheme are evaluated. It shows that the production of CH<sub>4</sub> increases by 68% over twenty years of injection and production. Furthermore, the factors that affect storage efficiency are analyzed to provide information for the optimization of PtG-based subsurface energy storage systems. The simulation can be applied to different geological systems and for parameter sensitivity studies to reduce energy losses and improve storage efficiency.</p><p>&#160;</p><p>&#160;</p><p><strong>Keywords:</strong> Power-to-Gas; Subsurface gas storage; Carbon dioxide; Methane</p><p>&#160;</p><p>[1] K&#252;hn M, Nakaten N, Streibel M, Kempka T. CO<sub>2</sub> Geological storage and utilization for a carbon neutral &#8220;Power-to-gas-to-power&#8221; cycle to even out fluctuations of renewable energy provision. Energy Procedia. 2014; 63:8044-9.</p><p>[2] Ma J, Li Q, K&#252;hn M, Nakaten N. Power-to-gas based subsurface energy storage: A review. Renewable and Sustainable Energy Reviews. 2018; 97:478-96.</p><p>[3] Ma J, Li Q, Kempka T, K&#252;hn M. Hydromechanical response and impact of gas mixing behavior in subsurface CH<sub>4</sub> storage with CO<sub>2</sub>-based cushion gas. Energy & Fuels, 2019; 33 (7), 6527-6541</p><p>[4] Streibel M, Nakaten N, Kempka T, K&#252;hn M. Analysis of an integrated carbon cycle for storage of renewables.&#160;Energy Procedia&#160;40 (2013): 202-211.</p><p>[5] K&#252;hn M, Streibel M, Nakaten N, Kempka T. Integrated underground gas storage of CO<sub>2</sub> and CH<sub>4</sub> to decarbonise the &#8220;power-to-gas-to-gas-to-power&#8221; technology.&#160;Energy Procedia&#160;59 (2014): 9-15.</p>
The Canadian Mackenzie Delta exhibits a high volume of proven sub-permafrost gas hydrates that naturally trap a significant amount of deep-sourced thermogenic methane (CH4) at the Mallik site. The present study aims to validate the proposed Arctic sub-permafrost gas hydrate formation mechanism, implying that CH4-rich fluids were vertically transported from deep overpressurized zones via geologic fault systems and formed the present-day observed GH deposit since the Late Pleistocene. Given this hypothesis, the coastal permafrost began to form since the early Pleistocene sea-level retreat, steadily increasing in thickness until 1 Million years (Ma) ago. Data from well logs and 2D seismic profiles were digitized to establish the first field-scale static geologic 3D model of the Mallik site, and to comprehensively study the genesis of the permafrost and its associated GH system. The implemented 3D model considers the spatially heterogeneously distributed hydraulic properties of the individual lithologies at the Mallik site. Simulations using a proven thermo-hydro-chemical numerical framework were employed to gain insights into the hydrogeologic role of the regional fault systems in view of the CH4-rich fluid migration and the geologic controls on the spatial extent of the sub-permafrost GH accumulations during the past 1 Ma. For >87% of the Mallik well sections, the predicted permafrost thickness deviates from the observations by less than 0.8%, which validates the general model implementation. The simulated ice-bearing permafrost and GH interval thicknesses as well as sub-permafrost temperature profiles are consistent with the respective field observations, confirming our introduced hypothesis. The spatial distribution of GHs is a result of the comprehensive interaction between various processes, including the source-gas generation rate, subsurface temperature, and the hydraulic properties of the structural geologic features. Overall, the good agreement between simulations and observations demonstrates that the present study provides a valid representation of the geologic controls driving the complex permafrost-GH system. The model’s applicability for the prediction of GH deposits in permafrost settings in terms of their thicknesses and saturations can provide relevant contributions to future GH exploration and exploitation.
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