The final report of the project COSMA describes the modeling results of four different scenarios regarding the pressure build-up in shallow aquifers due to the injection of CO2 into the sandstone aquifers of the Detfurth Formation. It is based on the “Technical Report on hydrogeological and static structural geological model implementation” (D 2.1) which focuses on the compilation of geological and hydrogeological background data (average values) and the development of a simplified conceptual hydrogeological model for a setting typical for the Northern German Sedimentary Basin as well as the model selection, model parameterization, definition of boundary conditions and implementation in hydrogeological flow model software packages. The hydrogeological model of the Cenozoic includes Quaternary and Tertiary aquifers down to the layer beneath the Rupelian clay. Moreover, a concept for modeling the interaction between deep, consolidated, saline aquifers with unconsolidated freshwater aquifers was developed. This report describes scenario analyses by using the numerical hydraulic model of the Detfurth Formation (Middle Bunter) and the simplified numerical groundwater model of the Cenozoic. The numerical models can be used to assess the key parameters, having an impact on the upconing of deeper saline groundwater beneath the well fields of water works (in shallow aquifer) due to imposed pressure signals.

Increasing subsurface activities like geothermal energy production, unconventional gas exploitation (EGR – enhanced gas recovery), enhanced oil recovery (EOR) or geological carbon dioxide storage (GCS) are potentially hazardous for the environment. Especially fresh water aquifers used as drinking water resources need to be protected. The first phase of the project COSMA focuses on potential hazards and hazardous events arising from those activities and aims at developing an approach for quantifying and comparing potential risks. A general description of hazards and hazardous events resulting from emerging subsurface activities is given in the first deliverable D1.1 “Geological CO2 Storage and Other Emerging Subsurface Activities: Catalogue of Potential Impacts on Drinking Water Production”. In this 2nd deliverable, reported hazards and hazardous events resulting from geothermal energy production in Germany are described. This report includes analyses of enquiries to experts from all federal states, State Geological Surveys, information from standardization committees, developers, planners, drilling contractors, expert committees, consulting engineers and regulatory authorities such as environmental agencies, water authorities and mining authorities as well as from media reports. It aims to list and categorize observed impacts arising from recent geothermal projects, as there have been increasing activities in this field in the past 10 years in Germany and because there are many similarities to other subsurface activities with respect to drilling processes, fracking methods and reinjection of fluids. The German classification of geothermal systems distinguishes between shallow or nearsurface (< 400 m depth) and deep geothermal energy (> 400 m depth) systems, which will be used in the following chapters. Table 1 shows the difference to international classification schemes, regarding enthalpies and temperatures. The reported case studies of failures potentially leading to contamination of freshwater aquifers are described in chapter 2 with respect to the setting and the reason for failure (if known). Chapter 3 gives some recommendations with respect to possible precautions and countermeasures to prevent such potentially hazardous events. Regardless of the drilling depth there are general hazards and hazardous events that must be taken into account for all subsurface activities. Amongst these are hazardous events during operation which can lead to a contamination of the site, hazardous events during drilling caused by wrongly selected drilling techniques, drilling into unknown caverns, cavities or caves or faulty casing, construction or plugging (sealing). Furthermore, unexpected chemical reactions between fluids and casing or sealing material (e.g. grout) can cause seepage or leakage and therefore hydraulic short circuits. Table 2 gives a summary of general impacts of drilling, especially when multiple aquifers are intersected, as well as from operation of geothermal facilities. Further details are given in COSMA-1 report D 1.1.

The overall goal of the project Cosma-1: “Geological CO2 storage and other emerging subsurface activities” is the assessment of potential impacts of subsurface activities on shallow aquifers used for drinking water production. The first two deliverables (D 1.1 and D 1.2) dealt with general approaches for risk assessment and a description of potential hazards and hazardous events, which might be a risk for shallow freshwater aquifers, as well as lessons learned from existing geothermal energy production and storage sites in Germany. This Technical Report describes the activities of the second phase of the project COSMA-1 and focuses on the compilation of geological and hydrogeological background data (average values) and the development of a simplified conceptual hydrogeological model for a setting typical for the Northern German Sedimentary Basin. The hydrogeological model of the Cenozoic includes Quaternary and Tertiary aquifers down to the layer beneath the Rupelian clay. On this basis, a numerical model with the program Modflow (PMWIN 5.3) was implemented as no complex geometries had to be considered. The structural geological model of the target formation for underground utilisation, the Detfurth Formation (Middle Bunter), incorporates four different fault systems with nine faults in total enclosing the area of interest. Further, a concept for modeling the interaction between deep, consolidated, saline aquifers with unconsolidated freshwater aquifers in a setting typical for the Northern German Sedimentary Basin was developed. This included the model selection, model parameterization, definition of boundary conditions and implementation in hydrogeological flow model software packages. In the further course of the project, a scenario analysis will be performed by using the numerical hydraulic model of the Middle Bunter and the simplified numerical groundwater model of the Cenozoic. The numerical models will be used to assess the key parameters, having an impact on the upconing of deeper saline groundwater beneath the well fields of water works (in shallow aquifer) due to imposed pressure signals.