Influences of hydraulic boundary conditions on the deep fluid flow in a 3D regional model (central Upper Rhine Graben)

Knowledge of groundwater flow is of high relevance for groundwater management or the planning of different subsurface utilizations such as deep geothermal facilities. While numerical models can help to understand the hydrodynamics of the targeted reservoir, their predictive capabilities are limited by the assumptions made in their setup. Among others, the choice of appropriate hydraulic boundary conditions, adopted to represent the regional to local flow dynamics in the simulation run, is of crucial importance for the final modelling result. In this work, we systematically address this problematic in the area of the central part of the Upper Rhine Graben. We quantify how and to which degree different upper boundary conditions and vertical cross-boundary fluid movement influence the calculated deep fluid flow conditions in the area under study. Robust results, which are insensitive to the choice of boundary condition, are: (i) a regional groundwater flow component descending from the graben shoulders to rise at its centre and (ii) the presence of heterogeneous hydraulic potentials at the rift shoulders. Contrarily, results affected by the chosen boundary conditions are: (i) calculated flow velocities, (ii) the absolute position of the upflow axis, and (iii) the evolving local flow dynamics. If, in general, the investigated area is part of a supra-regional flow system—like the central Upper Rhine Graben is part of the entire Upper Rhine Graben—the inflow and outflow across vertical model boundaries need to be considered.

Fracture Spacing Variability and the Distribution of Fracture Patterns in Granitic Geothermal Reservoir: A Case Study in the Noble Hills Range (Death Valley, CA, USA)

Scanlines constitute a robust method to better understand in 3D the fracture network variability in naturally fractured geothermal reservoirs. This study aims to characterize the spacing variability and the distribution of fracture patterns in a fracture granitic reservoir, and the impact of the major faults on fracture distribution and fluid circulation. The analogue target named the Noble Hills (NH) range is located in Death Valley (DV, USA). It is considered as an analogue of the geothermal reservoir presently exploited in the Upper Rhine Graben (Soultz-sous-Forêts, eastern of France). The methodology undertaken is based on the analyze of 10 scanlines located in the central part of the NH from fieldwork and virtual (photogrammetric models) data. Our main results reveal: (1) NE/SW, E/W, and NW/SE fracture sets are the most recorded orientations along the virtual scanlines; (2) spacing distribution within NH shows that the clustering depends on fracture orientation; and (3) a strong clustering of the fracture system was highlighted in the highly deformed zones and close to the Southern Death Valley fault zone (SDVFZ) and thrust faults. Furthermore, the fracture patterns were controlled by the structural heritage. Two major components should be considered in reservoir modeling: the deformation gradient and the proximity to the regional major faults.

The Crustal Stress Field of Germany - A Refined Prediction

Information about the absolute stress state in the upper crust plays a crucial role in the planning and execution of e.g., directional drilling, stimulation and exploitation of geothermal and hydrocarbon reservoirs. Since many of these applications are related to sediments, we present a refined geomechanical-numerical model for Germany with focus on sedimentary basins, able to predict the complete 3D stress tensor. The lateral resolution of the model is 2.5 km, the vertical resolution about 250 m. Our model contains 22 units with focus on the sedimentary layers parameterized with individual rock properties. The model results show an overall good fit with magnitude data of the minimum (Shmin) and maximum horizontal stress (SHmax) that are used for the model calibration. The mean of the absolute stress differences between these calibration data and the model results is 4.6 MPa for Shmin and 6.4 MPa for SHmax. In addition, our predicted stress field shows good agreement to several supplementary in situ data from the North German Basin, the Upper Rhine Graben and the Molasse Basin.

Thermodynamic and Kinetic Modelling of Scales Formation at the Soultz-sous-Forêts Geothermal Power Plant

Geothermal energy has been a subject of great interest since the 1990s in the Upper Rhine Graben (URG), where the first European Enhanced Geothermal System (EGS) pilot site has been developed, in Soultz-sous-Forêts (SsF), France. Several studies have already been conducted on scales occurring at the reinjection side at the geothermal plants located in the URG. It has been observed that the composition of the scales changes as chemical treatment is applied to inhibit metal sulfate. The purpose of this study was to model the scaling phenomenon occurring in the surface pipes and the heat exchangers at the SsF geothermal plant. PhreeqC, a geochemical modelling software, was used to reproduce the scaling observations in the geothermal plant during exploitation. A suitable database was chosen based on the availability of chemical elements, minerals, and gas. A thermodynamic model and a kinetic model were proposed for modelling the scaling phenomenon. The thermodynamic model gave insight on possible minerals precipitated while the kinetic model, after modifying the initial rates equation, produced results that were close to the expected scale composition at the SsF geothermal plant. Additional laboratory studies on the kinetics of the scales are proposed to complement the current model.

Hydro-Thermal Modeling for Geothermal Energy Extraction from Soultz-sous-Forêts, France

The deep geothermal energy project at Soultz-sous-Forêts is located in the Upper Rhine Graben, France. As part of the Multidisciplinary and multi-contact demonstration of EGS exploration and Exploitation Techniques and potentials (MEET) project, this study aimed to evaluate the possibility of extracting higher amounts of energy from the existing industrial infrastructure. To achieve this objective, the effect of reinjecting fluid at lower temperature than the current fluid injection temperature of 70 °C was modeled and the drop in the production wellhead temperature for 100 years of operation was quantified. Two injection-production rate scenarios were considered and compared for their effect on overall production wellhead temperature. For each scenario, reinjection temperatures of 40, 50, and 60 °C were chosen and compared with the 70 °C injection case. For the lower production rate scenario, the results show that the production wellhead temperature is approximately 1–1.5 °C higher than for the higher production rate scenario after 100 years of operation. In conclusion, no significant thermal breakthrough was observed with the applied flow rates and lowered injection temperatures even after 100 years of operation.

Feature-based Groundwater Hydrograph Clustering Using Unsupervised Self-Organizing Map-Ensembles

Hydrograph clustering helps to identify dynamic patterns within aquifers systems, an important foundation of characterizing groundwater systems and their influences, which is necessary to effectively manage groundwater resources. We develope an unsupervised modeling approach to characterize and cluster hydrographs on regional scale according to their dynamics. We apply feature-based clustering to improve the exploitation of heterogeneous datasets, explore the usefulness of existing features and propose new features specifically useful to describe groundwater hydrographs. The clustering itself is based on a powerful combination of Self-Organizing Maps with a modified DS2L-Algorithm, which automatically derives the cluster number but also allows to influence the level of detail of the clustering. We further develop a framework that combines these methods with ensemble modeling, internal cluster validation indices, resampling and consensus voting to finally obtain a robust clustering result and remove arbitrariness from the feature selection process. Further we propose a measure to sort hydrographs within clusters, useful for both interpretability and visualization. We test the framework with weekly data from the Upper Rhine Graben System, using more than 1800 hydrographs from a period of 30 years (1986-2016). The results show that our approach is adaptively capable of identifying homogeneous groups of hydrograph dynamics. The resulting clusters show both spatially known and unknown patterns, some of which correspond clearly to external controlling factors, such as intensive groundwater management in the northern part of the test area. This framework is easily transferable to other regions and, by adapting the describing features, also to other time series-clustering applications.

Interdisciplinary Fracture Network Characterization in the Crystalline Basement: A case study from the Southern Odenwald, SW Germany

The crystalline basement is considered a ubiquitous and almost inexhaustible source of geothermal energy in the Upper Rhine Graben and other regions worldwide. The hydraulic properties of the basement, which are one of the key factors for the productivity of geothermal power plants, are primarily controlled by hydraulically active faults and fractures. While the most accurate in situ information about the general fracture network is obtained from image logs of deep boreholes, such data are generally sparse, costly and thus often not openly accessible. To circumvent this problem, an outcrop analogue study with interdisciplinary geoscientific methods was conducted in the Tromm Granite, located in the southern Odenwald at the northeastern margin of the URG. Using LiDAR scanning, the key characteristics of the fracture network were extracted in a total of five outcrops, additionally complemented by lineament analysis of two different digital elevation models. Based on this, discrete fracture network (DFN) models were developed to calculate equivalent permeability tensors under assumed reservoir conditions. The influence of different parameters, such as fracture orientation, density, aperture and mineralization was investigated. In addition, extensive gravity and radon measurements were carried out in the study area, allowing for more precise localization of fault zones with naturally increased porosity and permeability. Gravity anomalies served as input data for a stochastic density inversion, through which areas of increased open porosity were identified. A laterally heterogeneous fracture network characterizes the Tromm Granite, with the highest natural permeabilities expected at the pluton margin, due to the influence of large shear and fault zones.

Hydro-Thermal Modeling for Geothermal Energy Extraction from Soultz-Sous-forêts, France

The deep geothermal industrial project at Soultz-sous-Forêts is located in the Upper Rhine Graben, France. As part of the MEET project, this study aims to evaluate the possibility of extracting higher amounts of energy from the existing industrial infrastructure. To achieve this objective, the effect of reinjecting fluid at lower temperature than the current fluid injection temperature of 70 ℃ was modelled and the drop in the production wellhead temperature for 100 years of operation was quantified. Two injection-production rate scenarios were considered and compared for their effect on overall production wellhead temperature. For each scenario, reinjection temperatures of 40 ℃, 50 ℃ and 60 ℃ were chosen and compared with the 70 ℃ injection case. For the lower production rate scenario, the results show that the production wellhead temperature is approximately 1-1.5 ℃ higher than for the higher production rate scenario after 100 years of operation. In conclusion, no significant thermal breakthrough has been observed with the applied flow rates and lowered injection temperatures even after 100 years of operation.