Item

Abstract

Assessing resources of enhanced geothermal (EGS) or medium deep geothermal systems (MDGS) for direct heat use and underground thermal energy storage (UTES) is a challenging task where usually diverse data sets of multiple origin and scale have to be compiled to obtain a comprehensive conceptual model of the subsurface, its structure and its properties. Within the research project “Hessen 3D 2.0” (BMWI-FKZ: 0325944), which aims to enhance the assessment of the prospective risk (‚Fündigkeitsrisiko’) for these kinds of geothermal projects, we established a workflow to implement and analyse such broad data sets. In a first step, comprehensive datasets of physical rock-, fluid- and reservoir properties are compiled which are based on investigations on relevant reservoir analogues, hydraulic test data from boreholes and borehole geophysical logs. The second step comprises the development of 3D geological models from a combination of borehole data, geological cross sections, seismic profiles, gravity and geomagnetic anomalies and geological maps to achieve the required detail on subsurface structure. This is prerequisite to distinguish the potentially usable reservoir units both within the crystalline or metamorphic basement and the sedimentary cover. Geostatistical analysis of the acquired comprehensive geothermal database is performed in a third step of the workflow; this allows for a parametrization of the geological model, for thermohydraulic subsurface modelling, and finally for the geothermal resource assessment. Such models, which consider the variability of rock and reservoir and fluid properties provide a thorough understanding of the subsurface temperature distribution, the dominant heat transport processes and hydraulic conditions. Finally, under consideration of both technical and economic boundary conditions and the statistics for the different relevant reservoir properties of the different geological units, assessment of hydrothermal, petrothermal and UTES potentials is performed directly with the 3D model. Therefore, a multiple-criteria approach, which assesses the quality of various rock and reservoir properties and their relevance for the different geothermal utilizations is implemented. This 3D-grid based method can be used for an identification and visualization of different geopotentials using various parameters to determine each potential. Thereby, to specify the grade of each potential under technical and economic requirements, threshold values for each parameter are defined. The approach described here allows for a stochastic assessment of the geothermal resources of a particular site of interest, including the determination of the probability of success and it provides the necessary numbers to attract investors to geothermal projects.