Modelling of Fractured Granitic Geothermal Reservoirs: Use of Deterministic and stochastic methods in Discrete Fracture Networks and a Coupled Processes Modeling Framework

The MEET project (Multidisciplinary and multi-context demonstration of EGS exploration and Exploitation Techniques and potentials) is an interdisciplinary study aiming at the improvement of Enhanced Geothermal System (EGS) technology across Europe. The present work contributes to the MEET project in the form of a 3D structural model and Discrete Fracture Network (DFN) analysis, coupled with multi-physics models along key transects, of an analog for the Soultz-sous-Forêts reservoir (Upper Rhine Graben, France). The Noble Hills (Death Valley CA, USA) are chosen as analog due to the common transtensional tectonic setting (rifting zone), an arid region of near total outcrop exposure and making it ideal for Structure from photogrammetry images. In comparison to the Soultz-sous-Forêts reservoir, the Noble Hills has a similar granitic lithology, which has been fractured and hydrothermally altered. The front of the range is bounded by the Southern Death Valley Fault Zone (SDVFZ), a regional dextral fault, which places Tertiary sediments against granite. The photogrammetric models were produced from a combination of ground-based, unmanned aerial vehicle (UAV), and aerial photography. This multi-scale approach of image collection provided resolutions ranging from cm to dm scale. Planar feature extraction and detection was conducted using a combination of multi-point analysis, automatic facet detection, and manual polyline extraction. This planar data was extrapolated to construct a geometric model of the Noble Hills fracture network and ultimately create a representative 3D DFN model. Given the range of fracture size and relative complexity of fracture network, we implemented multiple workflows to address different aspects of the methodology. The DFNs were generated in 3D using a deterministic approach and the fracture planes were further projected onto orthogonal 2D slices for computing flow through interconnected network of fractures.

In total, 13 models are retained for the fluid flow simulations, localized along the major faults which structure the Noble Hills analog. The number of polylines extracted from the models is comprised between 318 to 7207 fractures. The variability in the number of fractures is related to the difference of the deformation gradient between the south and the north part of the Noble Hills.

The preliminary fluid flow simulations resulting from the Noble Hills models allow us to understand the interaction between the fracture geometry and stress field. Temperature is one of the main parameters of each hydrothermal system, and must be included to the models.