Discrete fracture network analysis of Devonian carbonate rocks in Western Germany: Implications for deep geothermal energy, heat exploitation and anthropogenic fault reactivation

Faults and fractures in carbonate reservoirs strongly influence subsurface fluid movement and can determine the success or failure of geothermal energy and heat production projects. Characterizing their physical and hydraulic properties is therefore crucial. Firstly, because they strongly control the secondary porosity and permeability of the reservoir, which are key parameters for the estimation of the reservoir quality, and for the planning of injection/extraction strategies. Secondly, because upon anthropogenic reactivation, they can lead to felt seismic activity, which could interrupt operations and potentially lead to damage to infrastructure and endanger the population.

Devonian carbonate rocks underlie a vast portion of North Rhine-Westphalia in Western Germany. Their stratigraphic thickness (up to 1,300 m), location, and depth, make them a potential reservoir for deep geothermal heat and energy exploitation. While estimated at depths between 1.3 km and 6 km, outcrop analogues of these Devonian carbonates are exposed in a number of quarries in the region.

This work quantitatively characterizes the fracture and fault distribution and permeability of the Devonian limestones and dolostones exposed at the the Steltenberg quarry, located at the northern margin of the Remscheid-Altena Anticline. The units outcropping in the quarry are tectonically affected by splays of the WSW-ENE-trending Ennepe thrust (Variscan), and by post-Variscan NNW-SSE-trending normal faults. We combine field structural analyses and fracture characterization using scan lines with a 3D digital outcrop model and fracture analyses using UAV imagery, to produce 3D Discrete Fracture Network (DFN) models. Preliminary results show three main fracture sets: WSW-ENE-trending and S-dipping fractures parallel to the Ennepe Thrust, WSW-ENE-trending and N-dipping bedding-parallel fractures, and NNW-SSE-trending sub-vertical fractures parallel to the regional post-Variscan normal faults. Our DFN modeling suggests that the latter represent the main pathways for fluid flow with permeability values up to 10^-14 m².

As next steps we will use the DFN modeling results (e.g., fractures sets, permeability tensor) as input for a 3D thermo-hydro-mechanical finite element model aimed at predicting fluid flow, pressure, and stress changes in a potential geothermal reservoir. Modeling, together with fault-related parameters such as slip tendency and fracture susceptibility, will help estimate the potential for fault reactivation and induced seismicity in the region.