Using fractured outcrops to calculate permeability tensors. Implications for geothermal fluid flow within naturally fractured reservoirs.

Naturally fractured systems are an important component to fluid flow for a variety of applications, in particular geothermal energy extraction. Geothermal reservoirs often have low rock permeability (e.g. limestone reservoirs) where permeability anisotropy is governed at first order by fractured networks controlled by fracture density, orientation and connectivity. These are often difficult to assess and as such permeability estimates can lead to high uncertainties. Understanding how fracture networks influence permeability of reservoirs is an important aspect to geothermal exploration.

Where subsurface data (e.g. seismic and well) are limited, other data sources for characterising the reservoirs are required. Outcrop analogues are excellent areas for the analysis and characterisation of fractures within the host rocks found at depth. 2D fractured cliff faces and pavements provide information on variation in fracture arrangement, distribution and connectivity which can be utilised in thermohydraulic modelling of the geothermal system.

Through imaging of 2D fractured faces within target reservoir rocks and using efficient discretisation and homogenisation techniques, reliable predictions on permeability distributions in the geothermal reservoirs can be made. Using an example from an open pit quarry within the Franconian Basin, Germany, fracture network anisotropy in a geothermal reservoir (Malm) is assessed using detailed structural analysis and numerical homogenisation modelling of outcrop analogues.

Structural analysis shows several events affected the limestone reservoir unit in the area. The first major phase of deformation recorded are steep-angled reverse thrust and strike-slip faulting (stress orientated NNE-SSW) attributed to the Late Cretaceous Inversion. A second deformation phase causing normal faulting and fracturing within a NW-SE stress field is related to the European Cenozoic Rift System (e.g. Eger Rift). The final deformation phase recorded corresponds to the Alpine Orogeny where strike-slip faults and conjugate fractures are formed under a NW-SE compression and NE-SW extension. The faults and fractures are heavily influenced by the Kulmbach Fault, part of the Franconian Lineament Fault System that is observed 10m north of the quarry and active during the multiphase deformation culminating with a reverse throw of 800m.

2D imagery is used to capture the fracture networks interpreted through the structural analysis from which different sets of similar fractures are extracted. These are then digitised and meshed for numerical modelling and homogenisation using MOOSE Framework. Three fractured faces are imaged at increasing distance from the Kulmbach Fault to determine the fault impact on the potential flow within the system. The calculated permeability tensors from the homogenisation show differences in fluid transport direction where fracture permeability is controlled by orientation compared to a constant value which would be more pronounced for larger scale simulations. Therefore, for reliable predictions of geothermal flow within the networks, assigning permeabilities for sets is vital. As a result, it is observed that the orientation of the tensor is influenced by the Kulmbach Fault, and thus faults within the reservoirs at depth should be considered as important controls on the fracture flow of the geothermal system.