Geomechanical Model. Prior to having in situ fracture information for calibrating the numerical model (as part of WP7), we follow two different modelling approaches to adequately cover the range of possible sub- surface conditions. In the first approach we assume that the geothermal target is best characterized by a single, large scale fault. In this model, flow is mainly restricted to a planar fault structure. If drilling and logging data provide evidence for a single fault scenario, the hydraulic fault properties can be calibrated by well-testing and observed SHPM pressure. At the current stage (prior to drilling into the fault), we hypothetically assume hydraulic fault parameters for the single fault scenario. Using these models, we make forecasts regarding the hydraulic performance and induced seismicity response of the system. Our second modelling approach covers the scenario where the geothermal target is best characterized by a complex fracture network. Associated numerical models are based on statistical distributions of fracture parameters (orientation, density, length, aperture) which can be calibrated using logging data. These models are not suitable for making forecasts of the induced seismicity response, which is inherently determined by the fracture geometries. In particular, the largest expected earthquake magnitude is controlled by the maximum available fracture surface area, which enters as input parameter in the numerical models. The largest expected earthquake magnitude for the fracture network scenario is however expectedly much smaller than the one forecasted in the single fault scenario. For assessing the hydraulic performance of a fracture network, the orientation of the fractures in the current stress field is of primary importance. Fracture permeability can be qualitatively approximated by calculation of slip (Ts) and dilation (Td) tendencies of fracture surfaces in a given stress field (Figure 2). For example, Xxx and Zoback (2000) analyse fracture orientations at the KTB ultradeep borehole and find indications that flowing fractures are generally oriented favourable for shear. Figure 2: Slip and dilation tendencies.
