| Abstract |
A highly permeable medium allowing groundwater flow is needed for the production of geothermal energy. Permeability can be enhanced by the presence of natural fractures and faults. Permeability prediction of fault damage zones and fracture corridors simply by extrapolation or up-scaling of reservoir characteristics is de-validated by the heterogeneity, anisotropy and scale variation of these features. In addition, the evolution of damage zones in time may cause changes in the porosity-permeability due to changes in the tectonic deformation style and geochemical processes. Fault zones are generally considered for their sealing capacity, however (hot) springs and fumaroles are natural evidence of extended upward flow along faults. Geothermal activity in for example western Turkey and the Basin and Range province (USA) is related to regional extension with deeply penetrating faults and recent geothermal drilling towards several fault damage zones in the Upper Rhine Graben near Landau, Insheim, Brühl, and Bruchsal in Germany even resulted in better flow results than expected compared to the overall porosity and permeability of the Buntsandstein reservoir. Previous studies showed that mainly the host rock lithology, fault activity, magnitude of displacement, pre-existing structures, the depth, the tectonic stress field and the width of the damage zone are considered to be essential parameters in predicting the conductivity of fault damage zones. In order to make a valid but simple model of the conductivity of fault damage zones, these are the most relevant parameters which can be obtained relatively straightforward. These parameters should be measured and taken into a quantitative approach. They include fault plane orientation, displacement, intersections, the width of the damage zone, and the local stress field. Next to the measurable parameters, available geological and geomechanical parameters such as lithology, shear and tensile strength and clay content should be combined with the results from algorithms provided to obtain an indication of the potential flow along a fault damage zone. This approach results in a workflow that predicts permeability in fault damage zones by quantification of damage zone and rock properties. This approach can be used to select potential drilling targets. The workflow provides an overall first step in the exploration of potentially high productivity targets in fault damage zones. |