STRUCTURAL GEOLOGICAL CONTRIBUTION TO GEOTHERMAL EXPLORATION IN A CASE STUDY FROM THURINGIA

For geothermal reservoirs to be of economic use a high flow rate of hot water through the rock is needed. Most geothermal reservoirs are “fractured reservoirs”, that is, fluid transport is through rock fractures. The permeability of a reservoir can be increased through stimulation, either by shearing and opening of existing rock fractures or by creating new hydraulic fractures in the reservoir rock, resulting in Enhanced Geothermal Systems. Here we show how structural geology can contribute to increase the likelihood of success in geothermal projects. As an example, we present results of a case study on the prognosis of fracture systems and associated permeability in granite in the subsurface of Thuringia (Eastern Germany). To obtain information on the geometry of pre-existing fracture systems, necessary to estimate the potential permeability of a man-made geothermal reservoir, we analysed outcrop analogues, that is, outcrops of the same rock types as those supposed to host the man-made reservoir at geothermal depths. The outcrop studies were performed in the Thuringian Forest and represent the rocks expected at depth in the Thuringian Syncline. Important fracture parameters include attitude, aperture, and interconnectivity to fracture systems.

Our field results indicate that there are subhorizontal primary igneous joints as well as several sets of subvertical fractures. The orientations of the subvertical fracture sets, however, vary considerably between the outcrops. Also, fracture frequencies range from less than one fracture per metre up to more than four fractures per metre within minor fault zones or near igneous dykes. For all outcrops, the interconnectivity of the fracture systems is high, indicating a relatively high fracture-related permeability of the granite. We use these field data for analytical models of potential fluid transport through statistically simulated fracture systems in granite. These models allow estimates of potential fluid flow rates through virtual holes. Finally we present boundary-element models on the local stress fields for representative fracture systems subject to realistic remote stresses and fluid overpressure during stimulation that allow to estimate the connectivity of the created fracture systems and therefore the fluid transport in the geothermal reservoir.