How Structural Geology Can Contribute to Make Geothermal Projects Successful

Abstract

In most geothermal reservoirs, particularly in man-made reservoirs, fluid transport is through rock fractures, that is, the host-rock permeability is fracture-controlled (?fractured reservoirs?). The permeability of a geothermal 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 (EGS). Here we show how structural geology can contribute to maximise the likelihood of success in geothermal projects, particularly as regards EGS projects. To obtain information on the geometry of existing fracture systems, so as to be able to estimate the potential permeability of man-made geothermal reservoirs, we analyse outcrop analogues, that is, outcrops of the same rock types as those supposed to host the man-made reservoir at geothermal depths. We focus on rock heterogeneity and anisotropy, mainly mechanical layering, that is, changes in the mechanical properties, particularly rock stiffness (Young?s modulus) and how these heterogeneities influence fracture propagation. Because of such rock heterogeneities, the local stress field can be very different from the regional stress field. We therefore show how numerical models help to understand the local stress fields as well as the connectivity of existing and newly created fracture systems and therefore the fluid transport in geothermal reservoirs. We also present results of a case study on the prognosis of fracture systems and permeability in the Buntsandstein in the Northern German Basin. Our field results indicate that even very thin layers (cm-scale thicknesses) of shale, for example, may be responsible for arrest of many joints in sandstone-shale sequences such as the Buntsandstein.