HR: 11:30h
AN: T22A-05    [Abstracts]
TI: Fracture Propagation and Fluid Flow in Geothermal Reservoirs
AU: * Philipp, S L
EM: Sonja.Philipp@geo.uni-goettingen.de
AF: Geoscience Centre, University of Göttingen, Goldschmidtstr. 3, Göttingen, 37077 Germany
AU: Gudmundsson, A
EM: Agust.Gudmundsson@gwdg.de
AF: Geoscience Centre, University of Göttingen, Goldschmidtstr. 3, Göttingen, 37077 Germany
AB: In most geothermal reservoirs, fluid flow is largely controlled by the permeability of its fracture network. In order to generate permeability in man-made reservoirs, interconnected fracture systems, needed for significant permeability, are formed either by creating hydraulic fractures or by massive hydraulic stimulation of the existing fracture system in the host rock. For effective stimulation, the geometry of the fracture system and the mechanical properties of the host rock must be known. Studies of fracture systems in exposed paleogeothermal fields can help understand the permeability development in stimulated reservoirs. Here we present results of two field studies: 1) infrastructures of extinct fracture-controlled geothermal fields in fault zones in Great Britain, and 2) fracture systems in Mesozoic rocks that could be used to host man-made geothermal reservoirs in Germany. The mineral veins in the paleogeothermal fields are related to the faults, indicating that geothermal water was transported along the then-active faults into the host rocks. There is evidence that the veins were injected as hydrofractures (fractures generated by internal fluid overpressure) from the fault planes into limestone layers. Many mineral veins and most of the joints are confined to individual layers (stratabound). Similarly, in the Mesozoic rocks most joints are restricted to sandstone layers and do not propagate into adjacent shale layers. We supplement these field studies by many numerical models exploring how stress fields affect fracture propagation. We focus on the influence of changes in mechanical properties (particularly Young's modulus) between host rock layers in and around fluid reservoirs. The numerical models show that, depending on the external loading conditions, certain layers in a reservoir may become stress barriers to fracture propagation. Our results suggest that fluid flow along faults, and the propagation of hydrofractures, are important parameters in the permeability development of geothermal reservoirs. These studies provide a basis for models of fracture networks and fluid transport in future man-made reservoirs. We conclude that the likely permeability of a man-made geothermal reservoir can be inferred from field data, natural analogs, laboratory measurements, and numerical models.
DE: 5104 Fracture and flow
DE: 5114 Permeability and porosity
DE: 8010 Fractures and faults
DE: 8135 Hydrothermal systems (0450, 1034, 3017, 3616, 4832, 8424)
DE: 8168 Stresses: general
SC: Tectonophysics [T]
MN: Fall Meeting 2005