Sustainability of engineered fractured systems: An experimental study on hydro-mechanical properties

The Earth’s subsurface exhibits a high potential for generating and storing energy. Engineered fractured systems, for example geothermal or carbon storage reservoirs, highly depend on the capacity of rock to conduct and store fluids. Faults and fractures create the largest contrasts in flow in these reservoirs and can enhance the reservoir potential when being generated or engineered. While the scientific focus is mainly on the effectiveness of enhancements and the risks associated with them, the sustainability of these enhancements must be better understood. In this thesis, the dependence of fracture permeability on a variety of parameters is studied. The aim is to develop a better systematic understanding of the hydro-mechanical processes controlling the potential and sustainability of fractures to conduct fluids at a variety of conditions. Several parameters that are assumed to control fracture permeability are considered in laboratory experiments. These include the rock type (clastic vs. crystalline), the fracture type (shear vs. tensile), the fracture geometry (aperture and roughness) and effective stress changes (pore and external stress). Potential geothermal rocks are considered in order to directly relate the findings to potential geothermal exploration projects. The results demonstrate the complex dependency on a variety of parameters and highlights the different physical processes depending on mainly rock and fracture type. An attempt was made to assess the potential of fractures to act as fluid conduits in reservoirs, as well their hydraulic sustainability during effective pressure changes. From these results, general implications are made concerning the ability and sustainability of fractures to conduct fluids depending on rock and fracture type. The main controlling parameters are assessed and possible mitigation strategies are developed to reduce the risk of permeability losses. Generally, only reservoir enhancement strategies resulting in a sustainable productivity increase can guarantee the scientific and political breakthrough of geothermal energy supply.