The increasing demand for energy with the least possible environmental impact in the last years has led to an ever-increasing interest in geothermal energy world-wide. Most areas with elevated temperature gradients are hot but have a relatively low permeability. In such cases hydraulic stimulation is used to increase permeability and reservoir productivity.
The aim of hydraulic stimulation is obviously the maximization of productivity. It is however linked to risks, such as induced micro-seismicity or borehole stability, which in turn are likely to increase significantly production costs. For the stimulation the pumping volume rate and the applied pressure are parameters influencing the reaction of the rock mass formation in the sense of brittle or ductile behaviour. It is, as a result, necessary to develop methods of determining the exact stimulation procedure suitable in each specific case in order to enhance productivity, while keeping costs and risks to a minimum.
The present project suggests an approach to the subject from three different aspects: constitutive, experimental and numerical. The constitutive model is to result from examination of the physics at the micro-scale and up-scaling to the continuum by means of averaging techniques. Interaction of neighbouring cracks will be taken into consideration. As input to the model the probability density functions of the orientations and spacing of cracks will be used. The output is to be the evolution of the fracture matrix and, as a consequence, the evolution of the rock permeability.
The experimental program will focus on crack interaction and fracture propagation. The effect of high temperatures, as a rule present in geothermal reservoirs, will also be examined.
The numerical program will follow two different paths, that of DEM and that of FEM. The first will be used to provide insight on the fluid-solid interaction, while the second will be used for validation and case study simulations.