MODELING FLOW AND HEAT-TRANSFER IN FRACTURED ROCKS - CONCEPTUAL-MODEL OF A 3-D DETERMINISTIC FRACTURE NETWORK

This work examines 3-D advective-conductive heat transfer during forced water circulation through fractured crystalline rocks. The subject of the modelling investigation is the Hot-Dry-Rock (HDR) site at Soultz-sous-Forets, Alsace, France. Fracture data from borehole logging provides an insight to the complex structure of crystalline rocks. The proposed model is based on the general idea, that flow and advective transport in stimulated HDR reservoirs occur mainly in fractures, whose orientation is linked by the tectonic stress field. Therefore, a deterministic fracture network model is established which represents the mean geometrical features of a stimulated HDR system. A physical and mathematical model of a fractured porous medium is developed for describing flow and transport within the interacting system of fractures and rock matrix. The model is then translated into a discrete finite-element approach (ROCKFLOW). The numerical results are tested against analytical solutions for parallel fracture arrays. Fluid flow and heat transfer are simulated and illustrated. The 3-D model that is presented permits an effective calculation of the long-term performance of HDR systems. The thermal drawdown is computed with respect to the important reservoir parameters such as the borehole separation, the fracture spacing and the position of boreholes within the fracture network. A long-term prediction of the thermal performance of an HDR system is most important for an economical assessment of this type of geothermal energy.