The aim of the HDR project Soultz is the geothermal power production based on an artificially created heat exchanger at 5000 m depth.

During the years 1999-2004, three wells were drilled down to 5000 m, one injection (GPK3) and two production (GPK2 and GPK4) wells. After drilling, each well was stimulated by massive water injection with volumes up to 30000 m3 and typical flow rates of 30 - 50 l/s.

Subsequently, injection tests with significant lower rates were performed to determine the productivity (injectivity) of the wells. A good agreement was obtained between the productivity during and after stimulation for all three wells. The stimulated flow paths obviously retain their hydraulic conductivity completely in the post-stimulation period. This observation is very important for planning of hydraulic stimulation operations since the productivity enhancement is predictable. The pressure during stimulation is mainly controlled by rock stress. A higher injection rate leads to a higher productivity during stimulation and consequently afterwards.

At the wells GPK2 and GPK3 a slight increase of downhole pressure was observed during stimulation with constant rate (50 l/s). This characteristic is considered to be typical for shearing of fractures. In contrast, the stimulation of GPK4 was accompanied by a slight but continuous pressure decrease as it is usually observed in stimulation operations dominated by tensile fracturing. Further indication (flow profile) supports the assumption that jacking occurred at least in the vicinity of the borehole GPK4.

It is of special interest that the above mentioned coincidence between productivity during and after stimulation appears to be valid also for GPK4 where tensile fracturing seems to be a significant stimulation process. For the conditions in Soultz, it is generally assumed that the dominant stimulation process is shearing of natural fractures. In view of the observations at GPK4, this assumption should further be investigated.

In order to transfer the experiences of Soultz to other places it is very important to understand to which extent artificial fractures contribute to the reservoir development.

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