The main objective of the European HDR/EGS-project at Soultz is the installation of a geothermal pilot plant for power production by the end of 2008.
After drilling of the three 5000 m deep wells to form a triplet with two producers (GPK2 and GPK3) and a central injector (GPK3), the wells were hydraulically stimulated through massive water injection. In addition to hydraulic stimulation, a series of chemical stimulation operations were undertaken to achieve increasing the performance of the wells and the near-wellbore permeability of the geothermal reservoir. Several acid systems such as hydrochloric acid (HCl), Regular Mud Acid (HCl-HF) and Organic Clay Acid (C6H8O7-HF-HBF4-NH4Cl) were injected to dissolve minerals deposits in the wellbore as well as filling materials of fractures in the vicinity of the wells. Also a high-pH chelating agent (NTA) was tested as alternative to the more usual acid treatments.
Whereas acid treatments have resulted in a productivity improvement of the production wells (GPK2 and GPK4) up to 50%, almost no amelioration was obtained in the injection well GPK3. The use of chelating agents resulted in some productivity deterioration of GPK4.
The results of stimulation experiments were evaluated using short-term hydraulic tests, conventional pressure transient analysis, interference pressure data, microseismic monitoring, temperature and flow logs. This combination of evaluation techniques helped in getting insight into the origin of the productivity enhancement.

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