All three deep Soultz wells were subjected to chemical stimulations as a 
complementary method to hydraulic fracturing. The finding of fractures filled 
with calcite constituted the starting point of chemical stimulation in Soultz. 
Further, the potential improvement of the near wellbore performance without 
seismic events has been considered as advantage of chemical treatments.
In 2003, the first deep well (GPK2) was stimulated by injection of hydrochloric 
acid. A significant reduction of near wellbore friction losses was observed 
immediately after the acid front reached the open hole section. Nevertheless, 
the improvement is important only at high flow rates (30 l/s), whereas at lower 
rates (15 l/s) only a minor improvement remains. It is likely that turbulent 
friction losses inside the wellbore, where a fish is stuck, were reduced due to 
the acid injection. No clear indications were found for an improvement in the 
formation around the well GPK2.
During a circulation test between GPK2 and GPK3 in 2003, hydrochloric acid was 
injected in GPK3over a limited time period of 12 hours. No reduction of the 
injection pressure was observed during or after the acid injection. Three years 
later a so called organic clay acid (OCA) was pumped into the well. Two 
injection tests before and after the OCA-job showed almost no improvement of 
the well productivity due to OCA injection.
The failing of the acid stimulations in GPK3 coincides with the existence of a 
large infinite conductive fracture as the dominant outlet. There is no potential 
to improve the well by dissolving minerals from these fracture faces.
A series of chemical treatments were applied in GPK4, starting with the 
injection of HCl in 2005. This operation improved the wells injectivity by 50%, 
but it is questionable whether this improvement was achieved in the open hole 
or through leakages in the casing. The following chemical operations were 
injection of RMA (Regular Mud Acid), NTA (chelating agents) and OCA. While the 
RMA operation improved the wells injectivity further, the NTA reduced it again, 
probably by plugging fractures. The final OCA application led to an increase of 
the productivity by 25% up to 0.5 l/s/bar (determined after two days of 
injection). The origin of the improvements due to acid injections (HCl, RMA, OCA) 
can be twofold: Microseismicity was observed during these operations and is 
located only at the casing leakages at (4100 m and 4400 m TVD) which 
suggests, in combination with new flow logs, the stimulation of this leakage 
during the chemical treatments. On the other hand, the recent open ho  le flow 
profile ends at 4780 m TVD and does not allow an interpretation of the deeper 
flow distribution in the last 200 m of the open hole. Thus, the achieved 
productivity enhancement at GPK4 can not clearly be attributed to an 
improvement of the reservoir below the casing.

Grensasvegur 9
Reykjavik,
Iceland