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.