Page 249 - Geothermal Energy Systems Exploration, Development, and Utilization

4.10 Case Studies 225
Table 4.7 Summary of the chemical stimulation
operations carried out in the three 5-km deep wells.
at Soultz-sous-Forˆ ets EGS project.

Well Date Concentration of Results: injectivity/productivity
chemical agents increase

GPK2 February 2003 HCl 0.09 + 0.18% Wellhead pressure drop and
(production well) productivity increase (0.5 l s −1
−1
bar ).
GPK3 June 2003 HCl 0.45% No increase: 0.35 l s −1 bar −1
(re-injection well) February 2007 Organic clay acid OCA Weak impact: 0.4 l s −1 bar −1
HT
GPK4 February 2005 HCl 0.2% Productivity: 0.2–0.3 l s −1 bar −1
(production well) May 2006 HCl 15% (3 tons) – HCl Productivity: 0.3–0.4 l s −1 bar −1
October 2006 12% + HF 3%
Chelant: NTA 19% The formation of a plug
increased wellhead pressure.
March 2007 Organic clay acid OCA Productivity: 0.4–0.5 l s −1 bar −1
HT

allowed cleaning the well and neighboring fractures, and injecting NTA farer in
the fractures.
Although few data are available, the efficiency of a chemical treatment to improve
the injectivity or productivity of a well seems to be dependent on the distribution,
size, and hydrothermal deposits of the fractures located at the vicinity of this well.
So, the injection of HCl into GPK-2 and GPK-4 has increased the productivity of
these wells (G´ erard, Fritz, and Vuataz, 2005) whereas HCl and OCA injected into
GPK-3 had no effect in this well crossed by a major fault governing more than 75%
of its communication with the reservoir at 4750 m true vertical depth (TVD).
The chemical stimulation program performed at Soultz generated an improve-
ment factor of 1.1–2.5 of the injectivity/productivity (Nami et al., 2008). The
integration of results from seismic monitoring, temperature, and flow logging
helps in detecting the productive zones of the wells and their changes through
chemical stimulations. The effectiveness of chemical stimulation could be further
improved by using techniques to divert the treatment fluid toward selected zones
in the reservoir (drillpipe, coiled tubing, packers). Particularly in fractured crys-
talline formations, where the reservoir permeability is strongly controlled by the
pre-existing natural fracture network, a ‘‘focused’’ acidizing of these highpermeable
joints and fracture zones is essential.
Due to the recent and limited operations, the experience in chemical stimulation
in the geothermal wells acquired from the site of Soultz is not very important.
Regrettably, the same chemical treatments have not been used in GPK-3 and
GPK-4 and hence, it is not possible to compare the efficiency of these treatments.