Page 230 - Geothermal Energy Systems Exploration, Development, and Utilization

206 4 Enhancing Geothermal Reservoirs

Figure 4.10 Installation of a pumping unit for injection
of chemical compounds at Coso geothermal field (photo
P. Rose, EGI, Univ. of Utah).
acid concentrations have been shown to be effective in geothermal wells producing
from natural fractures not containing separate, large carbonate zones. Naturally
fractured volcanic formations can withstand high HF concentration. The HCl–HF
stage can be 10% HCl–5% HF, or 10% HCl–7% HF, for example. These acid
mixtures have been used successfully in stimulating geothermal wells in South
and North America (Figure 4.10), as well as in Southeast Asia (the Philippines),
where a large number of acid treatments have taken place. Acid volumes can vary
quite a bit. A summary of the main chemical stimulation experiments carried out
in geothermal fields is given in Table 4.5, showing variable results.
Only few chemical stimulation experiments and laboratory tests have been
attempted until now in EGS wells and reservoirs. Limited reported data were found
at the projects of Fenton Hill in USA (Sarda, 1977) and Fj¨ allbacka in Sweden
(Sundquist, Wallroth, and Eliasson, 1988; Wallroth, Eliasson, and Sundquist,
1999). At the EGS reservoir of Soultz-sous-Forˆ ets however, several consistent and
documented chemical stimulation tests have been carried out since 2003. Different
techniques were consecutively used in the three 5 km-deep wells: soft acidizing,
RMA, chelating agents (NTA), and organic clay acid (OCA). Although they were
not executed with the same comparable protocol, various but encouraging results
were observed after this first series of tests using chemical stimulation methods in
a fractured granitic EGS reservoir.

4.9
Sustainability of Treatment

4.9.1
Hydraulic Stimulation

4.9.1.1 Proppant Selection
Experiences from fracture stimulations (Legarth, Huenges, and Zimmermann,
2005; Zimmermann et al., 2009) highlight the importance of making the right