Page 248 - Geothermal Energy Systems Exploration, Development, and Utilization

224 4 Enhancing Geothermal Reservoirs
Although the public concern about induced seismic events during hydraulic
stimulation was one important reason for undertaking chemical treatments as
an additional or even an alternative method to hydraulic stimulations, the main
argument for chemical stimulation was the evidence of fracture filling carbonates
and other soluble minerals, based on drill cuttings and cores analysis, as well as on
geophysical logs.
Chemical stimulation and especially, the use of strong acids are rather known to
attack the rocks, minerals, or fractures located at the vicinity of a well. It was the
reason for which other chemical treatments such as NTA, chelating agent dissolved
in caustic soda solution or OCA, mixing of weak organic acids, that slower dissolve
rocks and minerals, were tested in GPK-4 well after the injection of HCl and RMA
in order to extend the chemical stimulation as far as possible.
Chemical stimulations were performed by injecting acid from the wellhead
through the casing string (9.5/8’’ for GPK-3 and GPK-4 and 7’’ for GPK-2). The
stimulation zone was therefore, the whole open hole section of the wells (500–650 m
length). Corrosion inhibitors were used to protect the inner casing string. With
exception to chemical treatments with HCl, the other operations were conducted
by specialized service companies. Hydraulic tests were performed before and after
the chemical stimulations, to evaluate the progress in productivity or injectivity. A
geochemical monitoring of the discharged fluid was carried out after most of the
chemical stimulation experiments (Sanjuan et al., 2007).
Although they were not executed with the same comparable protocol, different
but encouraging results were observed after these series of tests using several
chemical stimulation methods in a fractured granitic EGS reservoir. If GPK-3 well
has shown weak variations of its injectivity, GPK-4 well presented a real increase
of injectivity and productivity after the treatments (GEIE, 2006; Nami et al., 2008),
and GPK-2 well also presented a very sensible improvement despite the fact that
the treatments were limited in terms of time, volume, and concentration (G´ erard,
Fritz, and Vuataz, 2005). As a summary of the chemical stimulation tested in the
three deep wells of the Soultz EGS project, the Table 4.7 gives a synthesis of the
principal results.
HCl and OCA, the only compounds injected into GPK-3, were not efficient to
improve the injectivity and productivity indices of this well whereas the treatments
−1
used in GPK-4 increased the PI of GPK-4 from 0.2 to 0.5 l s −1 bar . However,
as the treatments used in each well were different and their combination in the
case of GPK-4 could bring more efficiency, it is difficult to compare their effects
between each well.
The injection of a caustic soda solution has been particularly efficient to clean
up the well GPK-4 and neighboring fractures by removing significant amounts of
drilling grease and drilling fragments such rock debris and cuttings. Consequently,
the injection of a caustic soda solution accompanied by a fluid production test
seems to be an efficient operation of cleaning and is recommended to remove
drilling wastes and residues from the geothermal wells and their vicinity just after
their drilling. Given the results obtained on GPK-4, it would have been preferable
to use NTA after a prior injection of caustic soda solution, which would have