Page 228 - Geothermal Energy Systems Exploration, Development, and Utilization

204 4 Enhancing Geothermal Reservoirs
4.8.2
Thermal Stimulation

An evaluation of treatment success may be performed by means of simply compar-
ing the ultimate productivity of a well to the productivity prior to the stimulation
treatment. It is also possible to relate the final well productivity to the cumulative
circulation losses during drilling. The results between both methods may vary
significantly, since they will contain different effects. While in the first case, the
observed improvements will primarily be attributed to reopening of feed zones,
which were clogged during drilling operation, in the second case the observed
improvement will most likely be attributed to the actual increase in feed zone per-
meability, for example, by raising the permeability of near well fractures. Unlike in
the case of low-temperature wells, no clear correspondence between the injectivity
at the end of the thermal stimulation operation and the productivity exists, for
high temperature wells (Table 4.4).

4.8.3
Chemical Stimulation

Over the past years, great improvements in matrix acidizing have taken place for
geothermal wells, paralleling the developments in hydraulic fracturing. Provided
that the forecasted production/injection results make economic sense, matrix
acidizing is still simpler, often less risky, and more economic to implement than

Table 4.4 Characteristics of production wells in the Reyk-
janes high temperature field, in SW-Iceland.

Well Depth Temperature Injectivity Injectivity Productivity
◦
(m) ( C) index 1 index 2 index
−1
−1
−1
(ls −1 bar ) (l s −1 bar ) (l s −1 bar )
RN-10 2 050 310 – 6.6 2.3
RN-11 2 250 295 – >10.0 10.0
RN-12 2 510 290 – 8.0–9.0 20.0–40.0
RN-13 2 460 290 – 4.0–5.0 1.0–2.0
RN-14 2 310 290 6.0 6.0–7.0 –
RN-15 2 510 280 3.5 4.0 1.0
RN-16 2 630 220 1.2 2.0 –
RN-18 1 820 >285 5.0 5.4 1.5
RN-19 2 250 250–260 5.0 5.0 –
RN-21 1 710 275 6.0 13.0 6.0
RN-22 1 680 305 10.0 10.0 15.0
RN-23 1 920 305 – 38.0–48.0 50.0
RN-24 2 110 >275 – 10.0–20.0 38.0

After Axelsson, Th´ orhallson, and Bj¨ ornsson (2006).
Injectivity index 1 : end of drilling, Injectivity index 2 : end of stimulation operations.