Conceptual Modeling and Tracer Testing at Ribeira Grande, São Miguel, Azores, Portugal
The Ribeira Grande geothermal reservoir on São Miguel Island,
Azores, is exploited by two different production/injection areas on the
northern side of the Lagoa do Fogo (Agua de Pau) volcano, both developed and
operated by Sociedade Geotérmica dos Açores, S.A. (SOGEO).
southern (upslope) area has been in continuous production since 1994 and now
has a 13 MW capacity.
- The northern (downslope) area has been in continuous
production since 1981, with an expansion from 3 MW to 10 MW that went on-line
To support the expansion and plans for continued development in the
northern area, the conceptual model of the reservoir has been updated, a tracer
test conducted, and a numerical model developed. The reservoir is fed by a
240°C upflow of relatively alkaline, Na-Cl water that appears to occur in
the southeastern quarter of the southern area. Outflow is to the north, and the
reservoir as a whole is marked by a striking degree of chemical homogeneity and
limited cooling along a horizontal distance of more than 3 km. The conceptual
model is constrained somewhat by limited drilling (one well) in the gap between
the southern and northern production areas.
Tracer injected at the northern
edge of the southern production field (one well) shows very limited returns to
the south (0 ~ 3 ppb), and was not seen to the north (1.75 km and further)
after 240 days. Tracer injected at the northern edge of the northern production
field (two wells) shows more rapid and substantial returns (10 ~90 ppb) to
wells of the northern field, and it is planned to relocate this injection
further to the NE.
Results of early exploration drilling, and locations of
surface manifestations, indicate that an eastward and southeastward expansion
of the northern production/injection area should be possible, whereas further
expansion of the southern production area is limited by severe surface
topography and limited access. SOGEO plans to conduct step-out drilling for
production and injection both, in late 2009.