Page 190 - Geothermal Energy Renewable Energy and The Environment

176 Geothermal Energy: Renewable Energy and the Environment

+1000
Steam horizon

0
Production wells

–1000

Injection well
Meters –2000

–3000

–4000
1 km

–5000

FIGUre 9.17 Distribution of microseismic events (open circles) recorded during November 2007, related to
injection at well 42B-33 (solid line labeled “Injection well”). The approximate location of the steam horizon
is indicated. Production wells are shown as the dashed lines. (Data from Smith, B., Beall, J., and Stark, M.,
Induced Seismicity in the SE Geysers Field, California, Proceedings World Geothermal Congress, Kyushu-
Tohoku, Japan, 2887–92, 2000.)

fluid escapes from the immediate vicinity of the volume of rock from which the production wells
are accessing steam. However, the fact that steam production stabilized over the long-term suggests
that repressurization of the reservoir was successful. The mass balance between injected liquid
and steam formation that contributed to recharging the reservoir cannot be rigorously determined,
however. In addition, although production has been stabilized, the overall reservoir performance
remains below that achieved during the late 1960s and early 1970s. It remains to be determined the
extent to which the new injection program will bring production back to those earlier levels.

synopsIs

Generating electrical power from geothermal resources requires no fuel while providing true baseload
energy at a capacity factor that is consistently above 0.9. Extracting geothermal energy requires the
ability to efficiently transfer heat from a geothermal reservoir to a power plant where the heat is
converted to electrical energy. This process is accomplished using production wells that penetrate
hundreds to thousands of meters into the subsurface, providing a flow path for the reservoir fluid to
ascend from depth. Injection wells are used to replenish the reservoir by recycling condensed water,
supplementing it (if needed) with other water sources. Reservoirs are of several types. Dry steam
reservoirs have sufficient enthalpy to vaporize all available water. Such systems are the simplest to
engineer and have the highest energy availability of all geothermal resources but they are geologi-
cally uncommon. Hydrothermal systems are more common. Such systems usually possess sufficient
heat (temperatures in the range of about 160°C–250°C) at elevated pressures to allow water to flash
to steam as at ascends the wellbore and approaches the turbine. Power is generated in dry steam and
hydrothermal generating plants by the expansion of the steam as it traverses through a turbine facil-
ity designed to extract as much energy from the fluid as efficiently as possible. Several designs have
been developed to take advantage of the steam enthalpy, including single flash and dual flash plants