Page 186 - Geothermal Energy Renewable Energy and The Environment

172 Geothermal Energy: Renewable Energy and the Environment

and alteration of the original rock. Such processes commonly lead to sealed fractures and clogged
pore spaces in the rock, with the ultimate consequence that rocks that were once permeable rocks
become much less permeable, reducing or eliminating fluid flow pathways. Such a process likely
occurred relatively quickly in the subsurface above the primary heat sources in rocks that probably
had low intrinsic permeability when they first were emplaced. Once the fluid flow pathways were
reduced or eliminated, convection of the hot fluid upward would be reduced. Since convection is
an effective heat transfer process, sealing of the fluid flow pathways would effectively trap heat
above the heat source. With sufficient energy input, the groundwater would boil and vaporize in the
subsurface. The impermeable barrier of rock that formed during water–rock interaction would then
act as a cap, trapping a zone of steam in the area that is now being utilized for geothermal power
production.
The volume of this system is large. Drilling has extended to well over 3000 meters and no liquid
zone has yet been found. This implies the total steam reservoir must be over a thousand meters thick
in places. The dry steam temperature is between 225°C and 300°C, depending upon location within
the field (Dalrymple et al. 1999; Dobson et al. 2006; Moore, Norman, and Kennedy 2001).

power GeneraTion hisTory
The first attempt to generate power using geothermal steam at The Geysers was in 1921 when a 35
kW generator was installed to supply power to a local hotel. Although power was successfully gen-
erated, the steam energy conversion system used for power production was damaged by particulates
in the fluid and the chemically aggressive nature of the fluid, and was abandoned after a relatively
short time.
In 1961 power production was again pursued, this time with more robust technology and a bet-
ter understanding of geothermal power production principles. The first plant was an 11 MW plant
that was built near the site of the original 35 kW power plant. Today, there are a total of 21 power
plants with an installed generating capacity of about 1400 MW. The locations of the power plants
are shown in Figure 9.14C.
Drilling in the region can be a complex undertaking. On the positive side, the hard, impermeable
cap rock that sealed in the resource allowed the wells to be completed without casing. These open
holes reduce costs since liners or other metal pipe need not be emplaced in the hole to keep it stable.
However, the terrain is rugged and the ground unstable, partly due to the geology and partly due to
the presence of aggressive gases and fluids that have reacted with the rock, reducing the cohesion of
the soils and shallow bedrock. These conditions have combined to make it economically expedient to
utilize a drill pad located in a stable location for wells that access resources some distance to the side
of the drill site. Such deviated wells, which are wells that angle off in a predetermined direction, are
one method that has been employed to tap the reservoir (Figure 9.15). Recently, such methods have
become more common in the geothermal industry, building on a long history of such drilling meth-
ods in the oil and gas industry. Another drilling innovation that has been utilized is the branched or
forked or multiple-legged well (Figure 9.15), which will have several wells deviate off of one vertical
well. Such methods gather the steam from several places in the reservoir and pipe it through a single
wellhead, increasing the per-well production and simplifying the above-ground piping.

emissions
In the course of exploration, it was discovered that the composition of the steam system varied from
place to place. Common noncondensable gases include CO , H S, and HCl. In the northern part of
2
2
the field, especially, it was found that the HCl concentration is particularly high, causing the steam
to be extremely aggressive. This acidic, low pH resource is currently not economical to develop
since exotic, costly materials would be required for the piping and turbine components. As a result,
the northern part of the resource has yet to be developed.