Page 272 - Geothermal Energy Renewable Energy and The Environment

14 The Geothermal Energy

Future: Possibilities and Issues

Throughout this book there has been an effort to provide an historical context for the various ele-
ments that contribute to the development and use of geothermal energy. Knowledge of the events
that have contributed to the growth and success of geothermal use provide perspective on underlying
issues that influence whether an application or concept will succeed. That historical context is brief,
barely spanning 100 years. Even so, significant advances in understanding and technological capabil-
ity have motivated rapid growth, particularly in the last 50 years. And yet, as suggested by much of the
material in this book that growth may well be little more than the initial stage of geothermal energy
applications playing a much larger role in the global energy landscape. This chapter considers some of
the important new resources and technologies that are likely to influence growth in the near future.

GeopressUred resoUrces

maGniTude of The resource
As noted in Chapter 7, drilling for oil and gas in the southern United States demonstrated the pres-
ence of horizons in the subsurface where the fluid pressure exceeded that expected for a simple
hydrostatic gradient. Since first recognized, these geopressured systems have been found in other
regions that are usually associated with oil and gas fields. They are attractive as a potential geother-
mal resource because they have temperatures suitable for power generation using binary generators.
In addition, these zones are distributed in regions where industrial demand for power is high, as in
the oil and gas industry. In many instances, such areas are also near major electric load centers, thus
making them potential resources for municipal power (Figure 14.1).
The power generation potential for these systems is significant. Fluids in these settings com-
monly occur in the temperature range of 110°C to 150°C. It has been reported (Garg 2007) that
the recoverable thermal energy in the northern Gulf of Mexico Basin, a region of geopressured
resources, is between 270 × 10 and 2800 × 10 J. Other regions, as indicated in Figure 14.1, are
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likely to possess significant additional potential. The total capacity for electrical power generation
is estimated to be greater than 100,000 MW (Green and Nix 2006).
Many of these locations have high methane concentrations associated with them. This energy-
rich hydrocarbon gas is an additional resource with an estimated recoverable energy content of
between 1 × 10 and 1640 × 10 J (Westhusing 1981; Garg 2007).
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why Geopressured reservoirs form
Geopressured zones form in sedimentary basins where subsurface fluid migration is impeded by
greatly reduced permeability. Generally, these low-permeability, or sealed, zones block what would
otherwise be a natural fluid migration pathway. They, therefore, form natural traps for migrating
oil, gas, and other fluids. As fluids continue to migrate into those regions, driven by burial, compac-
tion, and hydrological gradients, the pressure below these sealed zones increases beyond the nor-
mal hydrostatic gradient and approaches the local lithostatic pressure over geological time (see the
Sidebar in Chapter 3). The elevated pressures in these regions are sufficient to drive rapid transport
of fluid to the surface, thus eliminating the need for pumping and providing additional energy to the

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