Page 283 - Geothermal Energy Renewable Energy and The Environment

274 Geothermal Energy: Renewable Energy and the Environment

vicinity of fracture surfaces and act over a distance of 10 m, these assumptions result in (after rec-
onciling most of the units)

Q /Q = (300 J/s-m)/(5 J/s-m) = 60.
cv
cd
This result indicates that, in the immediate vicinity of a fracture, replenishing the heat removed
by convection with heat in the surrounding rock mass will take significantly longer than the time
it took to remove the heat. Accurately estimating the amount of time for an entire EGS system to
recover is a complex problem (Elsworth 1989, 1990) and is not possible without detailed information
about geometry of flow paths, heat capacities and thermal conductivities of the affected materials,
and the local heat flow. However, for reasonable assumptions about these parameters and the geom-
etry of an EGS stimulated region, Pritchett (1998) and Tester et al. (2006) have shown that the time
it takes to replenish 90% of the heat is approximately three times the operational period.
This result suggests two operational scenarios. One scenario would be to use an EGS reservoir
system for 30 years and then abandon it for about 100 years. The wells originally put in place could
then be refurbished at significantly lower cost than the original drilling effort, and the system could
then be restarted. An alternative approach, and one which would be less disruptive to the generation
effort, would be to use deviated wells to develop an area three to five times the size needed for the
operation. Then, systematically cycling fluid production through the various wells on a regular but
limited-time basis would allow operations to proceed in an uninterrupted fashion for 100 years
or more.

synopsIs
The development of technology suitable for generating power from geothermal systems has
improved significantly since the 1950s, when electrical power generation using geothermal energy
resources grew to an international enterprise. The growth of the power generation industry has been
steady since that time. Even so, geothermal resources of even greater magnitude are available but
have yet to be developed. Two resources, in particular, could importantly contribute to generating
power. One such resource is moderate temperature fluids associated with oil and gas fields. These
geopressured reservoirs could contribute significantly to meeting the energy needs in regions where
these resources exist. An even larger resource is the ubiquitous geothermal reservoir that exists at
depths of 3–10 km below the surface. Conservative estimates place the magnitude of this resource at
several thousand times the total energy needs of the United States and the globe. However, access-
ing that resource is currently a technological challenge that has yet to be met. Current research and
development efforts suggest this could be accomplished within the next 25 years.

Problems
14.1 What is a geopressured geothermal resource and how do they form?

14.2 Where are geopressured resources located? How might this distribution affect the
development of geopressured resources?
14.3 Why are geopressured resources difficult to develop? Consider in your discussion the
effects of local climate.
14.4 What generation technologies are suitable for generating power at geopressured sites?
What difficulties might this technology face in these settings?
14.5 What is EGS? How does it differ from other geothermal power generation technologies?
14.6 The potential EGS resource is very large. If you were an investor, what criteria would
you use to determine where to first deploy this technology? Why?
14.7 Of the principle challenges faced by EGS, what, in your opinion, is the easiest to over-
come? What is the hardest? Why?
14.8 If you were to develop an EGS site for a 60-year lifetime, what drilling schedule would
you institute to assure that the facility was sustainable for that time period?