Indonesia and lessons learned from the original HDR project located in the southwest of the United States.
The High Cost of Deep Heat The upside potential of geothermal energy may be enormous. In 2008, world electricity con- sumption was 2 terawatt years. The heat flux continuously flowing from the Earth’s core is equivalent to about 44 terawatt years.3
These
numbers are astronomical of course, but if only a small percentage of this potential were to be tapped, it would easily supply most of the world’s energy demands. Most geothermal resources are also truly renewable in that the same fluids can be reheated, produced, injected and recycled throughout the life of the reservoir. Besides the technological questions are finan-
cial ones that persist in the face of otherwise posi- tive investment factors (above right). Geothermal projects, with few exceptions, require a signifi- cantly higher initial capital outlay than do oil and gas, solar, wind and biomass projects. The risk is also higher, and the current experience with return on investment in geothermal installations is discouraging. For example, a 50-MW hydrother- mal project is estimated to yield an initial rate of return of less than 11% and a profit-to-investment (P/I) ratio of 0.8. By comparison, a large oil and gas project typically yields an initial rate of return of nearly 16% and a P/I of 1.5.4 These poor financial results are partially a
reflection of geography. Areas with favorable hydrothermal conditions tend to be sparsely populated and far from large electricity markets. Financial results are also hampered by the diffi- culty inherent in drilling and developing these formations. Geothermal resources are found in much harder and hotter rock than those for which petroleum and mining industry bits are designed, so drilling is slower and more costly. To be economic, geothermal wells must accommo- date relatively large flow volumes, and therefore wellbore diameters must be greater than those of most oil and gas wells. This adds considerably to well construction costs. The extreme temperature of geothermal environments forces operators to choose high-priced premium products for such things as cements, drilling fluids and tubulars. While in recent decades the oil industry
has greatly refined drilling and reservoir man- agement efficiencies—consequently reducing costs—it has often done so through such elec- tronics-based innovations as logging while drill- ing and subsurface monitoring. These tools are
Solar 24 to 33
Renewable Energy Sources
Geothermal Biomass Hydroelectric Wind
Capacity Factor, %
86 to 95 83 30 to 35 25 to 40
Reliability of Supply
Continuous and reliable
Reliable
Intermittent, dependent on weather
Intermittent, dependent on weather
Intermittent, dependent on weather
Environmental Impact
Minimal land usage
Minimal (noncombustible material handling)
Impacts due to dam construction
Unsightly for large- scale generation
Unsightly for large- scale generation
Main Application
Electricity generation
Transportation, heating
Electricity generation
Electricity
generation (limited) Electricity
generation (limited)
> Alternative energy comparative value. Among renewable energy sources, geothermal energy is one of the most attractive based on the capacity factor—the percentage of energy actually produced by a plant compared with its potential output when operated continually at full capacity. It also compares favorably with other alternative energy sources when different metrics are used. (Capacity factor data from Kagel A: A Handbook on the Externalities, Employment, and Economics of Geothermal Energy. Washington, DC: Geothermal Energy Association, 2006.)
currently restricted to temperatures below about 175°C [350°F] and are not available for use in high-temperature geothermal wells.
Finding and Defining With the exception of some “blind” deep, high- temperature systems, the search for hydrother- mal formations is made relatively easy by hot springs and fumaroles that are visible at the sur- face.5
Additionally, many hydrothermal fields are
in deep sedimentary basins where oil and gas drilling and, more importantly, data collection have already occurred. The geologic setting for hydrothermal reser-
voirs varies. The reservoirs in the largest fields contain a wide range of rocks, including quartz- ite, shale, volcanic rock and granite. Most of these reservoirs are identified not by lithology but by heat flow. They are convection systems in which hot water rises from depth and is trapped in reservoirs whose caprocks have been formed by the mixing of upwelling geothermal fluids with local groundwaters and by precipitation of car- bonate and clay minerals. Therefore, the search for a commercial near-
surface hydrothermal reservoir is based on iden- tifying tectonic activity, heat source, heat flow, water recharge and outflow of deep fluids to the surface. Permeability is typically characterized by a network of fractures or active faults held open by local in situ stresses.
2. “First Successful Coproduction of Geothermal Power at an Oil Well,” JPT Online (October 21, 2008), http://www.
spe.org/jpt/2008/10/first-successful-coproduction- geothermal-oil-well/ (accessed July 14, 2009).
3. Pollack HN, Hurter SJ and Johnson JR: “Heat Flow from the Earth’s Interior: Analysis of the Global Data Set,” Reviews of Geophysics 31, no. 3 (August 1993): 267–280.
The hunt for a hydrothermal reservoir begins
with an assessment of available regional data on heat flow, seismic activity, thermal springs and characteristic surficial elemental signatures from remote sensing and imaging. Geophysical, geologic and geochemical techniques that can provide information on the size, depth and shape of deep geological structures are then put into effect. Subsurface temperature measurements are
AUT09–RVF–03
the most direct method for ascertaining the existence of a hydrothermal system. Thermal- gradient holes can be as shallow as a few meters, but to exclude surface-temperature effects the preference is for a depth of more than 100 m [330 ft]. Temperature surveys can delimit areas of enhanced thermal gradients—a basic require- ment for geothermal systems. In volcanic terrains, high-temperature rocks may occur at relatively shallow depths, and it is likely that a heat source is present. In systems of deep circulation, high temperatures indicate thin continental crust, high rates of heat flow and deep permeable faults that transmit mantle heat close to the surface. Hydrothermal reservoirs require high tempera-
tures and effective permeability, which is offered by coherent rocks capable of supporting open frac- ture systems. These rocks have a relatively resis- tive signature. The associated clay-rich caprocks, however, have low resistivity. The resistivity con- trast at the base of the caprock, which can be
4. Long A: “Improving the Economics of Geothermal Development Through an Oil and Gas Industry Approach,” Schlumberger white paper,
www.slb.com/ media/services/consulting/business/thermal_dev.pdf (accessed September 15, 2009).
5. A fumarole is a vent or opening in the Earth’s surface through which steam, hydrogen sulfide or other gases escape.
Winter 2009/2010
7
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64