within a hydrothermal reservoir as geological cross sections and maps (right). An upward flow of water creates an upward isotherm pattern and indicates permeable rocks. When reservoir flow is vertical, temperatures increase significantly with depth. In an outflow zone the flow is hori- zontal and temperatures decrease with depth.11 Permeable zones have smaller temperature


gradients with depth than do impermeable ones and generally display a convective isotherm pat- tern. In very low-permeability formations, the temperature gradient is steep and is easily seen in a cross section as closely spaced isotherms that reveal a conductive thermal regime. The gra- dient helps determine the location of permeable and impermeable zones. Since low resistivity usually indicates low-permeability conductive clays, MT surveys may be used to locate the base of a geothermal caprock and, indirectly, its high thermal gradient. The dimensions of the reservoir can then be mapped and used to identify drilling targets and prospective locations of production and injection wells.


Enhancing Nature The hydrothermal fields that are now online and that were discovered through these techniques and models represent the geothermal industry’s low-hanging fruit. The future of geothermal energy lies in more-complex systems that must be coaxed into production and in recovering more heat from those already in existence through EGS projects (right). Similar to processes in oil and gas operations,


conceptual modeling may be used to plan and execute EGS projects for hydrothermal reservoir development. Using data gained from years of production to construct better models, engineers can assess the potential response of these geo- thermal fields to infill drilling, water injection and other processes that help extend the field and improve reservoir efficiency. At Desert Peak near Fernley, Nevada, a geo-


thermal field was discovered and defined in the 1970s and 1980s. It has been delivering power to a double-flash power plant since 1986 and is typi- cal of the deep-circulation, or fault-controlled, geothermal systems of the western USA.12


An EGS


project that would expand the operation through hydraulic and chemical stimulation is under study. The study will determine the distribution of rock types, faults, alteration minerals and mineralized fractures east of the existing hydro- thermal field to create a new structural model of the field.13


Acid sulfate fumarole


Unaltered


Argillic zone Zeolite-smectite


212°F zone


302°F 392°F


482°F


Propylitic zone


Upflow in fractures


150°C 250°C 200°C 572°F


Heat and gas from magma


300°C


> Isotherms from geothermometry. Cation geothermometry data from a fumarole and a chloride hot spring can be modeled using a geological interpretation to obtain a subsurface temperature profile. The hot spring is assumed to be close to the top of the water table. Propylitic alteration transforms iron- and magnesium-bearing minerals into chlorite, actinolite and epidote. (Adapted from Cumming, reference 8.)


Category of Resource


Conduction-dominated EGS Sedimentary rock formations Crystalline basement rock formations Supercritical volcanic EGS


Hydrothermal Coproduced fluids


Thermal Energy, in Exajoules [1 EJ = 1018 J]


100,000 13,300,000 74,100


2,400 to 9,600 0.0944 to 0.4510


> Enhanced geothermal systems potential in the USA. Estimates for the potential energy payout from EGS resources at depths between 3 and 10 km are more than 13 million exajoules (EJ). Recovery of even a small percentage would be more than enough to supply all the electrical needs of the nation. [Adapted from “The Future of Geothermal Energy,” http://geothermal.inel.gov/ publications/future_of_geothermal_energy.pdf (accessed June 30, 2009.)]


6. For more on MT: Brady J, Campbell T, Fenwick A, Ganz M, Sandberg SK, Buonora MPP, Rodrigues LF, Campbell C, Combee L, Ferster A, Umbach KE, Labruzzo T, Zerilli A, Nichols EA, Patmore S and Stilling J: “Electromagnetic Sounding for Hydrocarbons,” Oilfield Review 21, no. 1 (Spring 2009): 4–19.


7. Manzella A: “Geophysical Methods in Geothermal Exploration,” Lecture notes. Pisa, Italy: Italian National Research Council International Institute for Geothermal Research, http://www.cec.uchile.cl/~cabierta/revista/12/ articulos/pdf/A_Manzella.pdf (accessed August 10, 2009).


8. Cumming W: “Geothermal Resource Conceptual Models Using Surface Exploration Data,” Proceedings of the Stanford University 34th Workshop on Geothermal Reservoir Engineering, Stanford, California, USA (February 9–11, 2009).


9. Duffield WA and Sass JH: “Geothermal Energy—Clean Power from the Earth’s Heat,” US Geological Survey, Circular 1249, http://pubs.usgs.gov/circ/2004/c1249/ (accessed August 3, 2009).


AUT09–RVF–05


10. A geothermometer is a mineral or group of minerals whose composition, structure or inclusions are fixed within known thermal limits under particular conditions of pressure and composition and whose presence thus denotes a limit or a range for the temperature of formation of the host rock.


11. Cumming, reference 8.


12. A double-flash system uses brine separated from geothermal water before it was flashed. The brine is flashed a second time at a lower pressure, and the resulting steam is used to drive a separate turbine or is sent to the high-pressure turbine through a separate inlet.


13. Lutz SJ, Moore JN, Jones CG, Suemnicht GA and Robertson-Tait A: “Geological and Structural Relationships in the Desert Peak Geothermal System, Nevada: Implications for EGS Development,” Proceedings of the Stanford University 34th Workshop on Geothermal Reservoir Engineering, Stanford, California (February 9–11, 2009).


100°C Smectite clays Marine clays


Chloride spring


Winter 2009/2010


9


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