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product may contribute as well as the presence of equilibrium steam in the aquifer. In calculating aqueous species distribution in the aquifer water such equilibrium steam was assumed not to be present. Calculated variation in the calcite saturation state upon adiabatic boiling is considerably more accurate than calculation of the absolute saturation state. Upon boiling degassing of the water with respect to CO 2 generally causes an initially calcite saturated water to become supersaturated. This is particularly the case with waters high in CO 2 such as in Olkaria West and where aquifer temperatures are around 200°C but CO 2 solubility in water at minimum around 200°C. In relatively high temperature and low CO 2 waters, such as at Nesjavellir, early boiling may only cause limited degassing and not calcite supersaturation. Prolonged depressurization boiling, which leads to a decrease in fluid temperature, decreases the calcite saturation index due to the retrograde solubility of calcite with respect to temperature. This often causes boiled geothermal waters to be calcite undersaturated. Calcite scaling is not expected to be a problem at Olkaria and Nesjavellir and neither at Svartsengi and Reykjanes if the first depth level of boiling is outside wells i.e. in the producing aquifers. Scaling tests at Nesjavellir showed that at high temperatures of the wellheads of wells NJ-14 and NJ- 22, the two phase fluids deposited scales that were dominantly sulphide phases in well NJ-14 fluids and mixed sulphides and oxides in the case of well NJ-22. The scales were essentially amorphous to X-rays. There were however, some indications of some mineral phases in the scales such as chalcopyrite and clayey material. Separated waters after the heat exchangers, entry to the retention tank and at the injection well deposited mainly amorphous silica but there are also some indications of the presence of clays in the scales. The rate of deposition of ~0.261 mm/yr was highest at the entry to the retention tank and much lower of ~ 0.0168 mm/yr at the injection well. Silica polymerisation occurs in the retention tank and this reduces the rate of silica deposition at the injection well. At entry to the retention tank, the high rate of deposition could be due to supersaturation by monomeric silica which causes a higher rate of precipitation (Yanagase et al., 1970; Rothbaum, 1979). An intermediate test site between the steam separators and the wellheads could be chosen to investigate the type of scales that would form. The solute content of the water discharged from the atmospheric silencer of Olkaria well OW-34 is high The discharge enthalpy of the well fluids is close to that of dry steam leading to extensive evaporation of the water fraction in the discharge by depressurization boiling to a low pressure. This extensive evaporation is thought to be the cause of the high salt content of the water discharged at atmospheric pressure as well as the extensive amorphous silica scale formation. A solution to this scaling problem involves mixing of the discharge of well OW-34 with separated brine or condensate from another well e.g. well R-3 where separated brines from the Olkaria II plant are being re-injected to lower the discharge enthalpy of OW-34. This will reduce the amount of evaporation of water in well OW-34. Theoretical calculations based on mass balance equations, chloride concentration of spent brine being re-injected in well R-3 and that of well OW-34 for purposes of diluting the water have been tried. In this way aqueous silica concentrations will not build up to the same extent and amorphous silica deposition will be reduced or inhibited. 48
REFERENCES Allegrini, G., and Benvenuti, G., 1970: Corrosion characteristics and geothermal power plant protection (Collateral processes of abrasion, erosion and scaling) U.N. Symposium on the development and utilisation of geothermal resources, Pisa, Geothermics, 865-881. Ármannsson, H., 1989: Predicting calcite deposition in Krafla boreholes. Geothermics, 18, 25-32. Ármannsson, H., 2003: CO 2 emissions from geothermal plants. Proc. of the International Geothermal Conference IC2003, Reykjavik, 56-62. Ármannsson, H., Gíslason, G., and Haukson, T., 1982: Magmatic gases in well fluids aid the mapping of the flow of pattern in a geothermal system. Geochim. Cosmochim. Acta, 46, 167-177 Arnόrsson, S., 1978: Precipitation of calcite from flashed geothermal waters in Iceland. Contrib. Mineral. Petrol., 66, 21-28. Arnόrsson, S., 1981: Mineral deposition from Icelandic geothermal waters, environmental and utilisation problems. J. Petr. Techn., 33, 181-187. Arnόrsson, S., 1989: Deposition of calcium carbonate from geothermal waters – theoretical considerations. Geothermics, 18, 33-89. Arnόrsson, S., 1995: Scaling problems and treatment of separated water before injection. In: Rivera, J. (ed), Injection technology. World Geothermal Congress 1995; IGA pre-congress course, Pisa, Italy, May 1995, 65-111. Arnόrsson, S., 2000a: The quartz and Na/K geothermeters. I. New thermodynamic caliberation. Proc. of World Geothermal Congress 2000, Kyushu- Tohuku, Japan, 929-933. Arnόrsson, S., 2000b: Injection of waste geothermal fluids: Chemical aspects. Proc. of the World Geothermal Congres 2000, Kyushu- Tohuku, Japan, 3021-3025. Arnόrsson, S., and Stefánsson, A., 2005: Wet steam well discharges.I. Sampling and calculation of total discharge compositions. Proc. of World Geothermal Congress 2005, Antalya, Turkey, Arnórsson, S., D’Amore, F., and Gerardo-Abaya, J., 2000: Isotopic and chemical techniques in geothermal exploration, development and use. In: Arnórsson, S. (ed.), Isotopic and chemical techniques in geothermal exploration, development and use. Sampling methods, data handling, interpretation. International Atomic Energy Agency, Vienna. Arnόrsson, S., Geirsson, K., Andrésdόttir, A., and Sigurdsson, S., 1996: Compilation and evaluation of thermodynamic data on aqueous species and dissociational equilibria in aqueous solution I. The solubility of CO 2 , H 2 S, H 2 , N 2 , O 2 and Ar in pure water. University of Iceland, Science Institute, report No. RH-17-96, 74 pp. Arnόrsson, S., Sigurdsson, S., and Svavarsson, H., 1982: The chemistry of geothermal waters in Iceland I. Calculation of aqueous speciation from 0°C to 370°C. Geochim. Cosmochim. Acta, 46, 1513-1532. Benoit, W., 1989: Carbonate scaling characteristics in Dixie Valley, Nevada geothermal wellbores. Geothermics, 18, 41-48. Bertini, R., 2005: World geothermal power generation in the period 2001-2005. Geothermics, 34, 651- 690. 49
Page 1 and 2:
GEOTHERMAL TRAINING PROGRAMME Repor
Page 3 and 4:
INTRODUCTION The Geothermal Trainin
Page 5 and 6: ABSTRACT The Olkaria geothermal sys
Page 7 and 8: Page 9. CONCLUSIONS ...............
Page 9 and 10: 1. INTRODUCTION For many countries
Page 11 and 12: deposition does not occur at depth
Page 13 and 14: The rate of sulphide precipitation
Page 15 and 16: the Western Sectors. The Olkaria II
Page 17 and 18: 46.4 MWe and 125 MWt. A shallow ste
Page 19 and 20: 3. SAMPLING AND ANALYSIS 3.1 Sample
Page 21 and 22: 4. FLUID COMPOSITIONS The compositi
Page 23 and 24: 5. CALCULATION OF AQUIFER FLUID COM
Page 25 and 26: increases the difference in tNaK -
Page 27 and 28: The procedure for calculating aquif
Page 29 and 30: Concentrations of CO 2 at equilibri
Page 31 and 32: High concentrations of HCl can be g
Page 33 and 34: variation in chloride concentration
Page 35 and 36: A 240°C a steam cap overlies the l
Page 37 and 38: The solubility of CO 2 in water cha
Page 39 and 40: where it passes through mist elimin
Page 41 and 42: Coupon holder loosened the screws a
Page 43 and 44: Coupon # 20 (Re-injection well): A
Page 45 and 46: 1900 1800 C #10 Well NJ-14 1700 160
Page 47 and 48: 0,3 13 Weeks (06-07-2005 to 14-10-2
Page 49 and 50: 8. EVALUATION OF SCALES DEPOSITED A
Page 51 and 52: atmospheric pressure. The results d
Page 53 and 54: The IR spectra of scale samples #1
Page 55: 9. CONCLUSIONS Olkaria well fluids
Page 59 and 60: D’Amore, F., Truesdell, A.H., and
Page 61 and 62: Hurtado, R., Andritsos, N., Mouza,
Page 63 and 64: Saemundsson, K., and Fridleifsson,
Page 65: APPENDIX A: Tables with analyses, c
Page 74: APPENDIX C: Analyses of scales and
Page 80 and 81: Spectra for scales from coupon # 16
Page 82 and 83: Spectra for Olkaria OW-34 scale # 1
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