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Application of Geochemical Methods in Geothermal Exploration in Kenya
Available online at www.sciencedirect.com
Procedia Earth and Planetary Science 7 (2013) 602 – 606
Water Rock Interaction [WRI 14]
Application of geochemical methods in geothermal exploration in Kenya Samuel M. Mwangi Laikipia University College, P. O. Box 1100, Nyahururu 20300, Kenya
Abstract The major goals of geochemical exploration are to obtain the subsurface composition of the fluids in a geothermal system and to use this to obtain information on temperature, origin, and flow direction, which help in locating the subsurface reservoir. Subsurface waters have been classified into meteoric water, ocean water, evolved connate water, magmatic water, and juvenile water. Geothermal water is mostly meteoric and oceanic water, although andesitic waters near subduction areas often contain significant proportions of evolved connate and magmatic waters. Geothermal waters have been classified with respect to their anion and cation contents into alkali-chloride water, acid sulphate water, acid sulphate-chloride water, and bicarbonate water. Acid waters are generally unsuitable for elucidation of subsurface properties. Conservative constituents are used for tracing the origin and flow of geothermal fluids, stable isotopes (especially 2H and 18O), along with B and Cl being most important. Rock forming constituents (e.g. SiO2, Na, K, Ca, Mg, CO2, and H2) are used to predict subsurface temperatures and potential production problems such as deposition and corrosion. Kenya is located on the Great Rift Valley in Eastern Africa and has a huge potential for geothermal production of power and other direct uses. In this paper, we report on the geochemical methods used in Kenya to explore, develop, and monitor the available geothermal resources for power production and other direct uses. © 2013 Published by Elsevier B.V. B.V. © 2012The TheAuthors. Authors. Published by Elsevier Selection and/or peer-review under responsibility of the Organizing and Scientific Committee of WRI 14of– 2013 Selection and/or peer-review under responsibility of Organizing and Scientific Committee WRI 14 – 2013. Keywords: sub-surface waters, geothermal manifestations, geochemical methods.
1. Introduction The Olkaria Geothermal Area is located in the central sector of the Kenya Rift Valley to the south of Lake Naivasha and 120 km Northwest of Nairobi. The Kenya Rift Valley is an integral part of the East African Rift Valley System, which extends for over 3000 km from Southern Mozambique through Tanzania, Kenya and Ethiopia to join the Red Sea and the Gulf of Aden rifts at the Afar triple junction [1]. The Olkaria high temperature geothermal field is associated with a region of Quaternary volcanism. Twelve (12) similar Quaternary volcanic centers occur in the axial region of the rift and are potential geothermal resources [2-4].
1878-5220 © 2013 The Authors. Published by Elsevier B.V. Selection and/or peer-review under responsibility of the Organizing and Scientific Committee of WRI 14 – 2013 doi:10.1016/j.proeps.2013.03.220
Samuel M. Mwangi / Procedia Earth and Planetary Science 7 (2013) 602 – 606
Geothermal exploration provides a great understanding of the location, nature and origin of the geothermal waters in a geothermal system and generally geochemical exploration will measure the earth’s chemical properties in order to delineate geothermal fields, locate aquifers, site wells and provide useful information on which decisions of commercial viability can be made on a geothermal resource. During these measurements, emphasis is put on parameters that are sensitive to sub-surface temperatures, salinity of the fluids and gaseous chemical composition in the areas of interest. Ellis and Mahon [5] have classified subsurface waters into meteoric water, ocean water, evolved connate water, magmatic water and juvenile water. Water rock interaction is of importance when it comes to geochemistry because different components and constituents are produced that are of interest to geochemists. Conservative constituents are used for tracing origin and flow of geothermal fluids, stable isotopes (especially 2H and 18 O) along with B and Cl being most important. Rock forming constituents (e.g. SiO2, Na, K, Ca, Mg, CO2, H2) are used to predict subsurface temperatures and potential production problems such as deposition and corrosion. Kenya is endowed with huge geothermal potential with estimates in excess of 10,000MW (GDC Strategic Plan 2010) due to its location along the Great Rift Valley of Eastern Africa (Fig. 1).
Figure 1: Map showing location of geothermal area along the Kenyan Rift Valley. 2. Geochemical Methods
Geochemical methods are used in locating and sampling surface discharges which include: water (hot/cold) and fumarolic steam. Non- manifestation areas are surveyed using soil gas surveys and these
603
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Samuel M. Mwangi / Procedia Earth and Planetary Science 7 (2013) 602 – 606
include measuring chemical components associated with geothermal activity such as: CO2 gas, Hg and Rn-222 radioactivity. In many areas, geothermal indicators occur in form of fumaroles, hot (boiling) springs, mud pools, hot altered grounds and geysers. Three main methods [8] have been used in Kenya to obtain geochemical samples and they are: Water sampling - this involves collecting liquid water samples from drilled boreholes and natural springs so as to evaluate origin of the fluids, temperature estimations at depth, predict scaling and corrosion problems. Fumarole steam and condensate sampling – gases and condensates are sampled in order to compute reservoir temperature at depth where the steam is being formed. Soil gas survey (for Rn-222, CO2 and temperature) - Rn-222 and CO2 in the soil are indicators of permeability and possible location of a reservoir. 3. Results and discussion
The major gases in geothermal steam are CO2, H2S, H2, CH4, N2, NH3, CO, and O2. The noble gases in steam include He, Ne, Ar, etc. Carbon dioxide is generally the major gas component often comprising more than 80% of all non-condensable gases and its concentration in total increases with reservoir temperature. Hydrogen sulphide concentration commonly decreases as steam ascends to the surface due to reaction with wall rock, dissociation to sulphur or oxidation to SO4. In many geothermal fields of high temperature above 200 oC, temperature dependant gas and mineral-gas equilibrium control the concentration of the geothermal gases [6-7]. Geochemical studies of geothermal fluids involve three main steps: 1) sample collection, 2) chemical analysis and 3) data interpretation [11]. The types of samples collected in this study were condensates from four fumaroles (Table 1) in the Olkaria field and thereafter, full analysis as stipulated by Zhong-He and Armannsson [12] was done. Our main interest was in the chemical constituents that are volatile enough to partition into steam phase upon separation in boiling system as expected in a geothermal aquifer producing steam. These elements include the soluble gaseous compounds present in geothermal steam such as CO2, H2S, NH3, F, and B. In addition other non-volatile elements were considered such as major ions, Cl and SO4 and the pH and total dissolved solids (TDS) and the electrical conductivity in steam condensates. The samples were collected at the fumarole sites that showed higher temperatures and more altered features. Table 1 below shows results obtained in Olkaria [9]. Table1: Some geochemical results in Olkaria PARAMETER Co-ordinates (UTM) Sampling temp. (oC) pH/20oc SiO2(ppm) B (ppm) Na (ppm) K (ppm) CO2 (ppm) SO4 (ppm) Cl (ppm) NH4 (ppm) H2S (ppm) Conductivity (μS/Cm) TDS (ppm)
DF1 9897127, 0205186 91.5 5.76 1.53 2.61 1.59 0.165 62.92 4.7 0.45 0.11 0.04 37 18
DF2 ( BH) 9897780, 0204820 91 6.51 1.68 2.86 0.503 1.017 40.04 3.3 0.25 0.64 0.02 19 9
DF3 989689, 0205014 87.7 5.73 0.025 1.86 2.69 0.087 58.3 2.6 0.45 1.32 0.04 14 7
DF5 9898050, 0205195 91 5.68 0.25 3.94 0.215 0.194 45.54 3.51 0.05 0.04 0.06 2 1
Samuel M. Mwangi / Procedia Earth and Planetary Science 7 (2013) 602 – 606
Geothermometers are parameters used to determine sub-surface conditions since at particular temperatures, common assemblages of minerals will tend towards equilibrium with a given water chemistry. It has been noted that for certain parameters or ratios of parameters, the relationship between temperature and chemical composition will be stable and predictable. In this kind of sampling which is vapour dominated, gas geothermometers are used since nothing is known of the composition of the liquid phase from which the produced fluid originates. The concentrations and relative concentrations of gases in the geothermal steam are controlled by temperature dependent fluid-mineral and gas-gas equilibria that have been used as the basis of gas geothermometers [13]. A univariant CO2 gas-geothermometer developed by Arnórsson [10] is used in the final step of our geochemical study for data interpretation and also an H2S geothermometer to determine the sub-surface temperatures. The CO2 geothermometer gave us a more reliable reservoir temperature estimation ranging from (237 oC – 258 oC) while the H2S geothermometer temperature estimation was quite low ranging from (170.6 oC – 187.6 oC). This could be due to the oxidation and condensation it undergoes as it ascends to the surface. 4. Conclusion
Ellis and Mahon [5] have summarized all the above activities by saying that “the credibility and usefulness of geochemical data depends on the methods used and the care taken in the collection of samples.” Geochemical investigations involve the sampling and analysing of chemical and/or isotopes of the waters and gases from geothermal manifestations (hot springs, fumaroles, etc.) or wells in the study area [10]. Information from down-hole measurements of an exploratory well can be used to prepare well profiles and have these correlated to sub-surface temperatures with hydrothermal mineral assemblages. The reservoir temperature in Olkaria estimated by use of CO2 based geothermometry is above 230oC confirming the existence of a geothermal resource. Similar temperatures are registered in the Domes field drilled wells. Geochemical surveys can also be used to locate feeder zones where the hot fluids are entering the reservoir; thus geochemical and geological surveys provide useful data for planning explorations and their costs are relatively low compared to the more sophisticated methods of geophysics and exploratory drilling. 5. References [1] KenGen. Surface exploration of Kenya’s geothermal resources in the Kenya Rift. The Kenya Electricity Generating Company Limited, Internal report; 1998, 24. [2] Omenda PA. The geology and structural controls of the Olkaria geothermal system, Kenya. Geothermics 1998; 27(1): 55-74. [3] Omenda PA, Onacha SA, Ambusso WJ. Geological setting and characteristics of the high temperature geothermal systems in Kenya. Proceedings of the New Zealand Geothermal Workshop 1993; 15: 161-167. [4] Riaroh D, Okoth W. The geothermal fields of the Kenya rift. Tectonophysics 1994; 236: 117-130. [5] Ellis AJ, Mahon WAJ. Chemistry and geothermal systems, Academic Press; 1977. [6] Arnórsson S, Gunnlaugsson E. New gas geothermometers for geothermal exploration. Calibration and application. Geochim Cosmochim Acta 1985; 49: 1307-1325. [7] Giggenbach W.F. Geothermal gas equilibria. Geochim Cosmochim Acta1980; 44: 2021-2032. [8] Magnus O. Sampling methods for geothermal fluids and gases, Orkustofnun, National Energy Authority, Geothermal Division, October 1988. [9] Cyrus W, Karingithi CW. Geochemical characteristics of the Greater Olkaria Geothermal Field, in Kenya. UNU-GTP conference, Naivasha, 2008.
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Samuel M. Mwangi / Procedia Earth and Planetary Science 7 (2013) 602 – 606
[11] D’Amore F, editor. Application of Geochemistry in Geothermal Reservoir Development. United Nations Institute for Training and Research, New York, ISBN 92-1-157187-2, 1992: pp. 408. [12] Armannsson H, Zhong-He P. Analytical Procedures and Quality Assurance for Geothermal Water Chemistry, UNU-GTP 2006, Report 1. [13] Barbier E. Renewable and Sustainable Energy Reviews, Elsevier Science 6, 2002; 3-65: pp. 27-28.
Procedia Earth and Planetary Science 7 (2013) 602 – 606
Water Rock Interaction [WRI 14]
Application of geochemical methods in geothermal exploration in Kenya Samuel M. Mwangi Laikipia University College, P. O. Box 1100, Nyahururu 20300, Kenya
Abstract The major goals of geochemical exploration are to obtain the subsurface composition of the fluids in a geothermal system and to use this to obtain information on temperature, origin, and flow direction, which help in locating the subsurface reservoir. Subsurface waters have been classified into meteoric water, ocean water, evolved connate water, magmatic water, and juvenile water. Geothermal water is mostly meteoric and oceanic water, although andesitic waters near subduction areas often contain significant proportions of evolved connate and magmatic waters. Geothermal waters have been classified with respect to their anion and cation contents into alkali-chloride water, acid sulphate water, acid sulphate-chloride water, and bicarbonate water. Acid waters are generally unsuitable for elucidation of subsurface properties. Conservative constituents are used for tracing the origin and flow of geothermal fluids, stable isotopes (especially 2H and 18O), along with B and Cl being most important. Rock forming constituents (e.g. SiO2, Na, K, Ca, Mg, CO2, and H2) are used to predict subsurface temperatures and potential production problems such as deposition and corrosion. Kenya is located on the Great Rift Valley in Eastern Africa and has a huge potential for geothermal production of power and other direct uses. In this paper, we report on the geochemical methods used in Kenya to explore, develop, and monitor the available geothermal resources for power production and other direct uses. © 2013 Published by Elsevier B.V. B.V. © 2012The TheAuthors. Authors. Published by Elsevier Selection and/or peer-review under responsibility of the Organizing and Scientific Committee of WRI 14of– 2013 Selection and/or peer-review under responsibility of Organizing and Scientific Committee WRI 14 – 2013. Keywords: sub-surface waters, geothermal manifestations, geochemical methods.
1. Introduction The Olkaria Geothermal Area is located in the central sector of the Kenya Rift Valley to the south of Lake Naivasha and 120 km Northwest of Nairobi. The Kenya Rift Valley is an integral part of the East African Rift Valley System, which extends for over 3000 km from Southern Mozambique through Tanzania, Kenya and Ethiopia to join the Red Sea and the Gulf of Aden rifts at the Afar triple junction [1]. The Olkaria high temperature geothermal field is associated with a region of Quaternary volcanism. Twelve (12) similar Quaternary volcanic centers occur in the axial region of the rift and are potential geothermal resources [2-4].
1878-5220 © 2013 The Authors. Published by Elsevier B.V. Selection and/or peer-review under responsibility of the Organizing and Scientific Committee of WRI 14 – 2013 doi:10.1016/j.proeps.2013.03.220
Samuel M. Mwangi / Procedia Earth and Planetary Science 7 (2013) 602 – 606
Geothermal exploration provides a great understanding of the location, nature and origin of the geothermal waters in a geothermal system and generally geochemical exploration will measure the earth’s chemical properties in order to delineate geothermal fields, locate aquifers, site wells and provide useful information on which decisions of commercial viability can be made on a geothermal resource. During these measurements, emphasis is put on parameters that are sensitive to sub-surface temperatures, salinity of the fluids and gaseous chemical composition in the areas of interest. Ellis and Mahon [5] have classified subsurface waters into meteoric water, ocean water, evolved connate water, magmatic water and juvenile water. Water rock interaction is of importance when it comes to geochemistry because different components and constituents are produced that are of interest to geochemists. Conservative constituents are used for tracing origin and flow of geothermal fluids, stable isotopes (especially 2H and 18 O) along with B and Cl being most important. Rock forming constituents (e.g. SiO2, Na, K, Ca, Mg, CO2, H2) are used to predict subsurface temperatures and potential production problems such as deposition and corrosion. Kenya is endowed with huge geothermal potential with estimates in excess of 10,000MW (GDC Strategic Plan 2010) due to its location along the Great Rift Valley of Eastern Africa (Fig. 1).
Figure 1: Map showing location of geothermal area along the Kenyan Rift Valley. 2. Geochemical Methods
Geochemical methods are used in locating and sampling surface discharges which include: water (hot/cold) and fumarolic steam. Non- manifestation areas are surveyed using soil gas surveys and these
603
604
Samuel M. Mwangi / Procedia Earth and Planetary Science 7 (2013) 602 – 606
include measuring chemical components associated with geothermal activity such as: CO2 gas, Hg and Rn-222 radioactivity. In many areas, geothermal indicators occur in form of fumaroles, hot (boiling) springs, mud pools, hot altered grounds and geysers. Three main methods [8] have been used in Kenya to obtain geochemical samples and they are: Water sampling - this involves collecting liquid water samples from drilled boreholes and natural springs so as to evaluate origin of the fluids, temperature estimations at depth, predict scaling and corrosion problems. Fumarole steam and condensate sampling – gases and condensates are sampled in order to compute reservoir temperature at depth where the steam is being formed. Soil gas survey (for Rn-222, CO2 and temperature) - Rn-222 and CO2 in the soil are indicators of permeability and possible location of a reservoir. 3. Results and discussion
The major gases in geothermal steam are CO2, H2S, H2, CH4, N2, NH3, CO, and O2. The noble gases in steam include He, Ne, Ar, etc. Carbon dioxide is generally the major gas component often comprising more than 80% of all non-condensable gases and its concentration in total increases with reservoir temperature. Hydrogen sulphide concentration commonly decreases as steam ascends to the surface due to reaction with wall rock, dissociation to sulphur or oxidation to SO4. In many geothermal fields of high temperature above 200 oC, temperature dependant gas and mineral-gas equilibrium control the concentration of the geothermal gases [6-7]. Geochemical studies of geothermal fluids involve three main steps: 1) sample collection, 2) chemical analysis and 3) data interpretation [11]. The types of samples collected in this study were condensates from four fumaroles (Table 1) in the Olkaria field and thereafter, full analysis as stipulated by Zhong-He and Armannsson [12] was done. Our main interest was in the chemical constituents that are volatile enough to partition into steam phase upon separation in boiling system as expected in a geothermal aquifer producing steam. These elements include the soluble gaseous compounds present in geothermal steam such as CO2, H2S, NH3, F, and B. In addition other non-volatile elements were considered such as major ions, Cl and SO4 and the pH and total dissolved solids (TDS) and the electrical conductivity in steam condensates. The samples were collected at the fumarole sites that showed higher temperatures and more altered features. Table 1 below shows results obtained in Olkaria [9]. Table1: Some geochemical results in Olkaria PARAMETER Co-ordinates (UTM) Sampling temp. (oC) pH/20oc SiO2(ppm) B (ppm) Na (ppm) K (ppm) CO2 (ppm) SO4 (ppm) Cl (ppm) NH4 (ppm) H2S (ppm) Conductivity (μS/Cm) TDS (ppm)
DF1 9897127, 0205186 91.5 5.76 1.53 2.61 1.59 0.165 62.92 4.7 0.45 0.11 0.04 37 18
DF2 ( BH) 9897780, 0204820 91 6.51 1.68 2.86 0.503 1.017 40.04 3.3 0.25 0.64 0.02 19 9
DF3 989689, 0205014 87.7 5.73 0.025 1.86 2.69 0.087 58.3 2.6 0.45 1.32 0.04 14 7
DF5 9898050, 0205195 91 5.68 0.25 3.94 0.215 0.194 45.54 3.51 0.05 0.04 0.06 2 1
Samuel M. Mwangi / Procedia Earth and Planetary Science 7 (2013) 602 – 606
Geothermometers are parameters used to determine sub-surface conditions since at particular temperatures, common assemblages of minerals will tend towards equilibrium with a given water chemistry. It has been noted that for certain parameters or ratios of parameters, the relationship between temperature and chemical composition will be stable and predictable. In this kind of sampling which is vapour dominated, gas geothermometers are used since nothing is known of the composition of the liquid phase from which the produced fluid originates. The concentrations and relative concentrations of gases in the geothermal steam are controlled by temperature dependent fluid-mineral and gas-gas equilibria that have been used as the basis of gas geothermometers [13]. A univariant CO2 gas-geothermometer developed by Arnórsson [10] is used in the final step of our geochemical study for data interpretation and also an H2S geothermometer to determine the sub-surface temperatures. The CO2 geothermometer gave us a more reliable reservoir temperature estimation ranging from (237 oC – 258 oC) while the H2S geothermometer temperature estimation was quite low ranging from (170.6 oC – 187.6 oC). This could be due to the oxidation and condensation it undergoes as it ascends to the surface. 4. Conclusion
Ellis and Mahon [5] have summarized all the above activities by saying that “the credibility and usefulness of geochemical data depends on the methods used and the care taken in the collection of samples.” Geochemical investigations involve the sampling and analysing of chemical and/or isotopes of the waters and gases from geothermal manifestations (hot springs, fumaroles, etc.) or wells in the study area [10]. Information from down-hole measurements of an exploratory well can be used to prepare well profiles and have these correlated to sub-surface temperatures with hydrothermal mineral assemblages. The reservoir temperature in Olkaria estimated by use of CO2 based geothermometry is above 230oC confirming the existence of a geothermal resource. Similar temperatures are registered in the Domes field drilled wells. Geochemical surveys can also be used to locate feeder zones where the hot fluids are entering the reservoir; thus geochemical and geological surveys provide useful data for planning explorations and their costs are relatively low compared to the more sophisticated methods of geophysics and exploratory drilling. 5. References [1] KenGen. Surface exploration of Kenya’s geothermal resources in the Kenya Rift. The Kenya Electricity Generating Company Limited, Internal report; 1998, 24. [2] Omenda PA. The geology and structural controls of the Olkaria geothermal system, Kenya. Geothermics 1998; 27(1): 55-74. [3] Omenda PA, Onacha SA, Ambusso WJ. Geological setting and characteristics of the high temperature geothermal systems in Kenya. Proceedings of the New Zealand Geothermal Workshop 1993; 15: 161-167. [4] Riaroh D, Okoth W. The geothermal fields of the Kenya rift. Tectonophysics 1994; 236: 117-130. [5] Ellis AJ, Mahon WAJ. Chemistry and geothermal systems, Academic Press; 1977. [6] Arnórsson S, Gunnlaugsson E. New gas geothermometers for geothermal exploration. Calibration and application. Geochim Cosmochim Acta 1985; 49: 1307-1325. [7] Giggenbach W.F. Geothermal gas equilibria. Geochim Cosmochim Acta1980; 44: 2021-2032. [8] Magnus O. Sampling methods for geothermal fluids and gases, Orkustofnun, National Energy Authority, Geothermal Division, October 1988. [9] Cyrus W, Karingithi CW. Geochemical characteristics of the Greater Olkaria Geothermal Field, in Kenya. UNU-GTP conference, Naivasha, 2008.
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Samuel M. Mwangi / Procedia Earth and Planetary Science 7 (2013) 602 – 606
[11] D’Amore F, editor. Application of Geochemistry in Geothermal Reservoir Development. United Nations Institute for Training and Research, New York, ISBN 92-1-157187-2, 1992: pp. 408. [12] Armannsson H, Zhong-He P. Analytical Procedures and Quality Assurance for Geothermal Water Chemistry, UNU-GTP 2006, Report 1. [13] Barbier E. Renewable and Sustainable Energy Reviews, Elsevier Science 6, 2002; 3-65: pp. 27-28.