(PAPER)Task Week12 Resume Geothermal Raditya Yudha Satriaji Wahyu

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. Open
©
2012The
TheAuthors.
Authors.
Published
by Elsevier
B.V.access under CC BY-NC-ND license.
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. Open access under CC BY-NC-ND license.
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
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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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[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.