GEOTHERMAL SURFACE EXPLORATION APPROACH- CASE STUDY OF MENENGAI GEOTHERMAL FIELD,...
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GEOTHERMAL SURFACE EXPLORATION APPROACH: CASE STUDY OF MENENGAI GEOTHERMAL FIELD, KENYA John Lagat Geothermal Development Company Ltd, P. O. Box 17700-20100, Nakuru, Kenya
[email protected] ABSTRACT Geothermal exploration in a new prospect involves multi-disciplinary approach where surveys involving different geoscientific methodologies are conducted so as to be able to accurately develop a working model that will be used to site exploratory wells. These methods include geological mapping of the prospect area to study the volcanological evolution of the volcano and be able to model the heat source(s). Hydrogeological surveys and structural analyses are also carried out to relate their association with the development of the geothermal systems. Geophysical techniques normally employed included transient electromagnetic (TEM), magnetotellurics (MT), seismics, gravity and magnetics. Integrated geophysical surveys assist in imaging the subsurface to identify the geothermal reservoirs, the heat source and the depth, buried structures and the size and extend of the geothermal system. Geochemical surveys include sampling of gas and steam condensate from fumaroles, carrying out ground radon and CO2 traverses and hot spring and ground water sampling. Analyses and interpretation of geochemical data provide information on the subsurface temperatures, nature of geothermal reservoirs, origin of geothermal fluids and to map permeable zones. Surface heat loss surveys are carried out to determine the amount of heat lost by conduction and convection, hence understand the nature and size of the heat source. Integration of all the geoscientific data assists in the development of a geothermal model where the heat source and the size, the recharge of the system, the structures that control the geothermal system and the depth to the reservoir(s) are envisaged. This paper therefore describes the approach used to carry out surface exploration in Menengai geothermal field to t he development of a conceptualized geothermal and siting of exploration wells. Keywords: Exploration, conceptual model
INTRODUCTION Geothermal Energy Geothermal energy is the natural heat stored within the earth. The resource is manifested on the earth’s surface in the form of fumaroles, hot springs, steaming grounds and altered grounds. The economically usable geothermal energy is that which occurs close to the earth’s surface where it can be tapped by drilling wells up to 3,000 m below the earth’s surface. Such shallow heat sources are in most cases attributed to volcanic activity, which in many cases are associated with plate boundaries. The East African Rift is a good example of this. Thus, potential areas for the development of high temperature geothermal systems are associated with young Quaternary volcanoes similar to those that occur within the Rift valley.
As in the search for any natural resource, a strategy for geothermal energy exploration must be defined and followed as stipulated. Once a geothermal prospect area has been identified, the next step is to use the various exploration techniques to locate the most interesting geothermal area and identify suitable targets for resource exploitation. EXPLORATION METHODS To reduce the cost of exploration, it is normally approached in a prescribed sequence of steps, altering the order from time to time depending on prior knowledge of the area in question. In some cases, high costs will lead to the elimination of some steps in the sequence. This normally involves multi-disciplinary approach whereby all the geoscientific methods are applied. In Menengai, the following multi-disciplinary approach was followed:
Geological and hydrogeological surveys, Geochemical surveys,
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important geothermal controlling feature in the area.
Geophysical surveys, Heat Loss surveys and finally
An Environmental and Social Baseline Social Impact Assessment (ESIA) was also carried before drilling commenced so as to determine expected impacts and their mitigations. The discussions of the results from the multidisciplinary surveys carried out in Menengai geothermal prospect and the development of the conceptualized model to siting of the exploratory wells is described in the subsequent sections below. Geoscientific Surveys Results Geological mapping Geological surveys provided information on the evolution of Menengai volcano, hydrogeological controls and also the stratigraphic and structural framework of the area. Menengai geothermal reservoir is associated with a caldera volcano and therefore the heat source is associated with the partially emptied magma chamber below the volcano. Continued intra and post caldera eruptions estimated to be a few hundred years indicate that the magma body is still active. The hydrogeologic regime comprises of recharge from the higher rift scarps and the intense rift floor fracture/faulting resulting from extensional tectonics of continental rifting, provide for a good structural set-up that allows water from the rift scarps to penetrate deep into the crust and thus up flows into shallow reservoirs under Menengai. The regional TVA’s are important conduit of deep fluids thus an
The main structures in Menengai caldera volcano include the 88 km2 caldera, Solai and Ol Rongai TVA (Figure 1). The floor of the Menengai geothermal prospect depicts extensional tectonics with the main trough trending N-S over north of Menengai and NNW-SSE for section south of Menengai. The Ol Rongai structural system represents a part of the larger Molo TVA that has had a lot of volcanic activity including eruptions resulting to a buildup of NNW trending ridge referred to as Ol Rongai volcanoes. The Solai tectonic axis is a narrow graben averaging 4 km wide that runs on an N-S direction from the eastern end of Menengai caldera, through Solai. It is comprised of numerous fault/fractures all-trending in N-S direction. Geophysical mapping The rocks within the earth's subsurface have physical properties that vary from place to place. The common properties include electrical conductivity, seismic energy transmission, magnetic acquisition and gravity attraction. The geophysical methods employed in exploration for geothermal energy in Menengai are the electrical conductivity techniques, which include transient electromagnetic (TEM) and the magnetotelluric (MT), gravity, magnetic and seismic methods.
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Figure 1: Map showing the structural set-up of the Kenya rift floor.
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Gravity Surveys To examine gravity distribution beneath Menengai caldera several cross-sections were constructed along A-A´, B-B´, C-C´ and D-D´ as shown in Figure 2. The trend for all these sections (Figure 3) indicates low gravity within the caldera into which a high density body is superimposed. Peaks are also observed on this high density body suggesting shallow dykes or intrusions that form the heat sources. Electrical Resistivity Surveys Electrical Resistivity methods play a significant role in the investigation for geothermal energy since the methods probe deep into the subsurface. The geophysical techniques that were employed in Menengai geothermal field included TEM and MT. Joint TEM and MT inversions were carried out to image the subsurface for the existence of electrically conductive zones that form the geothermal reservoirs and the results used to interpret the data. Figure 4 shows MT resistivity cross -section E-W passing through the caldera. This profile shows generally higher resistivity near the surface which is probably due to un-altered rocks in the near subsurface. Underlain is a low resistivity layer about 1 km thick which run accross the entire cross -section.
This shallow low resistivity layer on this profile defines the clay layer formed due to hydrothermal alteration at the upper zone of the geothermal system in Menengai prospect and the outflow zones. A localized low resistivity anomaly is also observed at a depth of about 4 km. This low resistivity body could be associated with magmatic intrusion which is a probable source of crustal fluids for this prospect. Seismic Surveys Seismic studies (Young et al., 1991; Simiyu and Keller, 2001; Tounge et al, 1992) indicate that most of the activity is above the depth of 6-7 km (Figure 5), as shown by the seismic attenuation trends coinciding with the principal direction of the faults . Figure 5 shows depth event distribution along a NW-SE profile through Ol Rongai hills and Menengai caldera. Events are shallower below Ol-Rongai hills and Menengai caldera. Interpretation of these observations indicates that a geothermal system exists in the area and is shallower below Ol Rongai hills and Menengai caldera. Seismic events show attenuation below 4 km at Menengai caldera and below 4-5 km at Ol Rongai hills indicating the brittle-ductile transition zone, confirming the magmatic bodies forming the heat s ources for the geothermal system occur below those depths .
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Figure 2. Profiles of the gravity cross-sections through Menengai caldera (after Mungania et al, 2004).
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Figure 3. Gravity cross-sections through Menengai caldera as shown in Figure 1 (after Mungania et al, 2004)
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Menengai Caldera
Figure. 4. 2-D East-west MT resistivity cross-section (Lagat, et al., 2010) .
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Figure 5. Depth distribution of micro-seismic events through Menengai caldera (after Simiyu et al., 1997) Geochemistry Mapping Geochemical observations are particularly important for geothermal resource assessment in the stages of exploration. Prior to drilling, chemical geothermometers provide the only information on estimated reservoir temperatures. Similarly, geochemical surveys are also important to delineate the areal extent of the geothermal. Geothermometry One of the most important contributions of geochemistry to geothermal resource assessments is chemical geothermometery. Geothermometry is
the application of geochemistry to infer reservoir temperatures from the composition of geothermal fluids. Geothermal fluids that are found at the surface above Menengai geothermal systems are fumarole and steaming ground. Some geothermal systems do not have any fumarole or any other visible surface geothermal activity and in such areas soil gas mapping (degassing) which normally comprises radon and carbon dioxide is used. Following are the computed results for both the gases in the fumaroles and well discharge. The temperatures calculated using TH2 S using the Arnorsson and Gunnlaugsson (1985)
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geothermometer gave relatively high values over 270o C ranging between 271-300o C (Table 1). The calculated TCO2 temperatures ranges between 324340o C. From this well discharge geothermometers Menengai geothermal reservoir is estimated to be over 270o C (Table 2). Table. 1. Gas geothermometers of fumaroles Fumarole No. MF-1 MF-2 MF-6 MF-8 MF-9 MF-12
Gas Geothermometers (°C) TH2 S TH2 S-CO2 280 276 293 304 296 302 295 299 279 274 298 299
Table. 2. Gas Geothermometry of MW-01 Arnorsson and Gunnlaugsson (1985) S ample ID TCO 2 TH2S TCO 2/H (ºC) (ºC) 2 (ºC) M W-01-1 339 271 285 M W-01-2 340 275 285 M W-01-3 339 283 297 M W-01-4 334 275 299 M W-01-5 333 273 293
TH2S /H2 (ºC) 328 325 335 341 334
Radon/CO2 ratios The source of Rn-222 can be either magmatic where the uranium accumulates at the late stages of differentiation or from any other source containing radium, the immediate precursor of Rn-222. When Ra-226 is present in the hydrothermal system, it indicates that the hot water percolates through the host rock thus dissolving Rn-222, which is produced from alpha decay of Ra-226. Upon boiling of the water, the Rn-222 partitions into the steam phase and is transported to the surface through permeable zones. Due to its short half-life, Rn-222 has to travel for long distances within a relatively short period to be detected on the surface. Similarly, CO2 has to travel through a relatively permeable zone to avoid dispersion and subsequent dilution for it to be detected in high concentrations. CO2 may also originate from other sources like organic sources, which are likely to give false impressions of a geothermal source. The ratio of the two gases would be a good indicator of the magmatic source of the gases since the ratio is not expected to change if they are from the same source. High values of these ratios are found inside the caldera and towards the north and north western parts of the caldera. High values of Rn and CO2 ratios are found inside the caldera with a circular pattern like that of radon (Figure 6).
Figure 6: Radon/CO2 ratios distribution map
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Heat Loss Surveys Conductive heat loss and convective heat loss measurements were carried out in the prospect (Mungania et al., 2004). Heat flow data in the prospect covered an area of close to 900 km2 , where Shallow 1 m temperature gradient holes were drilled and temperatures obtained on the surface and at 25 cm, 50 cm and 1 m depths below the surface. Figure 7 shows the distribution of temperatures at 1 m depth in the Menengai geothermal prospect. From this figure, it is observed that areas with high temperatures at 1 m depth occur at the central parts of the caldera and around Ol Rongai and Ol Banita regions. Results indicate that total conductive heat loss from Menengai prospect is estimated to be in excess of 1,060 MWt with over 250 MWt being lost in the caldera. The convective heat loss is estimated to be more than 2,476 MWt with 2,440 MWt being lost in the caldera and this indicates that the heat source in Menengai is huge.
GEOTHERMAL MANIFESTATIONS Geothermal activity is manifested in this area by the occurrence of fumaroles (Plate 1), warm springs, steaming/gas boreholes, hot/warm water in boreholes, Fimbristylis exilis ‘geothermal grass’ (Plate 2) and altered rock/grounds (Plate 3). Fumaroles are located mainly inside the caldera floor. Three groups of active fumaroles found in the caldera have aerial extent ranging from a few m2 to less than a km2 . The two groups in the central and western portion of the caldera floor are located within fresh lava flow and close to their eruption centres. The steam emission has a mild H2 S smell. Some sulfatara deposition is evident on the surface. The other group of fumaroles located in the central eastern part of the caldera floor is found at the young lava/pumice contact and has extensively altered the pumiceous formation. The structural controls for these groups of fumaroles appear to be the eruption craters that may be the source of the pyroclastic deposits. The caldera floor is, however, almost covered by young lava flows.
Figure. 7. Temperature distribution at 1 m depth (modified from Munganiaet, al,. 2004)
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Plate. 1. Fumaroles on the caldera floor
Plate 3. Altered ground
GEOTHERMAL MODEL The heat source, the reservoir, the recharge area and the connecting paths through which cool superficial water penetrates the reservoir and, in most cases, escape back to the surface, compose the geothermal system. The multi-disciplinary surveys conducted in Menengai reveals that all these conditions have been met and a commercial geothermal system exists under the caldera and immediate surroundings . These factors are described in details below and summarized in Figure 8. Heat Source The presence of a caldera beneath Menengai volcano represents a collapse directly above an 88 km2 partially emptied vast magmatic chamber. Seismic surveys indicate a ductile brittle zone below 6 km indicating presence a molten body below that depth. Shallow intrusives conduct the heat to shallow levels, thus heating the geothermal fluids The continued post and intra caldera eruptions that are as young as several hundred years indicate that the body is still active. Plate 2. Fimbristylis exilis ‘geothermal grass’
Hydrogeology The location of Menengai prospect on the rift floor is lower than high rift scarps that form the recharge areas. Meteoric waters flow from the flanks and the intense rift floor fracture/faulting, provides a good structural set up that allows water to penetrate deep into the crust towards the magma bodies.
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Figure. 8: Geothermal conceptualized model of Menengai geothermal prospect (Lagat et al., 2010) SYSTEM CAPPING Menengai volcano eruption included eruptions of pyroclastics and lavas. Glassy component pyroclastics is usually very susceptible to alteration forming hydrated clays that are cause of selfsealing. Repeated eruptions of thick lavas also form very good capping for geothermal systems. Reservoir Rocks The rocks that occur at subsurface of Menengai geothermal field prospect where the geothermal reservoir is hosted are made up of faulted Pliocene flood lavas, which include mainly trachytes. The combination of the local and regional structures has enabled fracturing in the reservoir rocks to allow for permeability and storage of the geothermal fluid. WELL SITING Drilling of exploratory wells represents the final phase of any geothermal exploration programme and is the only means of confirming the characteristics and potential of a geothermal reservoir. Based on results of the surveys carried out in Menengai indicate a high potential. The prospect has 4 drilled wells currently and the exploration wells have proved presence of a viable geothermal resource. Accurate well siting is very important especially in a new field, which is still under exploration because it s aves costs. The exploratory well siting involves multidisciplinary surveys where all the results are incorporated for accurate results. The exploration wells sites were placed within the caldera floor close to the young volcanic centres in the centre of the caldera and within a local gravity high that trends coincident with the Molo TVA. The area has
Shallow intrusive, which act as the heat source for the geothermal system. The location being along the Molo TVA, high permeability is expected arising from fractures associated with the structure. Low resistivity anomaly in this locality indicates the presence of a geothermal system. High geothermal potential of the area is also indicated by high radon-222 radioactivity and CO2 values both in the soil gas and in the fumarole steam. Gas geothermometry from a fumarole close to the site gave temperatures of more than 270o C. Heat flow measurements indicate highest heat loss around this area indicating high subsurface temperatures at depth. Menengai Well MW-01, which was the first deep exploration well to be drilled in the Menengai geothermal field, is drilled to a depth of 2206 m. The well discharged on test therefore confirming a geothermal reservoir exists in Menengai geothermal field. Measured formation temperatures confirmed the results estimated from the fumaroles (Mibei, 2011). CONCLUSIONS • •
Successful exploration requires multidisciplinary approach incorporating all geoscientific disciplines Geothermometry temperatures from results of the already discharged MW-01 correlate very well with estimated fumarole geothermometers
REFERENCES Arnorsson, S. and Gunnlaugsson, E., (1985)., New gas geothermometers for geothermal exploration– calibration and application. Geochim. Cosmochim. Acta, 47, 567 – 577pp.
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Lagat, J., Mbia P., and Muturia, C., (2010)., Menengai Prospect: Investigations for its geothermal Potential. A GDC Geothermal Resource Assessment Project Report, Second Edition. Mariita, N.O. (2003). An integrated geophysical study of the northern Kenya rift crustal structure: implications for geothermal energy prospecting for Menengai area. A PhD dissertation, University of Texas at El Paso, USA. Convine, O., (2011)., Borehole Geology and Hydrothermal alteration mineralogy of well MW01 and MW-02, Menengai Geothermal Field, Central Kenya Rift. Report 30 in: Geothermal UNU-GTP training in Iceland 2011.
Mungania, J and Lagat, J. K., (2004)., Menengai Volcano: Investigations for its geothermal potential. Report prepared by KenGen and the Ministry of Energy. Simiyu, S. M. and Keller, G, R. (1997)., An integrated analysis of the lithospheric structure across the East African plateau based on gravity analysis and recent seismic studies. Tectonophysics Vol. 278
