NOTICE CONCERNING COPYRIGHT RESTRICTIONS This document may contain copyrighted materials. These materials have been made available for use in research, teaching, and private study, but may not be used for any commercial purpose. Users may not otherwise copy, reproduce, retransmit, distribute, publish, commercially exploit or otherwise transfer any material. The copyright law of the United States (Title 17, United States Code) governs the making of photocopies or other reproductions of copyrighted material. Under certain conditions specified in the law, libraries and archives are authorized to furnish a photocopy or other reproduction. One of these specific conditions is that the photocopy or reproduction is not to be "used for any purpose other than private study, scholarship, or research." If a user makes a request for, or later uses, a photocopy or reproduction for purposes in excess of "fair use," that user may be liable for copyright infringement. This institution reserves the right to refuse to accept a copying order if, in its judgment, fulfillment of the order would involve violation of copyright law.
Geothermal Resources Council Transactions, Vol. 22, September 20-23, 1998 The laguna Colorada (Bolivia) Project: A Reservoir Engineering Assessment Zen6n Delgadillo T.* and Hector G. Puente.** * ENDE Cochabamba, Bolivia * *C.F.E. Mexicali, Mexico ABSTRACT More than five years after drilling and testing the last well at the Sol de Maiiana Field, in the Laguna Colorada geothermal zone, the Empresa Nacional de Electricidad (ENDE) of the Republic of Bolivia, has restarted activities in the field to update and certifl the actual geothermal potential and propose its commercial development. ENDE requested that Mexico's Comision Federal de Electricidad (C.F.E.), carry out the tasks required for the electric potential certification at Laguna Colorada. C.F.E. conducted a geological assessment of the zone, to identi@ the main physiographic and structural features of the field, in addition to more detailed geological and geophysical studies. Six thermal springs close to the field were sampled to chemically characterize the discharged fluids. In the Sol de Maiiana existing wells, the casings were calibrated and pressure and temperature profiles of each well were measured. Afterwards, wells and were opened in order to evaluate production characteristics and determine the chemical composition of their fluids. In the well, a build-up transient pressure test was run to determine the permeability and storage characteristics of the reservoir. A numerical simulation study of the field was made using a three-dimensional grid to represent the main characteristics of the geothermal reservoir. Various extraction-injection scenarios for the development of Laguna Colorada were calculated and the results indicated that the field has excellent thermal and hydraulic characteristics, sufficient to generate at least 120 MWe for 25 years. Some of the most important results obtained for the certification of the geothermal potential of Laguna Colorada are described in this paper, as well as some proposals for the development of this project. Introduction In Bolivia, geothermal energy has been used for a long time, but almost exclusively in spas and resorts, mainly from low enthalpy resources (Paz, 1988). The initial geothermal energy potential assessment performed in 1970, showed a total potential for the country of 350 MWe. More recent studies carried out in the Sol de Maiiana Field, in the Laguna Colorada area, exceed those figures without including other geothermal prospects like Sajama, Empexa, etc. This is an important fmding given the growing reliability and potential of the high-enthalpy geothermal energy in Bolivia. The areas of geothermal interest in Bolivia are located mainly along the Western Los Andes mountain range that constitutes the border between Bolivia and Chile. This range has a North- South orientation and is formed mainly of andesitic rock and ignimbrite (composed of dacites and rhyodacites). The area of geothermal interest in Bolivia is part of the Bolivian high plateau (between 3,500 and 4,500 m.a.s.l), bounded to the West by chains of miocene and Pliocene volcanoes of up to 6,000-m elevation that form the Western Los Andes range. In this range there are active volcanoes such as Ollague, htana, Iruputuncu, etc. The area of Laguna Colorada is located to the South of the Republic of Bolivia, close to the Chilean border, approximately 340 km South of Uyuni City, Figure 1. In 1976, in collaboration with the United Nations Development Program (UNDP) within the framework of the Energy Resources Evaluation Program in Bolivia, the first comprehensive evaluation of geothermal resources in the country was performed. This evaluation gave way to all subsequent geothermal exploration efforts, classifying all surface geothermal outputs and identifying the zones of main interest. 257
~ ~ Delgadillo and Puente PERU BOLIVIA Figure 1. Location of the Sol de Manana geothermal field. After various geological and geophysical studies were carried out in the area, in collaboration with international institutions such as the UNDP, deep exploratory drillings were done during the 1988-1990 period, identifjling the Sol de Maiiana area as the zone with the greatest potential. Geological and Geophysical Settings The morphology of the area is dominated by the presence of volcanic structures of miocene-pleistocene age, as well as low relief terrain underlain by dacitic and ignimbritic flows and by the effects of glacial erosion (CFE 1997). In general terms, the rocks found in the subsurface are grouped into three main units: - Unit I, formed by a dacitic vitreous ignimbrite with andesitic lava intercalations. - - Unit II, formed by andesites. Unit 111, formed by dacitic ignimbrite with andesitic lava intercalations Lithological Unit I11 is made up of miocene volcanic rock. Its thickness is still unknown, since it has not been fully penetrated by the 6 drilled holes. Petrography studies show that in all wells, ignimbrite make the majority of the unit, with variable thickness of andesitic intercalations. In the Sol de Maiiana Field, the unit with the most permeability seem to be Unit 111, where the largest circulation losses were found during drilling of the higher temperature wells. However, well SM-4 was an exception as a non-productive well, although it penetrated an important body of ignimbrite with intercalations of andesitic lava. It has been suggested that the pekeability of the reservoir is associated with faults and fractures rather than to a specific type of rock such as the ignimibrites of unit III. Resistivity studies show a good correlation of surface thermal activity with the geothermal resource, as indicated by the temperature of the wells. The interpretation clearly shows the borderlines of the Sol de Maiiana area, in conjunction with the Apacheta Sector, indicating that geothermal activity probably extends to the west of both zones. In these zones, geothermal fluids are stored in the volcanic rock of Unit 111, which showed high permeability during drilling. However, it is possible that the low resistivities to the West of Apacheta and Northwest of Sol de Maiiana indicate the presence of a sealed layer where the Permeability of the volcanic rock has been reduced by precipitation of argillaceous minerals. This conclusion agrees with the drilling results, where in the fllrst 600m there were no partial circulation losses. The upward flow can be related to the 30 to 50 ohm-m resistivities observed in the area of the wells at a depth of 700 m, where the drilling operations had circulation losses. The map of gravimetric anomalies shows an undefined dense body to the West of Apacheta and a depression between wells Ap-1 and SM-1. As the composition and depth of the regional basement are unknown, further interpretation of the data might provide more information on the regional structure of the field. No. Ts OC ph E.C. CI B HCO3 Si02 SO4 Na K Li Ca Mg Rb M-1 72 M-2 82 M-3 79 M4 82 M4A 82 M-5 72 1.95 4 580 0.9 0.3 0.0 223 1403 13.0 11.2 0.01 0.3 4.20 0.03 2.27 2 540 0.2 7.8 0.0 256 1 067 41.0 11.0 0.01 6.8 18.00 0.08 2.02 4420 0.9 15.6 0.0 258 1 377 8.0 5.7 0.07 0.2 0.52 0.01 1.97 6 230 0.9 0.2 0.0 148 1 945 7.0 5.5 0.01 0.1 1.18 0.01 1.95 4 700 0.9 0.32 0.0 82 1 132 8.0 4.5 0.01 0.1 1.48 0.02 2.56 695 4.3 0.2 0.0 152 194 22.0 10.0 0.01 0.4 0.22 0.03 I 1 258
Delgadillo and Puente Nall 000 / FULL EQUILIBRIUM Figure 2. Relative contents of Na-K-Mg of wells and thermal springs of Laguna Colorada, Bolivia. WIOO Ceoc hemi st ry To characterize the type of water produced both in wells and in springs, the relative contents of Cl-SO, - HCO, (Giggenbach, W., 1991) were used. All samples of spring water collected were acid-sulfated type. On the triangular diagram (Figure 2) they are located in the steam heated zones, which is typical of shallow waters that interact with geothermal steam. The sulfate concentrations in these samples vary from 1067 to 1945 ppm, except for a concentration of 194 ppm in one spring, as shown on Table 1. The boron contents of 7.8 and 15.6 ppm in the M-2 and M-3 springs respectively, result from boric acid transport by steam and solution in the shallow condensation zone. Springs M-1, M-4, M-4A, and M-5 show boron concentrations lower than 0.4 ppm. To verify the equilibrium state of the system and the temperatures of the water-rock interaction process, the relative contents of Na-K-Mg (Giggenbach, W., 1991) are shown in (Figure 3), indicating that the samples from the springs originate from shallow waters or in the recently infiltrated zone. On the other hand, water from wells and showed a total equilibrium at a temperature of 290 C from a WNa geothermometer. (Table 2) shows the main chemical characteristics of the brine produced by wells and and (Table 3) shows the results of the chemical analysis of the gases fiom condensated steam analysis. The observed CVB molar ratio is near of 15, which is typical of geothermal brine and can be interpreted as the result of deep circulation processes. The low magnesium concentrations (0.07 ppm) also suggest high temperature water-rock interaction. 259 Reservoir temperature was calculated using the Na/K geothermometer giving a liquid enthalpy similar to the production enthalpy. This suggests that fluid in liquid phase enters the well fiom a liquid-dominated reservoir. In order to understand the origin of the fluids, the relative N,:He:Ar (Giggenbach, W., 1991) concentration was used (Fig- 2 W5M-2 5:WsM5 Figure 3. Relative contient of gases of wells of Laguna Colorada, Bolivia. 100 Ar
Delgadillo and. Puente WELL PRESSURE ALCAL. (PSI 1 C.E. TOTAL CI B Si02 Na K DATE WELLHEAD ph (nrnho/crn) (meqii) ( PPm 1 19/05/97 I - 16 500 0.47 8 450.0 200.0 887.0 4 680.0 840.0 S M-2 31/07/97 140 7.0 25 300 0.600 8 701.9 179.7 693.9 4 812.6 804.0 02/08/97 1 42 7.1 25 200 0.600 8 667.3 181.8 429.4 1 140.0 791.0 04/08/97 142 7.1 25 000 0.600 8 494.7 175.6 259.3 4 866.7 777.6 05/08/97 212 7.1 25 300 0.600 8 770.9 181.8 612.3 4 878.2 793.8 07/08/97 212 7.1 25200 0.600 8 632.8 181.8 519.6 4 888.9 788.7 19/05/97 - I 13 500 0.938 6780.0 160.0 887.0 3 800.0 680.0 31/07/97 150 7.2 19 800 0.800 7 182.0 152.9 593.5 4 032.7 627.5 02/06/97 21 5 7.4 21 700 0.800 7 424.2 152.9 508.9 4 159.7 643.3 03/08/97 21 5 7.4 21 700 0.800 7 320.6 152.9 508.1 4 169.1 648.3 ure 4). The observed enrichment in He might result fiom a long residence in the earth s crust. A positive feature in this field is that the concentration of non-condensable gas in steam produced by the wells, is less than 1%. A reservoir temperature of 263 C was estimated by using a gas geothennometer (H,/Ar) in the wells, which is in accordance with measured reservoir temperature. I I I I a0 (10 10 0 pa u11 Wellhead pressure [ h a ] Figure 4. Well Power output.. Well and Reservoir Parameters There are 5 geothermal wells in Sol de Maiiana field, drilled between 1988 and 1989, except well drilled in 1992. The depths of these wells vary fiom 1200 to 1700 m, and their completion is 8.5 diameter open hole, except for well SM-4 completed with a 7 diameter slotted liner. Another important feature of this field is that the static pressures (55 barg at 1250 m) and down-hole temperatures in the wells (230-245 C) are very similar among them. Also their chemical characteristics show a very homogeneous behavior. in the reservoir zone; which suggests a very good horizontal permeability in the area where these wells are located. and wells were opened for production after being closed for many years, in order to conduct an output test and collect chemical samples. In the well a build-up test was run in order to determine the & reservoir parameter characteristics. Pressure and temperature profiles under both static and flowing conditions were measured, allowing the evaluation of the productivity index of these wells. Well produced a total mass of 230 t/h at atmospheric pressure with a discharged enthalpy of 1168 Kjkg and a wellhead pressure.of 10.5 barg. The well was discharged over a period of 4 months showing very stable productive conditions indicating that the well is able to feed a 5 MW power plant. (Figure 5) shows the output curve for well. Well was also tested during this time, producing up to 22 1 t/h of total mass and a discharged enthalpy of 1082 KjKg. After the output test period, a build up transient test was conducted in order to assess the main thenno-hydraulic parameters. Results showed very high reservoir permeability and 260
Delgadillo and Puente WELL DATE cg He H2 Ar N2 0 2 CH4 COz H2S NH3 % Weight CONCENTRATIONS IN WEIGHT %,DRY BASE ~ ~ 31/07/97 04/08/97 O,l8 0,005 0,12 0,31 4,67 0,OO 0,07 84,14 7,40 3,27 0,26 0,000 0,14 0,11 5,31 0,OO 0,14 84,61 9,07 0,61 S M-2 05/08/97 06/08/97 07/08/97 02/08/97 03/07/97 0,16 0,020 0,26 0,11 10,87 0,OO 0,OO 81,51 7,23 0,OO 0,11 0,040 0,15 0,21 6,12 0,OO 0,13 82,50 6,34 4,51 0,08 0,002 0,13 0,05 5,27 0,OO 0,03 84,82 6,92 2,80 0,17 0,022 0,14 0,19 5,60 0,OO 0,13 83,30 8,76 1,86 0,15 0,011 O,O6 0,05 2,48 0,OO 0,21 88,73 5,71 2,75 02/08/97 0,010 0,29 10,74 0,OO 0,OO 2,32 82,62 2,85 1,17 03/08/97 0,09 0,006 0,08 0,05 2,88 0,OO 0,33 83,16 7,03 6,48 transmissibility value of 130 darcy-meter was estimated. A storage coefficient of 2.9 E-09 mlpa was also calculated. Reservoir Simulation An important part of the tasks carried out by CFE to certify the potential of the Sol de Maiiana field, consisted in developing a reservoir model that allowed estimation of the behavior of the reservoir under different production and injection conditions. This job was done with the use of the reservoir sirnulation code TETRAD 12, widely used by CFE in the commercial evaluation of the Mexican,geotheml fields. For the purpose of this study, the Laguna Colorada reservoir was considered a geothe&l system laterally isolated; that is, with impermeable and adiabatic side boundaries with no heat or fluid transfer through them. However, the reservoir model includes an inflow at the bottom and discharges to the surface. Mainly using the resistivity boundary and the main structural map through the faults, the reservoir grid was defined. At h s point, the Sol de Maiiana and Apacheta zones were considered to be part of the same reservoir, since the electrical response is similar in both zones. The area of well SM-4 was also included in the model as a reinjection area. A total area of 60 Kmz was defined by using a 12 by 5-km regular grid.. Calibration of the stationary states was done using pressure and static temperature profiles, and assigning different penneabilities and porosities to the different levels of the model. Sources and discharges of mass and heat were used in the reservoir model to correct calibration data. For the proposed model, a constant conductive heat source equivalent to 0.16 MW,bz was placed in level 5 of the grid, with a mass recharge located in the Sol de Maiiana field area, where the higher temperatures of the system were registered. The mass recharges were placed in blocks where there is a history of large hot springs, while the conductive heat discharge is also uniform and located in level 1 of the model. For the natural state adjustment, the penneability values used were between 0.5 and 25 md. Due to the lack of continuous production data in the Sol de Maiiana wells, the history adjustment consisted only in verifying that the production obtained through the reservoir model matches the production and enthalpy measured in the wells of the field. For this purpose, the permeability conditions in the blocks of the model were adjusted. The average well produc- so, 0 20 CI HCO,..._.. 100 80 60 40 20 80 100 0 HCo3 Figure 5. Relative contents of CI-SO,-HCO, pf well and thermal springs of Laguna Colorada, Bolivia. 261
Delgadillo and Puente tion curve in the field was considered similar to that of well measured during the well tests and adequately reproduced by numerical simulation. The forecast of a possible scenario for the development of the Laguna Colorada geothermal field was based on the potential of the field estimated by CFE. This work consisted of an analysis of the reservoir's behavior when producing for a 120 Mw power plant (2x60 Nw). Under this scheme, 20 productive wells are required to provide steam to the power plant, and 7 reinjection wells to dispose of approx~ately 4400 tm of residual water. The results indicate that the Sol de Maiiana reservoir can sustain a 120 MWe power plant running during a period of at least 25 years, with an estimated reservoir pressure drawdown of 1.5 bardyr in the central part of the field. One of the most sensitive parameters handled in this simulation, was the fbture- behavior of the reservoir's fluid enthalpy, which is strongly dependent on the permeability characteristics that control recharge into the geothermal system. Until now, the source and recharge system of the Sol de Maiiana reservoir remains unknown, and it will only be possible to study it in detail after exploitation begins. Conclusions A comprehensiv~ geological model was obtained for the. Laguna Colorada Geothermal Field, representing the main structural and lithological characteristics of the field. Geochemical analysis of thermal springs allowed characterization of geothermal fluids and an estimate of reservoir temperature. Geothermal wells at Sol de Maiianii were surveyed and tem- perature and pressure logs were conducted in order to measure static reservoir conditions. 55 barg at a depth of 1250 rn was measured in all the existing wells while a temperature of 230-245 C was registered at the main feed zones. Wells and were tested showing very similar production conditions. These wells are able to produce up to 5 MW each, feeding steam to a back-pressure turbine, taking advantage of the field's high altitude (4500 m.a.s.1). Based on the results obtained by numerical simulation ajld assessment of the wells, CFE veflied that the potential of Sol de Maiiana field is at least 120 MW for a period of 25 years, proposing that the development in the initial phase will be the installation of 2 condensing power plants of 60 Mw each. Acknowledgment Authors thank Cornision Federal de Electricidad of Mexico (CFE) and Empresa Nacional de Electricidad of Bolivia (ENDE) for pennission to publish this paper. Thanks are also due to Alfred Truesdell for his c o ~ eto n improve ~ the manuscript. References Paz Claure, Oscar L. 1988. Geotheml Resources in Bolivia. Geothermics, VOI. 17, NO 213, P.P. 391-399. Comisi6n Federal de Electricidad. 1997. Certificacih del Potencial Campo Sol de Maiiana, Repiiblica de Bolivia. lnforme Final. Giggenba~h, W. 1991. Chemicat Te~~i~ues in G~othe~aI Explo~tion. Application of Geochemistry in Geothermal Reservoirs Development. (ed. F. D' Amore). ~ I T ~ - ~ Rome. D P p.p.. 119-144. 262