Presented at “Short Course on Surface Exploration for Geothermal Resources”, organized by UNU-GTP and LaGeo, in Ahuachapan and Santa Tecla, El Salvador, 17-30 October, 2009.

GEOTHERMAL TRAINING PROGRAMME LaGeo S.A. de C.V.

GEOPHYSICAL EXPLORATION OF THE GEOTHERMAL FIELD,

Enrique A. Porras M. ORMAT Momotombo Power Company Centro FINARCA detrás antiguo SANDY’S Carretera a , Módulos 10 y 11, NICARAGUA [email protected]

ABSTRACT

An integrated geophysical exploration was executed in the Momotombo geothermal resource area under ORMAT administration as part of a power output recovery program. The geophysical surveys and studies included Self-Potential, gravimetry, aeromagnetics, microearthquake monitoring, analysis and integration of an existing resistivity survey, and remote sensing study including aerial photo. The purpose of the geophysical program was to define the production drilling target areas and specific drilling targets within the ORMAT Momotombo Exploration Concession. The program resulted in the identification of several production drilling target areas. Deep drilling took place in Momotombo between 2000 and 2002; four deep wells were drilled based on the proposed drilling target areas succeeding on one of the wells. Deep production drilling targets were not reached due to drilling difficulties. The identified reservoir features by geology and geophysics are the bases for proposing an updated conceptual model of the Momotombo geothermal system.

1. INTRODUCTION

The Momotombo geothermal reservoir has been developed for more than twenty years since 1983 when the first 35 MWe unit was commissioned. The second unit was installed in 1989 by increasing steam production rate. During this period, production wells showed marked changes in flow rates, fluid chemistry and specific enthalpies of produced fluids. By 1999, when the power plant output dropped to 9 MWe an international tender was issued for the rehabilitation of the project under a 15 year Concession. Ormat won the tender and undertook to implement a power output recovery program that included an integrated geophysical exploration program to define production drilling target areas and specific drilling targets within the Ormat Momotombo Exploration Concession, with focus on the areas in and adjacent to the existing wellfield. A description of geophysical methods applied by Ormat and its results are given in this paper.

2. LOCATION OF THE MOMOTOMBO FIELD AND DEVELOPMENT HISTORY

The Momotombo geothermal field is situated about 80 km northwest of Managua city (Figure 1). The Momotombo geothermal system, covering an area of about 2 km2, is related to the Momotombo volcano. This is one of the volcanic centres situated along the Pacific coast of Nicaragua where a chain

1 Porras 2 Geophyscial exploration Momotombo

MEXICO

GUATEMALA HONDURAS EL SALVADOR NICARAGUA

COSTA RICA PANAMA HONDURAS

EL SALVADOR AN NICARAGUA E COSIGUINA VOLCANO C

CA SITA SAN CRISTOBAL VOLCANO MOMOTOMBO VOLCANO STA CLARA VOLCANO O GEOTHERMAL ROTA VOLCANO MOMOTOMBO VOLCANO MANAGUA C EL HOYO VOLCANO MANAGUA FIELD LAKE

MASAYA VOLCANO ANTI

APOYO L

MOMBACHO VOLCANO

NICARAGUA AT VOLCANO LAKE CONCEPCION VOLCANO MADERA VOLCANO

COSTA RICA

FIGURE 1: Location of the Momotombo geothermal field (after Martínez et al., 1988) of active volcanoes is located (Figure 1). The field is situated on the northern shore of at the foot of the active volcano.

A total of 47 wells have been drilled as of 2002 during the four drilling stages: 1974-78, 1981-85, 1992-96 and 2000-2002. The well depths range from a few hundred meters to as deep as 2,839 m. Circulation losses were found during drilling at three different depths: 200-400 m b.s.l, 800-1400 m b.s.l and a deep zone at more than 2,000 m b.s.l., with temperatures of: 200-230°C, 250-290°C and higher than 320°C respectively.

3. GEOLOGICAL SETTING

Geologically the field is developed in the scope of the Nicaragua Depression within which the Momotombo volcanic complex is located (Figure 2). This volcanic complex consists of several small volcanic cones and a large located adjacent to and northwest of the Momotombo volcano. One of the most important geological features is the Monte Galán caldera with a diameter of 4.5 km (Figure 2). The eastern edge of this caldera extends to the northwest basal slope of Momotombo, which implies that the formation of the caldera was preceded by the formation of Momotombo volcano (Goldsmith, 1975). The Monte Galán caldera has a depth of slightly more than 100 m and there are four small lakes within the caldera that were not filled with the flows from Momotombo. There is a small cinder cone of 525 m height, Cerro Montoso, which is located on a fissure on the northwest caldera rim of Monte Galán. The cone has discharged pyroclastics and lava flows, some of which also occupy the floor of the Monte Galán caldera. One and half km northwest of Cerro Montoso is an old, heavily eroded cinder cone, Cerro Colorado, that rises to an elevation of 225 m, displaying thermal alteration in and near the summit crater (Goldsmith, 1975).

Geophyscial exploration Momotombo 3 Porras

LEGEND Qal Qce Qal N Qep SUPERFICIAL DEPOSITS Qce Qal Alluvian and lacustral plains; alluvian fans; colluvium El Qce Lagartillo SAN JUAN MOMOTOMBO-GALAN VOLCANIC COMPLEX CALDERA Qep 100°C Qal Qm2 Recent lava and pyroclastics from Momotombo Qpm Qce Qmo Lava and pyroclastics from Montoso Hill Qce Qm1 Old lava flows from Momotombo Qal CERRO Qal Qce Qgu Volcanic rocks from La Guatusa Hill COLORADO Qcl Qal Qce Deposits of re-worked volcanic materials Qvm Qcc CERRO MONTOSO LA SULFATOSA Qcp Tuffs from Galán Qal Qpm Qcl Galán Pre-calderic lava flows Qvh El Obraje MONTE GALAN Qm1 Qal CALDERA Qm2 Qm2 Qcc Cineritic cones of Cerro Colorado and Loma La Chistata Qcc Qmo Qm1 LA CHISTATA HILL LAGUNA LAGUNA Qal LAS PIEDRAS EL HOYO VOLCANIC COMPLEX BO U MONTE Qcl E ND Qcp Qep El Picacho l A GALAN O R Qcc b Y ra FA Qm1 j O U Qm2 SYMBOLS Qpm e B L R M T Qm2 iv O O MOMOTOMBO e T F Geological contact r G O A La California M R Qgu Border of a crater or caldera O BE 1297 M N 1100 O BOQUERON O T Qm2F 10 00 Lava flow Qal R M Qm1 E O 90 0 Fault U M 800 P O TO 700 F M Structural alignment O B 600 N O E 500 Fumarole B A MT-16 Qm1 R 400 Thermal manifestation G Qgu MT-33 300 MT-29 MT-32 Qm1 MT-31 MT-43 MOMOTOMBO TOWN MT-8 OM-51 Qgu IGNIMBRITES OF MALPAISILLO MT - 36 MT - 11 MT-10 OM-53 MT-7 MT - 34 MT - 4 1 MT-27 MT -3 8 MT-23 MT-28 MT - 6 MT-4 MT-14 MT-5 MT-37 MT-2 MT-40 MT-21 MT - 19 OM -5 2 MT-22 MT-1 LOMA LA GUATUSA MT-26 MT-35 MT-12 MT-17 Qpm Malpaisillo tuff and undefined products MT-13 MT-25 MT-9 MT - 24 Qpm OM-54 MT-20 MT-3 MT-42 T MT-18 L MT-15 U RMT-2 A RMT-1 F MT-30 NO DI AN PUNTA SALINITAS S PUNTA DEL DIABLO SCALE TO ER 0 3000 m PU MANAGUA LAKE FIGURE 2: Geological map and structural frame of the Momotombo area (after Geothermex, Three km south of Cerro Colorado is another, smaller, deeply eroded cone of 145 m height, Loma La Chistata, that rises above the plain that surrounds the Monte Galán caldera. It is cut by a fault in the south. On the south-eastern part of Momotombo and discontinuous with the slope of the volcano toward Lake Managua, is a mountain complex composed of andesitic lava flows and tuffs called Loma La Guatusa that seems to be a plateau formed by a lava flow from the Momotombo volcano. This volcanic complex is located close to the Momotombo volcano and also on the volcanic axis of the Cordillera Los Marrabios. Volcanic activity of La Guatusa occurred much earlier than that of the Momotombo volcano.

Figure 2 also shows the structural frame of the Momotombo geothermal field characterized mainly by three fault systems running NW-SE, NE-SW and N-S. The first two systems are correlated with the regional tectonics of the Nicaragua graben: the Puerto Sandino fault with a NE-SW direction and Puerto Momotombo Graben with a NW-SE direction. There are regional faults of N-S alignments. A set of these faults allows the circulation of fluids in a hydrothermal system. As the most productive shallow wells are located generally in a zone corresponding to structural crossings, the recharge zone for the shallow reservoir possibly presents in a fractured zone formed in the intersection of faults of these three systems. Figure 3 shows the location of the main faults within the well field. Fluid seems to be transported from depth through the western most fault of the NE-SW direction. Then it changes direction at a shallow depth after reaching the Bjornsson fault, and then fluid flows in a horizontal layer at a shallow depth after reaching Momotombo fault and the shallow reservoir located generally in a zone corresponding to structural crossings, the recharge zone for the shallow reservoir possibly presents in a fractured zone formed in the intersection of the faults of these three systems.

On the basis of the petrographic and macroscopic analysis of drilling cuttings and cores, the lithology of the Momotombo geothermal field has been divided into six units that consist of andesite and andesite-basaltic volcanic products, volcano-clastic deposits, different kinds of tuffs, volcano- sedimentary and sedimentary deposits, and some sub-intrusive bodies and dikes (Combredet et al., 1987). The geological interpretation of these units suggests that a long period of volcanic activity took Porras 4 Geophyscial exploration Momotombo

MT16 300

MT33 MT29 N

MT32

MT31

T MT43 L OM-51 U A MT8 F N OM51 O S MT36 MT11 S N R MT7 MT10 OM53 O J MT41 MT34 B MT27 MT38 MT28 RMT6 MT4 MT23 MT14 MT2 MT5 MT37 MT40 MT19 MT21 OM52 MT22 POWER PLANT MT1 MT26 MT35 MT12 MT17 LEGEND OMT54 MT9 MT13 MT25 MT24 MT43 MT20 MT3 WELL MT43 MT42 RMT18 20 0 ELEVATION, m RMT2 RMT15 MAIN FAULTS MANAGUA LAKE RMT1 BALL ON DOWN- RMT30 THROWN SIDE

DIRECTIONAL WELL

FUMAROLES/ 0 200 400 600 800 1000 m PUNTA SALINITAS HOT SPRINGS

FIGURE 3: Location map of wells and main faults in Momotombo place in the area. Such activity generated a 1,700 m thickness of different volcanic and volcano- sedimentary products, and the presence of sedimentary rocks combined with volcanic rocks in the deeper zones. All these units had been formed during early Miocene to the Quaternary age.

Figure 4 shows a petrological correlation by Combredet et al. (1987) in four wells located in the western part of the field. It also denotes the depth of the lithological contact between three of the six

SSW MT36 MT35 MT37 MT34 G ? Ground level 0 G ? Sea level v v 100 v >< UNIT I

200 Cl Cl < Cl LEGENDS oo Cl > o 300 oo Cl >< Andesite cz vvv 400 cz cz vv Basaltic/andesite cz ca vvv cz UNIT II 500 > < Altered lava cz cz cz Tuff 600 Cl Cl v

No data No Microbrecia v Cl 700 > < ?? Cl Ignimbrite in tuff ClvvCl + alc cz an 800 > < cz No information vvv v v Cl v v Cl cz Presence of inclusions 900 v v Cl Cl cz > v v < Measured depth (m b.s.l.) (m depth Measured analc+ Analcite v Cl v Cl > < 1000 < > < v Cl v Cl > cz Quartz > < cz > < 1100 v vv vvvv Chloritization of iron and cz > < < UNIT III Cl magnesium rich minerals > < > < 1200 >cz e pi Contact of lithological unit do < te < > 1300 > + < >

1400 > <

1500 > <

1600 FIGURE 4: Petrological correlations between wells in the western part of the well field (after Combredet et al., 1987) Geophyscial exploration Momotombo 5 Porras

lithological units in Momotombo. They also reported the depth of appearance for minerals such as epidote and quartz. The figure presents that epidote, which is an indicator of high temperature (>240°C) fluid circulation, is found at relatively shallow depths in well MT35 inferring a presence of a high temperature zone in this area.

3. OVERVIEW OF GEOPHYSICAL PROSPECTING

Geophysical explorations were carried out in Momotombo in seven different periods between 1970 and 2000. The most recent studies (DAL SpA, 1995 and TRANSPACIFIC, 2000) are closely related to defining new drilling sites for production wells as part of the power output recovery plan implemented by ORMAT and identified the main hydro-geological features of the Momotombo geothermal system. Table 1 presents a summary of the geophysical studies carried out so far.

TABLE 1: Geophysical surveys conducted in the Momotombo field (after Geothermex, 2001)

Profiles Depth Max. distance Number of Area Total Number Company Year Method research ab or dipoles measurem. (km2) No. length of surveys (m) (m) sites (km) Gravimetry 1 Magnetometry 1 ? 6 SEV Schlumb 268 1 8 ? Texas Instruments 1970 6 Dipole map ? 1000 3 Electro-Mag 1700 <3000 8 3 86 Audiomagnetoteluric ? 16 SEV Schlumb 35 6 United Nations 1974 Schlumberge Profile 2000 ? ? ? Dipole Maping ? SEV Schlumberger Electro-Consult 1974- 75 1000 3000 5 20 Gravimetry Phoenix Dipole-dipole Recon 3750 300 14 200 225 1977 1450 Geophysics Dipole-dipole detail 3750 5 6 48 44 Regional Gravimetry OLADE 1982 1500 Regional Magnetom DAL 1995 Frequency soundings 10 12 Self-Potential 886 50 44.3 ORMAT/ Gravimetry 525 2000 100 TRANSPACIFIC Aeromagnetics 370 Microearthquake Mo 13

During the 1970’s and early 1980’s the geophysical surveys were carried out as part of the pre- feasibility study for the development of the geothermal field. Among others, ELC (1976) and OLADE (1981) conducted gravity measurements that revealed the characteristic structure of the geothermal system. For example, these studies confirmed the existence of a high regional Bouguer anomaly in a NW-SE direction, which coincides approximately with the active volcanic axis from the Momotombo geothermal field up to the town of Malpaisillo (Figure 5).

Figure 5 shows a Bouguer gravity anomaly map by Cordon (1980) and then modified by DAL SpA, (1995). This gravity survey was conducted at 200 stations over an area of approximately 300 km2. The density values for the shallow layer and the underlying “Basement” are 2.3 g/cm3 and 2.6 – 2.8 g/cm3 respectively.

There are high anomalies in correspondence to the Galán and San Juan caldera (Figure 5); another anomaly is located to the southeast of the Momotombo volcano where the geothermal field is located. However, gravity survey in detail suggests that there are plateau like contours followed by descending gravity in the north-western part of the field with gravity values between 1.914 and 1.905 g/cm3 (Figure 6). Porras 6 Geophyscial exploration Momotombo

MALPAISILLO LEGEND 20 Iso-anomaly (mgals) 20 26 22 2 2 Positive anomaly zone 22 30 8 4 24 (high gravimetry) Negative anomaly zone (low gravimetry) 32 SAN JUAN CALDERA Fault Geophysical discontinuity Caldera edge 1380 34 36 Geothermal well 8 3

0 COLORADO HILL 6 GALAN CALDERA

0 MONTOSO HILL 4

28

26 16 MOMOTOMBO 1 24 20 8 2 22 2 20 MT16 MT29 24 2 2 MOMOTOMBO TOWN 8 6 RMT30

MANAGUA LAKE 550

FIGURE 5: Bouguer gravity anomaly map after Cordon (1980) and DAL SpA (1995)

1375000 N MOMOTOMBO 1374000 1923 VOLCANO 1921

1373000 1919

1917

1915 1372000

1913

GUATUSA 1911 1371000 PLATEAU 1909

1907

1370000 1905

SCALE 1903 0 200 400 600 800 1000 m (mgals) MANAGUA LAKE 1369000 PUNTA SALINITAS PUNTA DEL DIABLO 546000 547000 548000 549000 550000 551000 552000 553000 FIGURE 6: Bouguer gravity map (after TRANSPACIFIC, 2000) The high gravity in the eastern part of the well field near Punta del Diablo (Figure 6) may represent a body of high resistivity zone related with hydrothermal alteration activity (OLADE, 1981); the increase in gravity is associated with a more dense formation due to intense silicification. Another interpretation to this high local gravity is a presence of a basement with a thick sequence of lava (OLADE, 1981). The descending gravity to the north-western area may correspond to the upflow zone where a series of NW-SE faults have been described by geological studies (DAL SpA, 1995). Geophyscial exploration Momotombo 7 Porras

4.1 ORMAT geophysical programme

An integrated geophysical exploration program was executed in the Momotombo geothermal resource area that included four other different surveys such as self-potential, gravimetry, aeromagnetic and micro-earthquake monitoring. The purpose of the program was to define production drilling target areas and specific drilling targets within the Ormat Momotombo Exploration Concession, with focus on the areas in and adjacent to the existing wellfield. Figure 7 shows the resistivity contours at depths

80 0

A Resistivity contours at 1000 m 700 1372 60 0 N

50 0

GUATUS A P LA TEAU 550 54 9 40 0 MT 16

MT33 300 MT29 13 71 MT32300 MT 3 1 Fumaroles

MT 4 3 OM-51 MT8 MT 11 OM 51 MT10 MT36 MT7 OM53 MT 4 1 MT34 MT27 MT38 MT23 MT 28 RMT6 MT4 MT 1 4 MT 5 MT40 MT 2 MT19 OM52 MT 37 MT 21 MT1 MT26 MT22 MT12 MT17 MT35 MT 9 13 70 MT 13 OM 5 4 MT25 MT 2 4 MT20 MT 3 MT 42 RMT18 RMT2 RMT15 RMT1 MANAGUA LAKE MT30

0 200 400 600 800 1000 m PUNTA S ALIN ITAS SCALE

80 0

B Resistivity contours at 2000 m 700 1372 60 0 N k c Fu 50 0 o m GUATUSA 550 l 549 a r40o 0 PLATEAU MT16 B le n MT33 300 MT29B 1371 r 300 lo MT32 c te MT 31 k MT 43 OM-51 FUMAROLES s MT8 MT 11 OM 51 e MT 1 0 MT36 MT7 OM 5 3 MT 41 MT34 MT27 MT38 MT23 MT 28 RMT6 W MT4 MT 14 MT 5 MT40 MT2 MT19 OM 5 2 MT 37 MT21 MT1 MT26 MT22 MT12 MT17 MT35 MT 9 MT 13 OM 5 4 MT25 MT 2 4 1370 MT 2 0 MT 3 W MT42 RMT18 RMT2 MANAGUA LAKE e RMT15 llf RMT1 iel MT30 d B 0 200 400 600 800 1000 m loc PUNTA SALINITA S SCALE k FIGURE 7: Resistivity contours at 1,000 and 2,000 m depth (after DAL SpA and TRANSPACIFIC, 2000) Porras 8 Geophyscial exploration Momotombo of 1,000 m (A) and 2,000 m (B) from the surveys carried out in 1995 (DAL SpA, 1995). As shown in Figure 7, the field has been divided into three areas depending on resistivity values. They are the Wellfield, Western, and Fumarole blocks. The Wellfield block, where the wells are located, is characterized by generally low resistivity values. The Fumarole block is located to the north of the Wellfield block and is characterized by higher resistivity values, and there is a large fumarolic area present. Another high resistivity zone is located to the west of the Wellfield block. This area is represented by the Western block in Figure 7b.

The results show that the Fumarole block is characterized by high resistivity. Figure 7b shows that the line dividing the Fumarole block with the Wellfield block corresponds with a resistivity boundary where production wells are located (Figure 7a (MT7, 8, 32 and 33)). The resistivity boundary is also evident at 1,000 m depth. This resistivity pattern at depth suggests a presence of possible fractures along the various resistivity boundaries that are defined by discontinuities of the resistivity contours.

The program identified additional exploration targets (Figure 8), which are located outside the area capable of supplying the existing power plant but within the Ormat Momotombo Exploration Concession. These target areas currently contain no drill holes and nothing is known about the deep temperature regime.

Four deep wells were drilled by ORMAT between 2001 and 2002 following the geophysical study results. A total length of 7,097 m was drilled from which well OM53 resulted in being commercially productive and has become the best steam producer of the field. Another well, OM52, is now used as a brine injector.

Geophysical studies suggest deep drilling targets (deeper than 2,500 m) for production wells. None of the four drilled wells by ORMAT was able to reach more than 2,100 m, except OM52 (> 2,800 m).

500 4 00

MT16 300

MT33 MT29 MT32

MT31 MT43 MT8 OM51

MT36 MT11 MT7 MT10 OM53 MT34 MT41 MT27 MT38 MT28 RMT6 MT4 MT23 MT14 MT2 MT5 MT37 MT40 MT19 MT21 OM52 MT22 POWER PLANT MT1 MT26 MT35 MT12 MT17 LEGEND MT13 MT9 MT25 MT24 OM54 MT20 MT3 MT43 MT42 WELL MT43 RMT18

2 RMT2 00 ELEVATION, m RMT15 MANAGUA LAKE RMT1 RMT30 DIRECTIONAL WELL

RMT2 INJECTION WELL RMT2 PUNTA SALINITAS

DRILLING TARGET N TARGET AREA 0 200 400 600 800 1000 m OM53 WELL DRILLED BY ORMAT

FIGURE 8: Drilling targets for deep drilling in Momotombo Geophyscial exploration Momotombo 9 Porras

Such deep targets are still considered as promising for steam production in order to increase the current power output up to the current installed capacity of 77 MWe. ORMAT is willing to invest in new studies and drillings only if the concession term is extended beyond the remaining six years.

5. CONCLUSIONS

Characteristics of subsurface and physical phenomena were revealed by reviewing and interpreting the studies carried out since late 1960’s in the Momotombo area. Thus, features of the geothermal system in Momotombo can be summarized as follows. 1) Three main fault systems control fluid flow in the reservoir. 2) A low resistivity anomaly in the eastern part of Momotombo probably plays a role as a reservoir boundary of no flow to the east. 3) On the basis of the features suggested by geology and geophysics a conceptual model can be proposed for the Momotombo geothermal system. 4) Geophysical studies suggest deep drilling targets (deeper than 2,500 m) for production wells, from the four drilled wells by ORMAT, non of them was able to reach more than 2,100 m, except OM52 ( > 2,800 m).

REFERENCES

Combredet, N., Guilhaumou, N., Cormy, G., and Martínez, E., 1987: Petrographic correlation and analysis of fluid inclusions in hydrothermal quartz crystal in four wells in the Momotombo geothermal field, Nicaragua. Instituto Nicaraguense de Energía, Managua, Nicaragua, internal report. 32 pp.

Cordón, U., 1980: Momotombo field models at six stages in time. Internation Engineering Company, Inc (IECO). Geothermal Resources Council, Transactions, 4, 443-446.

DAL SpA, 1995: Estudios de evaluación del potencial energético: Factibilidad de la estabilización y de la tercera unidad geotermoeléctrica del campo geotérmico de Momotombo, Fase IA: Investigaciones científicas; Informe de síntesis. ENEL (Empresa Nicaragüense de Electricidad), Managua, Nicaragua, DAL SpA, Milano, Italia, internal report.

Electroconsult (ELC), 1976: Estudio de factibilidad de la primera planta geotérmica en Nicaragua. Momotombo geothermal field gravimetric survey. Reporte a ENALUF-Empresa Nacional de Luz y Fuerza, internal report.

Geothermex, 2001: Plan maestro geotérmico de Nicaragua, evaluación del área de Momotombo., Comisión Nacional de Energía de Nicaragua, Managua, Nicaragua, Geothermex, Inc., Richmond, California, USA, internal report, 97 pp.

Goldsmith, L.H., 1975: The Momotombo geothermal field, Nicaragua, development history. Geonomics, Inc., Berkeley, California, USA, internal report, 44 pp.

Martínez, E., Arcia, R., and Sabatino, G., 1988: Geothermal development in Nicaragua. Geothermics, 17- 2/3, 333-359.

OLADE, 1981: Estudio de reconocimiento de los recursos geotérmicos de la República de Nicaragua. Trans-Pacific Geothermal Corporation, 2000: Geophysical exploration of the Momotombo geothermal field, Nicaragua. ORMAT Momotombo Power Company, Trans-Pacific Geothermal Corporation, Oakland, California, USA, internal report, 54 pp.