Download Report

Proceedings World Geothermal Congress 2015
Melbourne, Australia, 19-25 April 2015
Origin of reservoir fluids from Borinquen Geothermal Field
Fernando Molina
Instituto Costarricense de Electricidad, Centro de Servicios Recursos Geotérmicos, Campo Geotérmico Miravalles.
P.O Box 10032-1000 San José Costa Rica
Fmolina@ice.go.cr
Keywords: Borinquen, Costa Rica, geothermal field, origin of fluids.
ABSTRACT
Several potential geothermal areas have been detected during geothermal exploration studies conducted by the Costa Rican Institute
of Electricity (ICE). One of the most important areas in the country is located within the Cañas Dulces Caldera, on the Pacific slope
of the Rincón de la Vieja volcano, Costa Rica.
In this sector, the presence of different hydrothermal manifestations that reveal the existence of a geothermal reservoir is
noteworthy. The analysis of the chemical and isotopic compositions of fluids have allowed us to elucidate the hydrological and
hydrogeological processes carried out in the hydrothermal system, as well as, to define the origin and temperature of the fluids that
constitute the reservoir of the Borinquen Geothermal Field, an area where two 55 MW plants will be installed.
1. INTRODUCTION
The Costa Rican internal magmatic arc rises parallel to the Mid-American Trench. The northeastern sector of this volcanic chain is
known as Guanacaste Volcanic Range which consists of four volcanic massifs (Orosí-Cacao, Rincón de la Vieja-Santa María,
Miravalles and Tenorio–Montezuma) separating the Atlantic and the Pacific slopes, with a maximum elevation of 2,000 m a.s.l. In
this area, the Costa Rican Institute of Electricity (ICE) carried out a series of exploration studies between the 1970s and up until
2008, which have identified different zones with geothermal potential in the surroundings of the Rincón de la Vieja-Santa María
volcanic massif. Along its southern foot, the development of the first phase of the 35-MW of the Las Pailas Geothermal Field was
inaugurated in 2011.
Nevertheless, in the eastern sector of the Rincón de la Vieja volcano, about 240 km northwest of the capital, San José (Figure 1),
another area with great geothermal potential (known as Borinquen) is located within Cañas Dulces Caldera (CDC), a structure of
collapse of 120 km2 (Molina et. al., in press). In this sector, the existing thermal anomaly is shown by the emergence of different
thermal sources (fumaroles, silicified rocks, soils, and hot springs). However, the hydrothermal manifestations generate more
information about the geothermal system because along the route of the fluids in the subsoil, several processes took place changing
the physical and chemical parameters of the fluids and the rocks, and this information is transported to the surface. Thus, it is very
important to analyze the chemical and isotopic compositions of the thermal fluids. This data is compared with that obtained from
the samples from the reservoirs and meteoric springs with the aim of explaining the hydrological and hydrogeological processes
that are carried out in the area and to elucidate the origin of the fluids that form the Borinquen Geothermal Field (BGF) reservoir.
Figure 1: Geodynamic setting of Costa Rica and location of the study area.
1
Molina
2. METHODOLOGY
Although several geochemical campaigns have been carried out in the western sector of the Rincón de la Vieja Volcano, only the
most consistent data matching with the samples collected during the dry season in both slopes is selected.
In order to determine the water types present in the study area, 16 samples from the hydrothermal manifestations and 3 samples
from the Borinquen geothermal wells were analyzed (Table 1), and these are classified using the Piper ternary diagram. Besides,
Cl/B relationship of mature waters is compared as an additional criterion to establish a relationship between the fluids. Furthermore,
the isotopic composition (δ18O and δD) of meteoric springs, hydrothermal manifestations, and Borinquen and Pailas geothermal
field reservoirs are analyzed with the aim to determine the origin of the fluids of the geothermal reservoir.
3. CLASSIFICATION OF WATERS
Hydrothermal manifestations in Borinquen are distributed in an area of 100 km2, starting from an elevation of 276 m up to 900 m
a.s.l. Temperatures range from room temperature to boiling point (24–97°C), pH values range from acidic to neutral (2.4–7.57),
and in some cases together with sinter and travertine deposits. Three types of waters have been identified through concentrations of
SO4, Cl and HCO3 anions using the Piper ternary diagram (Figure 2),
Sulfate waters: Sulfate waters have pH values from 2.4 up to 5.4 and temperatures ranging from 57° to 97°C. These waters come
from or nearby the fumaroles that topographically range from 560 to 900 m a.s.l. They are located in the up flow zones without any
relation with a geothermal reservoir of high enthalpy. They are immature waters that were formed through interaction of
underground waters with high oxygen content and magmatic H2S, resulting in oxidization and formation of SO4. These fluids reach
the surface with a temperature close to boiling point at atmospheric pressures.
Figure 2: Piper diagram used to classify different types of waters.
Bicarbonate waters: Bicarbonate waters have slightly acidic to neutral (5.0–7.6) pH values and temperatures ranging between 30.5°
and 50°C, and they were derived from hot springs located between 276 and 630 m a.s.l. These waters are made up of shallow
aquifers with little time of residence in the subsoil (Mg concentrations range between 4.8 and 65 mg/L), heated or mixed with
steam condensation, without relationship with a geothermal deposit. They turn into slightly acidic with a small input of H2S.
Chlorate waters: Chlorate waters have slightly acidic to neutral (4.01 – 6.46) pH values, and temperatures ranging between 36.6°
and 73.0°C, and they were deroved from hot springs located between 259 and 270 m a.s.l. in the Pacific slope and from 410 to 740
m a.s.l. in the Atlantic slope. Samples from the Salitral Norte Spring (SN1 and SN2) that emerge near the Fortuna dome and those
taken from geothermal wells (PGB1 and PGB3) plot in the mature waters field on the Piper diagram (Figure 2). In addition, due to
Na, K and Mg concentrations (Figure 3), it is determined that waters from SN spring have reached equilibrium with the rock at a
temperature close to 235°C, suggesting their relationship with a high enthalpy reservoir. However, samples from BGF wells
indicate an average temperature of 270°C, which is consistent with the maximum temperature directly measured at PGB1 (270°C).
2
Molina
Figure 3: Na-K-Mg ternary diagram in which the Salitral Norte springs have a temperature of 235°C in equilibrium with
the rock and the wells near 270°C.
Once in the liquid phase, the Cl/B ratios can only be affected by dilution or precipitation processes but not by chemical equilibrium
because both elements are of magmatic origin and conservative, and also no evidence for sedimentary rocks of marine origin,
which could contribute to B and Cl concentrations, was observed during the geological surveys (Molina et al., in press) or
geothermal drilling. Therefore, the Cl/B relationships indicate that the samples from Salitral Norte springs (SN1 97.9 and SN2 95.5)
and those from PGB 1 (PB1-1 102.6 and PB1-2 100.2) could be related, but not those from PGB3 (PB3 79.6).
4. STABLE ISOTOPES δ18O AND δD
In the study area, deuterium distribution (in meteoric water) in the Pacific slope (55 to 1,790 m a.s.l.) shows two particular trends
(Figure 4). Between 55 and 235 m a.s.l., a normal distribution of D concentration is observed due to effect of local altitude since the
isotope values decrease with increasing elevation. The opposite happens between 715 and 1,790 m a.s.l., as there is an increase in D
concentrations at these elevations with respect to altitude, up to the point at which the values from Monte Galán1 (located 6 km
away from the Pacific coast and at 60 m a.s.l.; Figure 5) are in coincidence with the values corresponding to 1,150 m a.s.l.
Figure 4: Deuterium distribution in the meteoric water at the Pacific slope of Rincón de la Vieja Volcano.
1
Monte Galán meteorological station is operated as a part of Global Network for Isotopes in Precipitation (IAEA/WMO; taken
from Lachniet and Patterson, 2002).
3
Molina
Figure 5: Digital elevation model with the distribution of the sampling points of the study area and its surroundings.
When stable isotope data from springs of meteoric water (without thermal effect) is plotted on a δ18O versus δD diagram together
with the surface water line (SWL; δD = 7.6‰ * δ18O + 10.5; r2 = 0.96; n = 66) defined for Costa Rica by Lachniet and Patterson
(2002), little relationship is observed with the local data is observed, since this line was defined for the entire country. Therefore, a
curve of best fit was plotted using spring meteoric water samples, and for the purposes of this work, it is used as the local meteoric
water line (LWL, Figure 6).
Figure 6: Diagram of stable isotopes (δ18O versus δD) showing the Local Water Line (LWL), the Surface Water Line
(SWL), and meteoric water and geothermal samples.
4
Molina
By adding data from samplings with thermal influence: hydrothermal manifestations, Pailas Geothermal Field (PGF) reservoir and
Borinquen reservoir, it was determined that the fluids of the PGF reservoir consist of heavier isotopes with respect to the waters
from the BGF reservoir, thus indicating two independent deposits. On the other hand, it seems that there is a relationship between
the samples from well PGB1 and Salitral Norte thermal springs (SN1 and SN2) since these are associated in a dilution line.
5. DISCUSSION
In order to determine the origin of the fluids from the BGF reservoir, it is necessary to know the origin of recharge water, in other
words, meteoric water. Thus, it is important to consider that Costa Rica is narrow. Additionally, the area of study is located in the
northwest, where the volcanic arc dividing the country reaches a maximum elevation of 2,000 m a.s.l., with humidity passages
between the volcanic massifs causing the area under investigation to be affected by rain from both slopes. However, the weather in
the Atlantic slope gets more precipitation (heavier isotopes), while the Pacific side is more arid (lighter isotopes), generating
regional trends. This behavior is evident in SWL (δD = 7.6‰, δ18O = 10.5‰) as determined by Lachniet and Patterson (2002) for
Costa Rica, since data from the Caribbean slope is usually over the SWL, whereas those from the Pacific slope are below SWL.
This explains distribution of D with respect to altitude in the study area (Figure 5); in the Pacific slope meteoric water between 55
and 235 m a.s.l. is the result of precipitation caused by humidity from the Pacific, whereas precipitation between 715 and 1,790 m
a.s.l. are from the Caribbean slope. On the other hand, D concentrations between 235 and 715 m a.s.l. could be the result of a
mixture of waters from both slopes. However, the above mentioned elevation ranks may vary since no meteoric water analysis have
been carried out between 235 and 715 m a.s.l.
Three types of waters were identified through SO4, Cl, and HCO3 anions concentrations. Nevertheless, from the different waters
discharged by hydrothermal manifestations only Salitral Norte springs (SN1 and SN2) are determined as mature waters, which have
reached balance with the rock at a temperature close to 235°C. This data suggests its relationship with a reservoir of high enthalpy.
Samples from geothermal pits (PGB 1 and PGB 3) indicate an average temperature of 270°C. Lower temperatures of SN springs
obtained through the Na-K-Mg geothermometer may be due to rebalancing reactions during ascent. On the other hand, Cl/B
relationship suggests that waters from SN springs and those from PGB 1 could be linked. This relationship is confirmed through an
analysis of δ18O and δD isotopic concentrations, since they are associated in a dilution line, confirming that these manifestations are
due to discharge of BGF reservoir. This reservoir is rechargeable with 83.6% of meteoric water with an isotopic composition of
δ18O = -6.75‰ and δD = -42.25‰.
6. CONCLUSIONS
Although they are located in the slopes of the same volcanic massif and separated by a distance of 8.5 km, fluids from Las Pailas
and Borinquen Geothermal Fields reservoirs are isotopically different, thus indicating independent reservoirs.
Salitral Norte springs are an outflow of Borinquen Geothermal Field reservoir. This reservoir contains 83.6% of meteoric water
with an isotopic composition of δ18O = -6.75‰ and δD = -42.25‰, corresponding to an elevation close to 950 m a.s.l. in the
Pacific, but recharged with waters from the Atlantic slope.
REFERENCES
Lachniet, S., and Patterson, W.: Stable Isotope Values of Costa Rica Surface Waters, Journal Of Hydrogeology, 260, (2002), 135150.
Molina, F., Martí, J., Aguirre, G., Vega, E., and Chavarría, L.: Stratigrafhy and Structure of the Cañas Dulces Caldera (Costa Rica),
Geol. Soc. Amer. Bull. (In Press).
5