Proceedings World Geothermal Congress 2015 Melbourne, Australia, 19-25 April 2015 1 Update on the Stable Isotope Geochemistry of the Leyte Geothermal Production Field Lorena M. Daco-ag, Ma. Regina Cabahug, Irene Marice Fernandez, Danilo Dacillo and Edwin Alcober Energy Development Corporation, Leyte Geothermal Business Unit, Tongonan, Ormoc City, Leyte, Philippines [email protected]Keywords: stable isotope, isotope geochemistry, Leyte, Philippines. ABSTRACT Isotopic data monitoring from geothermal wells, springs, ground water and rainfall aims to update the stable isotope systematic of the Leyte Geothermal Production Field, Philippines. Isotope collection was conducted during the exploration stage and in 1990- 2002 but was terminated due to the breakdown of the mass spectrophotometer isotope analyzer in 2002. With the procurement of a new isotope analyzer, sampling in Leyte resumed in 2010 and now becomes a routine geochemistry activity. Isotope data, correlated with the geochemical characteristics of the fluids, elucidates the reservoir processes affecting the performance of the Leyte Geothermal Production Field including Tongonan-1 Sector which continues to produce for over 30 years. For example, production wells affected by incursion of injected brine show enrichment of stable isotope (from -1.19 to 0.22 δ 18 Ores‰) coinciding with reservoir chloride increase. Conversely, for some wells affected by dilute, cooler waters depict relatively depleted oxygen-18 isotope (at -1.6 to -1.70 ‰) which corresponded to lower chloride and gas levels. In areas where boiling is prevalent, there is an observed enrichment of stable isotopes, increasing total discharge CO 2 (CO2 TD ) and discharge enthalpies greater than 2000 kJ/kg. Monitoring of isotopic trends and distribution in the area, together with geochemical trends, aids in mapping out the evolution of these reservoir processes. This also serves as a useful tool in evaluating the effectiveness of implemented strategies. As such, isotope geochemistry has been considered a valuable tool in managing the production field and in formulating resource strategies. Thus, isotope monitoring correlated with geochemical data must be continued to help us understand changes in field hydrology with continued mass extraction. 1. INTRODUCTION Historically, isotope collection in Leyte Geothermal Production Field (LGPF) was conducted during the exploration stage and in 1990-2002 but was terminated due to the breakdown of the mass spectrophotometer isotope analyzer in 2002. Upon the procurement of a new isotope analyzer, sampling in Leyte resumed in 2010. Comparing the two instruments, each has its own benefits and drawbacks. In terms of accuracy and precision, they have comparable ranges such as shown below: Criteria Mass Spectrometer Isotope Analyzer Accuracy +2 H: <1.00 ‰ +18 O: <0.10 ‰ +2 H: 1.00 – 2.00 ‰ +18 O: 0.25 - 0.50 ‰ Precision +2 H: <1.00 ‰ +18 O: <0.10 ‰ +2 H: 1.00 – 2.00 ‰ +18 O: 0.25 - 0.50 ‰ Since the acquisition of the isotope analyzer, EDC has built up a good number of deuterium (δD or δ 2 H) and oxygen-18 (δ 18 O) database. With these stable isotopes being part of routine sampling and monitoring, it is but appropriate to maximize their use in our operations. Figure 1 shows the correlation of some LGPF isotope data collected from different sources – thermal areas, surface waters and production wells. The black solid line is the Local Meteoric Water Line (LMWL) of the Greater Tongonan as established by Isidro and others in 1992. The black and blue dashed lines are the regression lines for Tongonan and Mahanagdong wells, respectively. Note that these lines intersect with the LMWL at the point where the surface waters plot suggesting that the fluids from the wells are mixture of andesitic waters (δ 18 O of 10 ± 3‰ and δ 2 H of -20 ± 10‰) and meteoric waters. These are sources of natural waters where andesitic is defined by Giggenbach as recycled sea water which has been subducted in regions with andesitic volcanism or convergent plate volcanism, while magmatic waters is defined as that equilibrated with magma regardless of its initial origin. Meteoric water, on the other hand, refers to water which has been recently involved in atmospheric circulation, including pluvial, vadose and other groundwaters (Cindrich and Gundmundssori, 1984).
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Proceedings World Geothermal Congress 2015
Melbourne, Australia, 19-25 April 2015
1
Update on the Stable Isotope Geochemistry of the Leyte Geothermal Production Field
Lorena M. Daco-ag, Ma. Regina Cabahug, Irene Marice Fernandez, Danilo Dacillo and Edwin Alcober
Energy Development Corporation, Leyte Geothermal Business Unit, Tongonan, Ormoc City, Leyte, Philippines