SolGeo: A new computer program for solute geothermometers and its application to Mexican geothermal fields
Introduction
A plethora of geothermometric equations put forth for estimating subsurface temperatures from solute concentrations in geothermal waters are difficult to use without suitable computer software. In the 1970s, Truesdell (1976) reported computer program GEOTHERM, which was capable of calculating temperatures from seven geothermometric equations (see also Henley et al., 1984). Since then, numerous new equations have been proposed and used in geothermometric temperature calculations (see D’Amore and Arnórsson, 2000, or Verma, 2002, for relatively recent compilations of such equations).
Commercial program AquaChem (Version 5.1.33; http://www.swstechnology.com/software_product.php?tab=3&ID=1#index) contains 18 geothermometric equations (four for Na–K; one for K–Mg; one for Mg–Li; three for Na–Li; two for Na–K–Ca; four for quartz; three for sulfate-waters) and provides for modifying regression coefficients corresponding to most of the programmed equations, but does not allow changes to the sign (+ or −) of the coefficients. More recent equations such as those proposed by Nieva and Nieva (1987), Fournier (1991), Verma and Santoyo (1997), and Can (2002) are absent from AquaChem. El-Naqa and Zeid (1993) presented a computer program (GEOTHERM) for applying Na–K, Na–K–Ca, and silica (or quartz) geothermometers, but so far temperatures computed using this program have not been reported in the published literature.
Most geochemists use some kind of spreadsheets or hand calculators that probably lead to frequent errors, as discussed below. Therefore, there is a special need for developing freely available software that could facilitate these computations in an easy, friendly, efficient, and reliable way. With this need in mind, we developed computer program Solute Geothermometers (i.e., SolGeo) that includes 35 geothermometric equations for solute geothermometers (Table 1). A salient feature of the program is that the concentration of each chemical variable is converted to the measurement units required by each geothermometric equation (see Table 1 for these requirements) using updated values of atomic weights (e.g. Vocke, Jr., 1999; Verma et al., 2003, Wieser, 2006) and proper geothermal water density calculations and corrections (McCutcheon et al., 1993, Nicholson, 1993). Here, we present validation of SolGeo from a worldwide geothermal database and results from application to four Mexican geothermal fields.
Section snippets
SolGeo: a new computer program
Computer program SolGeo (for Solute Geothermometers) was written in Visual Basic 6.0; the main functions of the code are summarized in Fig. 1. It accepts input data as an Excel (*.xls) or a Statistica (*.sta; Version 5) file (example files are available from any of the authors on request). Thirty-five geothermometric equations and their respective applicability constraints used in SolGeo are listed in Table 1.
For each sample, the chemical variables Cl, HCO3, SO4, Na, K, Li, Ca, Mg, and SiO2
Geochemical databases for program validation and application
Geochemical data on well waters and measured bottom-hole temperatures (BHTs) were compiled from the following geothermal fields: (1) Berlin, El Salvador (D’Amore and Mejia, 1999); (2) Fushime, Japan (Okada et al., 2000); (3) Geysir, Iceland (Pasvanoglu, 1998); (4) Kizildere, Turkey (Gôkgôz, 1998); (5) Miravalles, Costa Rica (Fung, 1998); (6) Salihli, Turkey (Tarcan et al., 2000); (7) Seferihisar, Turkey (Tarcan and Gemici, 2003); (8) Takigami, Japan (Furuya et al., 2000); (9) Xiaotangshang,
Results and discussion
We processed both the worldwide and the Mexican databases with SolGeo. The information from the worldwide database was used for performance validation by comparing geothermometric temperatures given in the literature (tlit) against SolGeo computed temperatures (tSolGeo); the results are shown in Fig. 2 in the form of “Difference” values (see the figure caption for explanation). In Table 3 are listed the miscalculated samples showing greater than ±4 °C difference between tlit and tSolGeo.
Conclusions
In order to recognize and reduce errors when estimating subsurface temperatures in geothermal systems we developed computer program SolGeo. Here we discuss and illustrate its application to geothermal well waters from around the world, including Mexico.
It was concluded that static formation temperatures in geothermal wells should always be reported, which could be of significance when developing more reliable geothermometric equations. We suggest that K–Mg, Na–Li, Na–K–Ca, Na–K–Mg, and
Acknowledgements
We are grateful to Alfredo Quiroz Ruiz for the efficient computer maintenance. The second and third authors thank DGAPA-UNAM for providing financial support through the PAPIIT project IN-108408. We thank Mark Reed, an anonymous reviewer and the editors Sabodh K. Garg and Marcelo J. Lippmann for the numerous suggestions to improve the manuscript.
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