Elsevier

Applied Thermal Engineering

Volume 75, 22 January 2015, Pages 1244-1261
Applied Thermal Engineering

Geochemometric modeling and geothermal experiments of Water/Rock Interaction for the study of alkali-feldspars dissolution

https://doi.org/10.1016/j.applthermaleng.2014.09.011Get rights and content

Abstract

Water/Rock Interaction (WRI) experiments, Na–K geothermometry and geochemometrics modeling have been used for the study of the kinetic behavior of the volcanic rock dissolution under geothermal conditions. Ionic exchange reactions between Na–K and alkaline-feldspar minerals were monitored at 90 °C and 150 °C for a period of nearly 24 and 3 months, respectively. Na/K ratios (inferred from WRI experiments and Na–K geothermometers) and reaction times were used for estimating the most probable quasi-steady state conditions.

A geochemometrics modeling was also performed to predict the reaction times required to achieve quasi-steady state conditions in the WRI experiments. This modeling was validated with WRI experimental data reported in the literature for which quasi-steady state conditions were known with accuracy. New WRI experiments were additionally carried out using batch reactors under controlled pressure and temperature conditions. Volcanic rock (basalt and dacites) crushed samples (500–1000 μm grain size) were reacted with distilled water at 90 °C and 150 °C using a W/R mass ratio of 5. Rock and fluid samples were collected and analyzed for major composition, before and after each experiment. The experimental results were subsequently used to calculate log(Na/K) values for describing the kinetic behavior of the alkaline-feldspar mineral dissolution. Values of log(Na/K) and reaction time at quasi-steady state conditions were reproduced with good accuracy by using the rational polynomial and logarithmic transformation regression models. These results were compared with those values inferred from the Na–K geothermometry, which theoretically assume deep geothermal equilibrium conditions.

Introduction

The immense amount of energy stored in both “convective” hydrothermal and “conductive” hot dry rock systems makes geothermal energy an important renewable energy to be considered for the future energy demand [1], [2], [3], [4]. In this promissory energy scenario, the development of improved geochemical tools for a better exploration and exploitation of the geothermal resources is still required. The knowledge of reservoir temperatures is recognized as a fundamental task for the reliable estimation of geothermal reserves [5], [6], [7], [8], [9], [10].

Solute geothermometers are practical and low-cost tools used to predict deep equilibrium temperatures of hydrothermal systems [5], [6], [7], [8]. Fluid sampling and chemical analysis are used as primary data to estimate reservoir temperatures [5]. Geothermal reservoirs are complex systems in which both mineral assemblages and the fluid chemistry greatly depend on a wide variety of Water/Rock Interaction (WRI) processes [11], [12], [13]. Solute geothermometers are based on the temperature-dependent chemical equilibrium reactions that occur in WRI processes, which are associated with the dissolution of primary and secondary minerals [12], [13], [14], [15]. The most frequently used geothermometers are cation (e.g., Na–K, Na–K–Ca, Na–Li, among others) and silica (SiO2) geothermometers [5], [6], [7]. Although, a plethora of geothermometers have been proposed in the literature, significant statistical differences still exist among their predictions when these estimates are compared with bottomhole temperature (BHT) measurements [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26]. The prediction capability of these geothermometers is commonly affected by several processes, such as: (i) the steam loss [7], [16]; (ii) the ionic exchange with clay minerals [7], [20]; (iii) the no re-equilibration, mixing or boiling processes after the fluid leaves the reservoir towards the surface or the sampling point [16], [21]; (iv) the lack of chemical equilibrium into the reservoir [11], [12], [15]; (v) the representative sampling of geothermal fluids [5], [17], [24]; (vi) the precision and accuracy of the fluid chemical analyses [5], [9], [17]; and (vii) the total propagated errors of the geothermometer [7], [8], [24], [27].

Among the solute geothermometers, the most used tool in the geothermal industry is the Na–K geothermometer because it has demonstrated a much better confidence for predicting deep equilibrium temperatures of high-enthalpy systems in comparison with other tools based on silica, Na–Li, and Na–K–Ca contents [6], [7], [8], [26]. Na–K geothermometers are actually less affected by the geochemical processes above described (i–v). Na–K geothermometers are based on the temperature-dependent variation of Na–K compositions due to an ionic exchange between fluids and coexisting alkali-feldspar minerals [25], [28], [29], [30]. The thermodynamic activity of the Na/K ratio is controlled by equilibrium conditions between alkali-feldspar minerals and geothermal fluids [12], [25]. The development of Na–K geothermometers is based on the following assumptions [25], [31]: (i) the fluid composition changes in Na and K follow the reaction of albite replacement by K-feldspar; (ii) the fluids in deep-seated reservoirs are in equilibrium with alkali feldspars; and (iii) the mixing of deep geothermal fluids with shallow waters, and the degassing of the fluids during the route towards the surface, do not affect the Na/K ratios. Na–K geothermometers usually overestimate the temperatures of low-enthalpy geothermal systems probably due to the lack of reliable field and experimental calibration data [7], [8], [26], [29]. Empirical calibrations of these geothermometers have been proposed to face such problems using geochemical databases and advanced geochemometric techniques [7], [8], [19], [22], [23], [24], [25], whereas a few theoretical thermodynamic calibrations have been also carried out [10], [20], [21], [25].

With the goal to overcome such overestimation problems, and mainly to improve the prediction performance of these geothermometers, experimental calibrations based on controlled WRI reactions are needed [9], [20], [25], [30]. Geochemical processes occurring in low-enthalpy geothermal systems may be better understood with the aid of WRI experiments and fluid geothermometry [31], [32], [33], [34], [35]. WRI experiments and fluid/mineral equilibrium modeling have been previously used for the study of hydrothermal systems [36], [37], [38], whereas the application of fluid geothermometry to the geochemical monitoring of WRI reactions has been rarely reported [39], [40]. Kinetic rates of complex reactions are typically studied by controlling the following thermodynamic and geochemical parameters [41], [42], [43], [44]: (1) temperature and pressure; (2) kinetic mechanisms of rock–mineral dissolution; (3) reactive surface of the rock–mineral and the crystal dislocation; (4) rock–mineral and fluid (pH) compositions; (5) chemical affinity of dissolving solution; and (6) reaction times.

WRI experiments are controlled by dissolution reactions of primary minerals and the precipitation of secondary minerals under full-equilibrium conditions [45], [46]. Reaction rates generally decrease with time, and full-equilibrium conditions are rarely achieved because long reaction times are needed for the dissolution of rock-forming minerals [47], [48]. WRI reactions and the approaching to quasi-steady state conditions may be described by the kinetic behavior of the reaction products and reaction times [49].

In this paper, WRI experiments and a geochemometrics modeling have been applied, for the first time, to reproduce the kinetic behavior of the alkali-feldspars dissolution, and to estimate the reaction times needed for reaching the quasi-steady state conditions. A new application of the Na–K geothermometer at low-to-medium temperatures was also used for supporting the experimental studies.

Section snippets

General methodology

A general schematic diagram showing the work methodology used in this study is shown in Fig. 1. The objective of the methodology was to develop a new geochemometrics approach both to describe the kinetic behavior of WRI experiments, and to estimate the reaction times at theoretical quasi-steady state conditions. The developed methodology consisted of four major tasks (Fig. 1):

  • i)

    Creation of a world geochemical database with measurements logged from WRI experiments which are mainly based on mineral

Results and discussion

According to the methodology described in Fig. 1, Fig. 2, the study of the kinetic behavior and the prediction of the reaction times (θ) at quasi-steady state conditions for previous and new WRI experiments were performed.

Conclusions

A new geochemometric methodology based on WRI experiments, regression models (RPM and LogTM) and Na–K geothermometry was developed for describing the kinetic behavior of the alkali-feldspars dissolution reaction. Reaction times (θ) at quasi-steady state conditions were also inferred from these analyses.

The applicability of this methodology was successfully demonstrated through the study of five WRI experiments already reported in the literature, where the log(Na/K) and reaction time

Acknowledgements

The authors want to thank to PAPIIT-UNAM (IN 115611) and the research project P09 of the CeMIE-Geo (CONACyT-SENER) for the financial support provided during the research.

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