Mahendra Pal Verma

Chemical thermodynamics of silica: a critique on its geothermometer

Abstract The chemical thermodynamic concepts used in the calculation of solubility data of silica (quartz) are presented taking into account the PVT characteristics of water. The temperature-dependence trends between the thermodynamically calculated and the experimental quartz solubility data are very similar, but the values are widely different at high temperatures. The experimental solubility, especially along the saturation curve at high temperature and thermodynamic data for silica need to be reevaluated in order to use silica chemistry to understand geological processes. There could exist a wide range of values for silica solubility at a specified temperature, depending upon the amount of water in the reaction vessel. Thus the silica contents in geothermal fluid, in general, cannot be used as a geothermometer to estimate the reservoir temperature. The derivation of a silica geothermometer needs an extra assumption about the total amount of water in the system. The solubility data for the two extreme cases, i.e. when the vessel (bomb) is completely filled with water and when there is just enough water to make the total specific volume equal to the critical volume of water at room temperature (25°C), are considered here. These lie on the two respective straight lines of log (SiO 2 (ppm)) against temperature (K). The equations for the two straight lines are log (SiO 2 (ppm)) = 0.0179 T (K)−4.3214 and log SiO 2 (ppm) = 0.0088 T (K)−1.6513, respectively. In the case of the well M-19A at Cerro Prieto, the silica concentration in the reservoir liquid is higher than the experimental solubility, but is lower than the calculated solubility value.

Geochemical exploration of the Chipilapa geothermal field, El Salvador

Abstract Results of the geochemical exploration of the Ahuachapan-Chipilapa area are presented. The procedure for interpreting the chemical composition of very dilute thermal waters is emphasized. Three groups of thermal waters are described, two with a geothermal brine component and one resulting from steam condensation. The model for one of the groups (Type 2) leads to predictions of temperature and chloride concentration that are reasonably close to those of the geothermal liquid feeding Chipilapa well CH-7B. In particular, it was predicted correctly that the salinities in the Chipilapa area would be considerably lower than those in the Ahuachapan field.It is shown that the simultaneous modeling of the carbon dioxide concentration and isotopic composition of fumarole steam allows discrimination between primary and secondary steam. The composition of all fumarole samples is described as steam originating from a single reservoir fluid at 250°C, and composition δ 18 O = −4.1, δ D = −46, CO 2 = 5 × 10 −5 molar fraction.The total discharge composition of CH-7B confirms the trend observed in the Ahuachapan field of decreasing reservoir salinities towards the east. Postulating the existence of a “deep” reservoir brine in the eastern (Chipilapa) section of the system, with lower salinity but otherwise similar temperature and isotopic composition to the Ahuachapan brine, allows for the generation of relatively simple models that explain the formation of the CH-7B brine, and the three groups of thermal waters. Type 1 waters are noteworthy in the sense that they result from a ternary mixture of meteoric water, geothermal brine and high-temperature steam condensate.The possibility that the east-west trend in salinity results from a process of dilution of brine with condensate from steam separated at very high temperatures is discussed. The distribution of the different types of hydrothermal manifestations delineates a lateral discharge system, with the steam upflow zone to the south of the study area, on the northern slope of the volcanic range, and thermal water discharges several kilometers to the north. It is deemed reasonable that exploratory drilling should be directed towards the southern edge of the geothermal system, as far as topography and the indicators of probable secondary permeability permit.

An interlaboratory calibration of silica for geothermal water chemistry

The results of an interlaboratory calibration of silica performed using commercial standards as samples are presented. The analytical values for silica concentration are consistent for lower concentration samples, but there are significant variations among the values for the higher concentration samples. The dilution technique is better than direct injection of high concentration samples to the atomic absorption spectrometer, although at present it is not possible to define the highest permissible silica concentration appropriate for direct measurements. High dilution factors also produce a high uncertainty in the analytical results. Therefore, the need still exists for conducting multi-laboratory calibrations over a wide range of silica concentrations to refine the precision and accuracy of silica analyses at high concentrations.

Origin of rainwater acidity near the Los Azufres geothermal field, Mexico

The chemical and isotopic compositions of rainwater were monitored at Los Azufres geothermal field (88 MWe) and its surroundings during May – September 1995, which is the rainy season. Samples were collected from eight sites: three within the field, three in its surroundings and two sufficiently far from the field such that they have no geothermal input. The concentrations of Cl−, SO42− and NO3− were measured in about 350 samples and found to be generally <5 ppm. Chloride concentrations remained constant with time, but sulfate and nitrate concentrations decreased, which suggests a nearby industrial source for the sulfate and nitrate. A mixing model for Cl−, SO42− and δ34S also suggests an industrial source for the rainwater sulfur. The determination of pH was found to be necessary, but is not sufficient to characterize rainwater acidity. The Gran titration method was used to determine alkalinity with respect to equivalence point of H2CO3∗. Values of alkalinity were found to range from 10−4 to 10−6 eq/L, and were negative only for some samples from Vivero and Guadalajara. Thus, SO42− and NO3− are in general not in acidic form (i.e. balanced by Na+, Ca2+, etc. rather than H+). Sulfate δ34S values were about −1.5‰ in Los Azufres and its surroundings, and in Morelia, but differed from the value of −0.2‰ for Guadalajara. The δ34S values for H2S from the Los Azufres geothermal wells are in the range −3.4 to 0.0‰. The δ34S ranges for the natural and anthropogenic sources for environmental sulfur overlap, making it difficult to differentiate between the contribution of different sources. However, a similarity of values of δ34S at Los Azufres and Morelia (85 km distant) suggest a regional source of sulfate that is not associated with geothermal emissions from Los Azufres.

Isotopic zoning and origin of the aquifers in the discharge area of the geothermal fields of ahuachapán and Chipilapa, El Salvador

Abstract The northern discharge areas of the Ahuachapan, and Chipilapa geothermal fields can be subdivided into four different zones based on their structural position, and the isotopic and chemical composition of their waters. In general, the contribution of geothermal waters from these two fields was estimated to be less than 10%. Elevation effects are of little importance, whereas a slight trend towards higher isotopic values with increasing water temperatures may exist. The NNW-SSE-trending Escalante and Agua Caliente faults represent lateral groundwater barriers, and provide vertical conduits for the ascending geothermal waters. The western discharge areas seem to be more influenced by the Ahuachapan, geothermal field, whereas those to the east are more influenced by the Chipilapa field. Groundwaters in the Northern Plain are mainly from shallow northward-flowing aquifers. These waters show temperature effects, mixing with geothermal waters and are affected by the geology of the area. However, none of these factors alone can explain the isotopic variations observed in the waters of the northern discharge areas.

Inter-laboratory test for oxygen and hydrogen stable isotope analyses of geothermal fluids: Assessment of reservoir fluid compositions

RATIONALE Knowledge of the accuracy and precision for oxygen (δ18 O values) and hydrogen (δ2 H values) stable isotope analyses of geothermal fluid samples is important to understand geothermal reservoir processes, such as partial boiling-condensation and encroachment of cold and reinjected waters. The challenging aspects of the analytical techniques for this specific matrix include memory effects and higher scatter of delta values with increasing total dissolved solids (TDS) concentrations, deterioration of Pt-catalysts by dissolved/gaseous H2 S for hydrogen isotope equilibration measurements and isotope salt effects that offset isotope ratios determined by gas equilibration techniques. METHODS An inter-laboratory comparison exercise for the determination of the δ18 O and δ2 H values of nine geothermal fluid samples was conducted among eleven laboratories from eight countries (CeMIEGeo2017). The delta values were measured by dual inlet isotope ratio mass spectrometry (DI-IRMS), continuous flow IRMS (CF-IRMS) and/or laser absorption spectroscopy (LAS). Moreover, five of these laboratories analyzed an additional sample set at least one month after the analysis period of the first set. Statistical evaluation of all the results was performed to obtain the expected isotope ratios of each sample, which were then subsequently used in deep reservoir fluid composition calculations. RESULTS The overall analytical precisions of the measurements were ± 0.2‰ for δ18 O values and ± 2.0‰ for δ2 H values within the 95% confidence interval. CONCLUSIONS The measured and calculated δ18 O and δ2 H values of water sampled at the weir box, separator and wellhead of geothermal wells suggest the existence of hydrogen and oxygen isotope-exchange equilibrium between the liquid and vapor phases at all sampling points in the well. Thus, both procedures for calculating the isotopic compositions of the deep geothermal reservoir fluid - using either the analytical data of the liquid phase at the weir box together with those of vapor at the separator or the analytical data of liquid and vapor phases at the separator -are equally valid.