REVISED QUARTZ SOLUBILITY TEMPERATURE …

1 REVISED QUARTZ SOLUBILITY TEMPERATURE dependence EQUATIONALONG THE WATER-VAPOR SATURATION CURVEM ahendra P. VermaGeotermia, Instituto de Investigaciones Electricas, Apdo. 1-475, Cuernavaca 62001, MexicoKey words: chemical thermodynamics, QUARTZ SOLUBILITY , PVTcharacteristics of water, regression equation, quartzgeothermometer, the application of chemical thermodynamics a quartzsolubility regression expression along the water-vaporsaturation curve for whole range of TEMPERATURE from 0 to374 C was developed. The equation is the following()() )( )(log2 + =KTppmSiO The correction of total discharge silica composition from awell for the vapor fraction in the geothermal reservoir is afundamental limitation to use this equation as ageothermometer.

2 For knowing the vapor fraction there isindirectly need of knowing the reservoir TEMPERATURE . It ispossible to calculate the deep reservoir TEMPERATURE withiteration process, but a basic assumption is that the reservoirfluid is in equilibrium with QUARTZ . The concept is elucidated incase of the Cerro Prieto geothermal INTRODUCTIONThe use of silica content as a geothermometer is presently anintegral part of almost all the geochemical investigations ofgeothermal systems around the world. The derivation of suchgeothermometers is basically based on the regression ofexperimental SOLUBILITY data. White et al. (1956) found firstthat the silica content could be used as a geochemical indicatorof geothermal reservoir TEMPERATURE , as the silica concentrationin hot springs at Steamboat, Nevada was very close to theexperimental SOLUBILITY of amorphous silica.

3 Since thenenormous contributions have been made to gather more fieldevidences and to create a systematic approach to understandthe geothermal reservoir characteristics from the fluidgeochemistry of silica. Mahon (1966) showed that theconcentration of silica in hot water discharged from drillholesat Wairakei in New Zealand is in agreement with SOLUBILITY ofquartz after corrections for adiabatic steam loss. From theexperimental QUARTZ SOLUBILITY data Fournier (1977) presentedthe first geothermometer in equation form. Henley et al. (1984)compiled all the existing silica geothermometers for manysilica phases, including the effects of adiabatic and conductivecooling processes. Fournier and Potter (1982) derived apolynomial equation for the QUARTZ geothermometer using therevised QUARTZ SOLUBILITY data, which is applicable up to 330 and Sontoyo (1997) applied a statistical data treatmentmethod and theory of error propagation in improving this silicageothermometer equation.

4 They had to eliminate the datapoints for TEMPERATURE higher than 300oC as those points wereoutlier according to their statistical , Rimstidt (1997) compiled all the QUARTZ solubilitydata along the water-vapor saturation curve and derived aregression expression that is valid up to 300 C. In manygeothermal fields the reservoir temperatures have beenmeasured above than 300 C. Therefore, it is necessary toknow the QUARTZ SOLUBILITY data at temperatures higher than300 C in order to deal the geochemistry of high temperaturehydrothermal (1999) presented the chemical thermodynamiccalculations for QUARTZ SOLUBILITY for a wide range oftemperature and amount of water in the reaction vessel.

5 It wasobserved a wide difference between calculated andexperimental SOLUBILITY . With a critical evaluation of thisdiscrepancy he concluded a need of creating internal consistentthermodynamic data for aqueous silicic species andreevaluation of QUARTZ SOLUBILITY data at higher temperaturesalong the water-vapor saturation this article the chemical thermodynamic concepts are usedto understand the causes of decrease in the QUARTZ solubilityafter 300 C along the water-vapor saturation curve in theexisting data in literature. Similarly a combined evaluation ofthe QUARTZ SOLUBILITY data along the water-vapor saturationcurve and in the compressed liquid region is presented in orderto derive a QUARTZ SOLUBILITY regression expression along thewater-vapor saturation for whole range of TEMPERATURE (0-374 C).

6 The limitations for using this expression as ageothermometer is discussed considering the well CP-M-19 Aat Cerro Prieto as an EXPERIMENTAL QUARTZ SOLUBILITY DATAIn the SOLUBILITY determination experiments, a certain amountof water and silica is closed in a reaction vessel (or bomb) andthe system is heated to a certain TEMPERATURE . Let us assumethat there is only pure water in the vessel in order tounderstand the effects of heating on the thermodynamic stateof such systems. Figure 1 shows the PT relations for wateralong different isochores. These curves have been constructedwith using the steam tables of Haar et al. (1984). For the caseswhen the total specific volume ( the total volume ofcontainer divided by the total weight (mass) of water andvapor) is greater than the critical specific volume of water( cm3/g), there is only vapor at a certain high temperatureand vice versa (Verma, 1997).

7 On the other hand, if the totalspecific volume of water is just equal to the critical volume ofwater, there will be water and vapor along the water-vaporsaturation curve up to the critical point. After the critical pointthere will not be any distinction between water and vaporalong the V= cm3/g line, but there will be compressedliquid at any point above this line and superheated steam belowit even in the supercritical region. Therefore there are twoextreme cases for conducting SOLUBILITY determination1927 Vermaexperiments when there will always be water in the reactionvessel at high TEMPERATURE : i. the vessel is just filled equal tothe critical volume of water (V= cm3/g) and ii. the vesselis completely filled at the initial room TEMPERATURE (V= ).

8 And there could be any proportion of water and vaporalong the saturation the QUARTZ SOLUBILITY data from literature are divided in twogroups: a) along the water-vapor saturation curve (compileddata from Rimstidt, 1997 and Verma, 1999) and b) in thecompressed liquid region (Compiled data from Verma, 1999).Let us first analyze the QUARTZ SOLUBILITY along the water-vaporsaturation curve. In very early works the QUARTZ SOLUBILITY atroom TEMPERATURE (25 C and 1 bar) is reported as 12 ppm(Brisco et al., 1936-7; van Lier et al., 1960). Fournier andPotter (1982) and Flemingo and Crerar (1982) accepted thevalue 1 ppm. But recently Rimstidt (1997) conducted thesolubility determination experiments for a long period and gotthe value of ppm.

9 To attain equilibrium betweenwater and QUARTZ at room TEMPERATURE requires doingexperiments for geological time period without supersaturatingthe solution at any instant, to prevent equilibrium with othersilica phases. If all the limitations involved in the quartzsolubility determinations are considered, it is justifyconsidering all the above values within analytical errors(Gerardo-Abaya et al., 1997).Figure 2 shows the TEMPERATURE dependence of quartzsolubility along the water-vapor saturation curve. Rimstidt(1997) critically analyzed the SOLUBILITY data and fitted theregression expression up to 300 C. He did not consider thequartz SOLUBILITY data from Fournier and Potter (1982), becausethose data were obtained by regression.

10 He himself measuredquartz SOLUBILITY at four temperatures 21, 50, 74, and 96 C, buthe considered ten values for his QUARTZ SOLUBILITY regression: 2for 21, 2 for 50, 2 for 74 and 4 for 96 C. Similarly he alsotook repeated values of silica SOLUBILITY by other authors fromliterature (Siever, 1962; Crerar and Anderson, 1971).Therefore, the SOLUBILITY data are refitted here removing theduplicated values given by the same author (Figure 3). Therefitted expression is more or less same as proposed byRimstidt; but it is more realistic, because it avoids a biasedstatistical evaluation of the dataset due to repetition of datapoints. Here I have included all the existing values of quartzsolubility at 25 C between to 12 ppm.