Vol. 48, 01, 05ok.qxd 145 ANNALS OF GEOPHYSICS, VOL. 48, N. 1, February 2005 Key words Los Azufres geothermal field – gas geo- chemistry – geothermal gas equilibria – reservoir ex- ploitation 1. Introduction Gas geochemistry is useful in geothermal ex- ploration and exploitation, especially in vapor- dominated fields where very little is known about the deep reservoir liquid. Several gas geother- mometers have been proposed (D’Amore and Panichi, 1980; Arnórsson and Gunnlaugsson, 1985), based on the variation of gas ratios with temperature in producing aquifers. Also methods to estimate the reservoir excess steam have been proposed (Giggenbach, 1980; D’Amore and Celati, 1983), which are useful when the reser- voir temperature is available. The reservoir ex- cess steam is very important in the assessment of vapor-dominated geothermal fields to calculate the in situ liquid saturation. Methods to estimate both the reservoir temperature and the reservoir excess steam were developed by considering gas equilibria and mineral buffers in the reservoir (D’Amore and Truesdell, 1985, 1995; D’Amore, 1998). D’Amore (1998) developed a method based on equilibria for the Fischer-Tropsch reac- Mailing address: Dr. Rosa María Barragán Reyes, In- stituto de Investigaciones Eléctricas, Gerencia de Geoter- mia, Reforma St. 113, Col. Palmira, 62490 Cuernavaca, Morelos, México; e-mail: rmb@iie.org.mx Gas geochemistry for the Los Azufres (Michoacán) geothermal reservoir, México Rosa María Barragán (1), Victor M. Arellano Gómez (1), Enrique Portugal (1), Fernando Sandoval (2) and Nuria Segovia (3) (1) Instituto de Investigaciones Eléctricas, Gerencia de Geotermia, Cuernavaca, Morelos., México (2) Comisión Federal de Electricidad, Residencia Los Azufres, Campamento Aguafría, Los Azufres, Michoacán, México (3) Instituto de Geofisica, Universidad Nacional Autónoma de Mexico (UNAM), México D.F., México Abstract Gas data of the Los Azufres geothermal field were analyzed using a method based on equilibrium of the Fisch- er-Tropsch (FT) reaction: CH4 + 2H2O = 4H2 + CO2 and on the combined pyrite-hematite-magnetite (HSH2) re- actions: 5/4 H2 + 3/2 FeS2 + 3/4 Fe2O3 + 7/4 H2O = 3 H2S + Fe3O4 in order to estimate reservoir temperature and excess steam. The solution of equilibrium equations produces a grid (FT-HSH2). This method is suitable for reservoirs with relatively high H2S but low H2 and NH3 concentrations in the fluid as is the case of the Los Azufres well discharges. Reservoir temperature and reservoir excess steam values were estimated for initial and present conditions in representative wells of the field to study the evolution of fluids, because of exploitation and waste fluids reinjection. This method was very useful in estimating reservoir temperatures in vapor wells, while in two-phase wells it was found that as the well produces a smaller fraction of water, the reservoir temperature estimation agrees qualitatively with results from cationic or silica geothermometers. For liquid-dominated wells the reservoir temperature estimations agree with temperatures obtained from the well simulator WELFLO. This in- dicates that FT-HSH2 results provide the temperature of the fluid entering the well where the last equilibrium occurs. Results show a decrease in reservoir temperatures in the southern zone of the field where intensive rein- jection takes place. With exploitation, it was also noted that the deep liquid phase in the reservoir is changing to two-phase increasing the reservoir steam fraction and the non-condensable gases in well discharges. 146 Rosa María Barragán, Victor M. Arellano Gómez, Enrique Portugal, Fernando Sandoval and Nuria Segovia tion and the combined expressions for pyrite- magnetite and pyrite-hematite mineral buffers. In this method more local oxidant conditions are as- sumed, implying the occurrence of high concen- trations of H2S and relatively low concentrations of both H2 and NH3 in the fluid. The method was suitable for gas data from the Los Humeros geot- hermal system in order to understand the dynam- ics of the reservoir at initial and present condi- tions (Arellano et al., 1998; Barragán et al., 1999, 2000a,b). Siega et al. (1999) described this method and an additional method based on the pyrite-pyrrhotite reaction. They concluded that the method based on FT-HSH2 equilibria best fits mature and magmatic systems data. The ob- jective of this work is to investigate the effects of exploitation in Los Azufres reservoir with time through the changes in both the reservoir tem- perature and the excess reservoir steam of wells. 2. The Los Azufres geothermal field The Los Azufres geothermal field is an in- tensely fractured, two-phase, volcanic hydrother- mal system located in the northern part of the Mexican Volcanic Axis, in the state of Michoacán at an average elevation of 2800 m a.s.l. (fig. 1). At present it is the second in the country generating 188 MWe (Torres, 2003, pers. comm.). The field Fig. 1. Location of the Los Azufres geothermal field and wells. AZ-was removed of the name of wells. 147 Gas geochemistry for the Los Azufres (Michoacán) geothermal reservoir, México Table I. Average chemical composition of vapor phases of the Los Azufres geothermal wells. TS – separation temperature (°C); ySEP – steam fraction at separation conditions (dimensionless); Xg – ratio of millimoles of non condensable gas per mol of water vapor and concentration of gas species in ‰ molar (dry basis). Average val- ues of FT and HSH2 are also given. Well Year TS ySEP Xg CO2 H2S H2 CH4 N2 NH3 FT HSH2 AZ-5 1985 180.7 0.709 13.020 978.200 14.600 3.140 0.724 2.684 0.642 –15.02 –5.94 AZ-5 1986 174.3 0.720 8.583 976.267 15.457 3.007 0.662 2.953 1.664 –15.75 –6.15 AZ-5 1987 179.7 0.706 11.007 976.833 14.557 3.191 0.965 3.062 1.428 –15.42 –6.08 AZ-5 1998 176.1 0.850 9.898 972.915 17.625 1.560 0.542 6.662 0.700 –16.27 –5.38 AZ-5 1999 178.9 0.860 15.360 972.023 16.000 2.680 0.738 7.661 0.880 –14.68 –5.46 AZ-5 2000 174.3 0.871 10.183 965.901 18.836 2.382 2.086 7.885 2.887 –16.04 –5.49 AZ-5 2001 175.0 0.892 13.916 902.032 12.813 11.298 1.246 66.408 5.987 –12.55 –6.58 AZ-6 1983 192.0 1 31.700 989.000 5.900 0.900 0.490 3.700 0.400 –14.87 –5.50 AZ-6 1985 182.3 1 35.333 982.000 6.567 1.100 0.807 6.100 3.200 –14.56 –5.39 AZ-6 1986 179.7 1 34.367 979.333 6.033 1.100 0.860 8.533 3.833 –14.63 –5.52 AZ-6 1987 169.3 1 31.525 979.750 6.875 1.450 0.943 9.550 1.200 –14.34 –5.57 AZ-6 1988 170.7 0.98 42.193 978.650 5.000 1.700 0.617 13.254 0.639 –13.41 –5.86 AZ-6 1998 178.1 0.940 43.280 983.950 3.200 1.200 0.221 10.763 0.617 –13.60 –6.27 AZ-9 1984 148.0 0.456 2.100 920.100 62.300 7.412 0.385 1.083 8.685 –17.22 –6.24 AZ-9 1987 182.5 0.407 2.335 944.250 42.970 3.673 0.494 6.232 2.383 –18.55 –6.35 AZ-9 1999 175.9 0.410 1.640 913.340 64.900 5.810 1.612 14.279 0.025 –18.88 –6.32 AZ-9 2000 175.3 0.424 1.350 859.076 62.455 10.600 1.340 59.896 5.714 –18.06 –6.82 AZ-9 2001 173.5 0.467 1.565 774.574 57.755 25.747 0.536 99.396 41.744 –15.74 –7.22 AZ-13 1985 172.5 0.5785 7.630 971.600 21.000 3.748 0.560 1.781 1.311 –15.89 –6.12 AZ-13 1986 172.3 0.584 5.733 961.933 27.303 5.854 0.730 2.044 2.150 –15.71 –6.23 AZ-13 1987 171.0 0.586 7.270 967.800 20.125 6.691 0.897 2.062 2.454 –15.15 –6.52 AZ-13 1998 182.3 0.940 6.268 953.860 31.375 2.640 1.457 9.167 1.488 –16.42 –5.19 AZ-13 1999 175.8 0.945 9.080 959.440 28.500 2.470 1.289 5.805 2.490 –15.82 –4.99 AZ-13 2000 184.0 0.999 9.164 950.878 29.613 2.960 1.566 7.053 7.855 –15.49 –4.99 AZ-13 2001 188.5 0.992 9.969 894.289 29.350 9.487 1.546 58.146 6.661 –13.35 –5.58 AZ-17 1984 174.0 1 17.850 987.500 6.450 2.450 0.195 1.400 1.600 –13.73 –6.37 AZ-17 1985 159.0 1 33.675 980.000 9.175 2.275 0.363 5.625 2.200 –13.03 –5.39 AZ-17 1986 200.0 1 15.967 975.667 11.300 1.967 0.303 5.500 5.200 –14.50 –5.60 AZ-17 1987 185.8 1 15.900 973.500 12.875 3.400 0.290 7.625 1.725 –13.54 –5.73 AZ-17 1988 169.0 1 12.367 968.333 13.500 4.033 0.473 12.167 1.433 –13.90 –5.95 AZ-17 1998 173.5 1 9.700 883.860 12.500 7.030 0.197 96.323 0.124 –13.01 –6.54 AZ-17 2000 173.5 1 12.550 875.190 12.800 7.130 0.309 104.177 0.444 –12.74 –6.32 AZ-17 2001 179.0 0.999 13.552 859.988 12.440 10.515 0.540 109.734 6.283 –12.18 –6.51 AZ-33 1984 206.0 0.842 27.570 986.600 7.640 1.956 0.610 2.424 0.815 –14.16 –5.82 AZ-33 1998 180.3 0.560 13.560 961.030 8.900 2.080 0.629 25.384 1.989 –16.02 –6.51 AZ-33 1999 177.6 1 9.010 942.830 19.500 6.820 0.062 30.193 0.633 –12.66 –6.00 AZ-33 2000 181.7 0.530 16.430 934.820 9.000 3.540 0.435 52.008 0.227 –14.71 –6.68 AZ-33 2001 178.4 0.608 16.519 937.275 10.240 6.210 0.964 39.919 5.290 –13.83 –6.71 148 Rosa María Barragán, Victor M. Arellano Gómez, Enrique Portugal, Fernando Sandoval and Nuria Segovia is divided into two zones: Maritaro a liquid-dom- inated zone in the north and Tejamaniles a steam- dominated zone in the south. Iglesias et al. (1985) found that in its natu- ral state the Los Azufres geothermal field con- sists of a deep aquifer where the ascending flu- id starts boiling at about 1200 m a.s.l. The two- phase liquid dominant region extends upwards from 1200 m a.s.l. to about 1700 m a.s.l. where steam becomes the dominant phase. The two- phase steam dominated region extends up to about 2400 m a.s.l. where a region of dry or su- perheated steam is located. According to Cathelineau et al. (1985), the most important hydrothermal minerals occur- ring at the Los Azufres geothermal system are: chlorite, pyrite, hematite, epidote, calcite, al- bite, adularia, zeolite and quartz which were formed by alteration of primary minerals: olivine, pyroxene/ amphiboles, biotite, feldspar and rock-matrix. The Los Azufres reservoir geochemical mod- el was proposed by Nieva et al. (1987). This mod- el was based on the spatial distribution of chemi- cal and isotopic species at reservoir conditions. The reservoir excess steam y was estimated for the wells by a method based on equilibrium of the Fischer-Tropsch reaction (Giggenbach, 1980; Nieva et al., 1987). The y values were obtained at the reservoir temperature estimated by the cation- ic (CCG; Nieva and Nieva, 1987) and silica (Fournier and Potter II, 1982) geothermometers in two-phase wells, whereas measured temperatures were taken in vapor wells. The y values were used to correct the chemical and isotopic composition of the total discharge to obtain the reservoir «ref- erence» values. As the concentration of volatile species (CO2) was larger in shallower strata and concentrations of non-volatile species (chlorides and oxygen-18 in the fluid) increased with depth, the model establishes the occurrence of a reser- voir steam up-flow with partial condensation process, to explain the distributions of species as observed. According to the geochemical model, this behavior seems to be dominant in the south- ern zone while for the north, the presence of two different liquid phases (with slightly different iso- topic composition) was proposed. The separated water in the Los Azufres two- phase wells is of sodium-chloride type with neutral pH at separating conditions. The deep reservoir fluid contains up to 1600 ppm of chlo- ride and the molar fraction of CO2 has been cal- culated between 0.3 to 8.3 (Nieva et al., 1987). The pH of fluids at the reservoir is neutral with values between 5.5 and 7.4 (Barragán et al., 1988). The CCG geothermometer (Nieva and Nieva, 1987) provided reservoir temperatures of more than 300°C for the north zone, where- as slightly lower values between 270 and 290°C were estimated for the south. The well discharges in the Los Azufres geot- hermal field contain a relatively high amount of non-condensable gases compared to other fields (table I), i.e. the vapor well AZ-6 contains more than 40‰ molar of non condensable gas in steam, though liquid dominated wells such as AZ-4 con- tain only about 2‰. Typically, the main con- stituent in dry gas is CO2 (average 94 vol%), then H2S (average 2.5 vol%) while H2, CH4, N2 and NH3 concentrations are minor (average 3.5 vol% all of them). In wells affected by reinjection, N2 concentration has increased since a mixture of water/air is injected into the reservoir. 3. Gas equilibria The chemical composition of geothermal gases depends mainly on temperature, pressure, vapor/liquid gas distribution processes and those secondary processes caused by different kinetic responses to changes in temperature and redox potential during the rise of the fluid to the sur- face (Giggenbach, 1980). Thus, large variations in gas compositions occur in different geother- mal fields, but also in the same field among dif- ferent wells. Giggenbach (1980); D’Amore and Truesdell (1985, 1995) and D’Amore (1998) de- veloped methods based on gas equilibria con- sidering mineral buffers in order to estimate physical characteristics of reservoirs. The min- eral buffers controlling the hydrogen-hydrogen sulfide geothermal reaction at temperatures above 300°C were pyrite, pyrrhotite and mag- netite while for temperatures around 240°C iron-aluminium-silicate minerals replace mag- netite. Calcite and anhydrite can also be used as buffers for geothermal gases since they are com- monly found in geothermal systems, but anhy- Fig. 2. RH values for total discharge fluids of Los Azufres wells. 149 Gas geochemistry for the Los Azufres (Michoacán) geothermal reservoir, México drite is likely to reveal more oxidizing condi- tions occurring at the periphery of geothermal systems (Giggenbach, 1980). However the flu- id-mineral reactions may be different for differ- ent geothermal reservoirs depending not only on temperature but also on the rock type (D’Amore and Truesdell, 1985). Giggenbach (1987) and Taran et al. (2002) suggest evaluating the ratio of fugacities RH = = log ( f H2 /f H2O), of H2 and H2O in order to in- vestigate predominating redox conditions within the reservoir. For most hydrothermal fluids the redox state is controlled by FeO- FeO1.5 and hematite-fayalite-quartz assem- blages, RH values are temperature independent at –2.8. For vapor dominated geothermal sys- tems values of RH close to –2.8 are found. Val- ues above –2.8 are considered to be typical of reducing conditions and are found in high-tem- perature (about 800°C) volcanic gases (Taran et al., 2002) whereas values below –2.8 are considered oxidizing conditions and are char- acteristic of geothermal systems. Figure 2 (modified from Giggenbach, 1987 and Taran et al., 2002) shows total discharge RH values for some Los Azufres wells versus the reser- voir temperature. As expected, the RH values indicate oxidizing conditions at reservoir (RH values less than –2.8) for all the wells. D’Amore (1998) developed a method us- ing the Fischer-Tropsch and another reaction for the H2S-H2 couple which was obtained from the combined pyrite-hematite, pyrite- magnetite equilibrium named HSH2. This method was fully described by D’Amore (1998), Siega et al. (1999), Barragán et al. (1999, 2000a, 2001). It is based on reactions (3.1) and (3.2) ( )FT CH H O H CO2 44 2 2 2+ = + (3.1) ( ) / / / / . HSH H FeS Fe O H O H S Fe O 5 4 3 2 3 4 7 4 2 3 2 2 2 3 2 2 3 4 + + + + = + (3.2) The thermodynamic equilibrium constant K for each reaction is given in eqs. (3.3) and (3.4), in terms of the partial pressures log log log log log P K P P P 2 4FT H CO CH H O 2 2 4 2 + + - - = (3.3) / / log log log log P P P K 3 5 4 7 4 HSH H S H H O 2 2 2 2 = + - - (3.4) and writing the constants in terms of the water partial pressure, according to (D’Amore, 1992) /log log log logP n n A Pi i iH O H O2 2= - +^ h (3.5) where (ni/nH2O) is the molar ratio of i compo- nent regarding the total water. The coefficient A for every species i is defined as a function of temperature and the steam fraction y ( ) ( ) < A y y B y A B y yB y if if 1 0 1 1 0 i i i i i $= + - = + -^ h Bi is the distribution coefficient for every gas and it is a function of temperature (Giggenbach, 1980; D’Amore, 1992). For temperatures be- tween 100 and 340°C (t in °C) . . . . . . log log log B t B t B t 4 7593 0 01092 6 0783 0 01383 4 0547 0 00981 CO CH H S 2 4 2 = - = - = - 150 Rosa María Barragán, Victor M. Arellano Gómez, Enrique Portugal, Fernando Sandoval and Nuria Segovia . . .log B t6 2283 0 01403H2 = - By substituting in eqs. (3.3) and (3.4) each P expression, as given by eq. (3.5) log log log log log log log log K A A A P n n n n n n 4 2 4 FT H CO CH H O H H O CO H O CH H O 2 2 4 2 2 2 2 2 4 2 + + - + - = + + - ` ` ` j j j (3.6) / / . log log log log log K A A n n n n 3 5 4 3 5 4 HSH H S H H S H O H H O 2 2 2 2 2 2 2 + - = -` `j j (3.7) The left side of eqs. (3.6) and (3.7) are defined as the FT and HSH2 parameters log log log log log A A A P FT K 4 2 FT H CO CH H O 2 2 4 2 + + + - - = (3.8) / .log logK A AHSH 3 5 42 HSH H S H2 2 2= + - (3.9) According to D’Amore (1992), the expressions for the equilibrium constants (logKFT and logKHSH2) are given by . ( / ) . ( )log logK T T4 33 8048 4 635FT = - - + (3.10) . ( / ) . ( )log logK T T7 609 6087 0 412HSH2 = - - (3.11) . ( / )log P T5 51 2048H O2 = - (3.12) where T is given in K. The graphic solution of eqs. (3.8) and (3.9) pro- vides a grid in the coordinates (HSH2, FT). The parameters FT and HSH2 are obtained from the gas composition according to eqs. (3.13) and (3.14) log log logFT H H O CO H O CO H O 4 2 2 2 2 4 2 + + - = _ _ _ i i i (3.13) / log5 4-logHSH H S H O H H O32 2 2 2 2= _ _i i (3.14) where concentrations of gas species are taken in the total fluid. The following trends and interpretations were given by D’Amore and Truesdell (1995): – Increase T, decrease y: contribution of flu- id from a hotter and deeper source with high liquid saturation. – Increase T, increase y: apparent increase in T and y due to lateral source of steam, with practically zero liquid saturation and with a strong local accumulation of gas. – Decrease T, decrease y: local source of pure and low temperature water with no gas content as in the case of reinjection fluids or fast meteoric water. – Decrease T, increase y: caused by either recharge from peripheral fluids rich in gas; or sulfides precipitation caused by local over-pro- duction with blockage of main fractures. 4. Results 4.1. Tejamaniles steam-dominated south zone The southern zone has been intensively ex- ploited since 1984. The wells AZ-6, AZ-17 and AZ-33 were selected as representative. Table II gives the height (m a.s.l.) of producing zones of the wells. In fig. 1 the location of wells in the field is given. Figure 3 shows the FT-HSH2 grid diagram for the steam well AZ-6. In the diagram, the dark mark corresponds to the reference point found by Nieva et al. (1987), calculated through equilibri- um of the FT reaction for the measured reservoir temperature. The points represent average values for the indicated years, all data available were in- cluded. The figure shows that the grid tempera- ture of about 290°C obtained for 1983 probably indicates that of the source of steam, since the reference temperature based on measured reser- Table II. Height of producing zones of studied wells. Well Elevation (m a.s.l.) AZ-5 1740 ± 330 AZ-6 2015 ± 100 AZ-9 945 ± 350 AZ-13 1620 ± 100 AZ-17 2200 ± 30 AZ-33 2190 ± 40 151 Gas geochemistry for the Los Azufres (Michoacán) geothermal reservoir, México voir temperatures is only 260°C. For the 1983 point, the vapor fraction (y) was very small, about 3% compared to that obtained for the ref- erence 25%. An overall tendency with 1983- 1998 data shows an important decrease in tem- perature, of about 30°C and an increase in y from 3 to 12%. According to D’Amore and Truesdell (1995), this trend is related to a decrease in the ratio H2S/H2O which could be caused either by recharge from peripheral fluids rich in gas or by sulfides precipitation. In well AZ-6 there is evi- dence of interference of reinjection fluids which are rich in gas. However the increase in y oc- curred only for the period 1983-1988, since points for 1988-1998 show a constant value of 13%. Thus, for the period of 1988-1998 the drop in temperature was due to exploitation that low- ered the reservoir pressure. Figure 4 shows the FT-HSH2 grid for well AZ-17. The figure shows that the grid tempera- ture of starting conditions was about 290°C while the reservoir steam y was about 10%. As in well AZ-6, the reference point indicates a lower temperature obtained from temperature logs. The overall tendency of the points in the grid shows a drop in the temperature but an in- crease in reservoir steam, which is the same trend as observed in well AZ-6. These results are a consequence of exploitation that causes pressure drop and boiling. Deep liquid boiling produces two-phase fluids rich in non condens- able gases that increases y values at reservoir. Data for 2001 in the grid, where y is about 50%, could reflect drilling operations in the field that caused anomalous results regarding gas equilib- rium. The anomalous point with a high y value (about 40%) in fig. 4 corresponds to the sample taken one week before the September 1985 earthquake (MS = 8.1). According to the grid, the reservoir steam/liquid ratio temporarily changed as a result of the tectonic event (Bar- ragán et al., 2001). Well AZ-33 produces two phase fluids with an average steam fraction of 0.71 at wellhead Fig. 3. FT-HSH2 grid diagram for well AZ-6. The dark dot shows the conditions taken as reference before ex- ploitation (Nieva et al., 1987). Circles show the average values for the indicated years. See text for discussion. 152 Rosa María Barragán, Victor M. Arellano Gómez, Enrique Portugal, Fernando Sandoval and Nuria Segovia Fig. 4. FT-HSH2 grid diagram for well AZ-17. The dark dot shows the conditions taken as reference before ex- ploitation (Nieva et al., 1987). Circles show the average values for the indicated years. See text for discussion. Fig. 5. FT-HSH2 grid diagram for well AZ-33. The dark dot shows the conditions taken as reference before ex- ploitation (Nieva et al., 1987). Circles show the average values for the indicated years. See text for discussion. Fig. 6. FT-HSH2 grid diagram for well AZ-5. The dark dot shows the conditions taken as reference before ex- ploitation (Nieva et al., 1987). Circles show the average values for the indicated years. See text for discussion. 153 Gas geochemistry for the Los Azufres (Michoacán) geothermal reservoir, México and it is affected by reinjection. The grid for well AZ-33 is given in fig. 5. The reference point taken in 1987 indicated an initial reservoir temperature of 270°C and an excess steam y of 20% while results in the grid for the year 1984 show a temperature of 283°C and a y of 6.5%. The scattering shown by data in the grid is re- lated to the influx of cooler fluids in an inter- mittent way, depending on reinjection. The da- ta for 1984-1988 show a decrease in both the temperature and y. According to D’Amore and Truesdell (1995), this pattern indicates recharge of pure and low temperature water with no gas content as is the case with reinjection. For 1997-1998 data the CCG reservoir temperature was 256°C while the grid provided 265°C. The relatively low value obtained by CCG is due to the admixture of lower temperature waters to the reservoir from reinjection. CCG provides the temperature «far» from the well, although the reinjected fluid has been heated during flow to the reservoir. The trend of variation found for 1998-2001 data shows the same pattern ob- served at wells AZ-6 and AZ-17, a decrease in temperature and an increase in y. The decrease in temperature is caused by the mixing of reser- voir-cooler reinjection fluids whereas the y in- crease is due to the deep boiling process in- duced by the drop in pressure that also releases non condensable gases. The same pattern ob- served for this well is shown at well AZ-46 which is also affected by reinjection. 4.2. Maritaro liquid-dominated northern zone Wells AZ-5, AZ-13 and AZ-9 have been se- lected as representative of the north zone. Figure 6 shows the FT-HSH2 grid of well AZ-5. This well produced two-phase fluids in the past with an average steam fraction of 0.73 at separating con- ditions but at the present time the steam fraction has increased towards 0.89 in average. In fig. 6, the points corresponding to 1985-1987 indicate reservoir temperatures of 275-280°C while the reference temperature (obtained by CCG) was 300°C. Overall tendency for the points indicates an increase in temperature and a decrease in y. 154 Rosa María Barragán, Victor M. Arellano Gómez, Enrique Portugal, Fernando Sandoval and Nuria Segovia This pattern is interpreted as a result of the con- tribution of a hotter fluid with high liquid satura- tion from a deep source. However, production da- ta indicate that an important boiling process takes place in this zone of the reservoir. The estimations of reservoir temperature obtained by silica and CCG geothermometers and the temperature ob- tained by the well simulation using the simulator WELFLO (Goyal et al., 1982) with time were com- pared according to Truesdell et al. (1995), (fig. 7). The silica geothermometer provides the tempera- ture «close» to the well while the simulator pro- vides the temperature of the fluid entering the well, thus both temperatures agree in most cases. In contrast, CCG provides the temperature «far» from the well because of its relatively slow re- equilibration rate. As is seen, for 1985-1987 the grid temperatures are close to the silica estima- tions, indicating that temperature at which the last gas equilibria occur. The comparison of the grid- CCG temperatures for Los Azufres wells suggest that as the wells produce higher steam fractions both temperatures approach each other (Barragán et al., 2002) as in the case of the 1998-2000 data. As noticed, a very low grid temperature for the year 2001 (230°C) was found. The average values of CO2 at 2000 and 2001 changed from 966 to 902‰ molar (dry basis) whereas the N2 changed from 7.9 to 66.4‰ molar (dry basis). This change is related to drilling operations in this zone during 2001, that allowed the influx of air to the reservoir causing gas deviations regarding equilibrium. Reservoir steam values y range between 0.5 and 5%, indicating a high liquid saturation in the reservoir fluid. The FT-HSH2 grid for well AZ-13 is given in fig. 8. In the past this well produced two-phase fluids but at present it emanates only steam. The figure shows that the grid reservoir temperature for initial conditions is 270°C which is rather low compared to the reference temperature of 300°C, but, well-bottom temperature obtained by the Fig. 7. Estimation of reservoir temperatures in well AZ-5 with time. TCCG: temperature estimated by CCG ge- othermometer; TSiO2: temperature estimated by silica geothermometer and TSIM: temperature estimated by the well simulator WELFLO. 155 Gas geochemistry for the Los Azufres (Michoacán) geothermal reservoir, México Fig. 8. FT-HSH2 grid diagram for well AZ-13. The dark dot shows the conditions taken as reference before ex- ploitation (Nieva et al., 1987). Circles show the average values for the indicated years. See text for discussion. Fig. 9. FT-HSH2 grid diagram for well AZ-9. The dark dot shows the conditions taken as reference before ex- ploitation (Nieva et al., 1987). Circles show the average values for the indicated years. See text for discussion. 156 Rosa María Barragán, Victor M. Arellano Gómez, Enrique Portugal, Fernando Sandoval and Nuria Segovia well simulator yields 275°C. As the well changed towards vapor phase the grid method indicates reservoir temperatures close to (in 1998) or high- er than (in 1999, 2000) CCG estimations given in the past. The temperatures higher than CCG esti- mations found for 1999 and 2000 (grid tempera- ture of 310°C) could be due to near-well boiling processes which increase the gas content in the fluid. The comparison of temperature estima- tions according to the method proposed by Truesdell et al. (1995) confirmed that near-well boiling occurred within the well until 1997 (Bar- ragán et al., 2002). Data of 2001 were affected by drilling thus show a decrease in temperature (295°C) and a higher value (6%) of y. Figure 9 shows the FT-HSH2 grid for well AZ-9. This well produces two phase fluids from the deep compressed liquid zone of the reser- voir. The steam fraction at separating conditions was 0.32 in 1998. The grid temperature found for initial conditions was 270°C while the refer- ence value based on CCG was 320°C. The grid temperature in this well compares well with the temperature obtained by the well simulator (280°C) providing the temperature of the fluid entering the well. The detailed analysis of chemical and isotopic data (Arellano et al., 2003) suggested that in spite of exploitation, this well shows stable conditions with time. This is also seen in fig. 9 since the points for 1987 and 1999 are located very close to each other in the grid. The point for the year 2000 in- dicates a reservoir temperature of 260°C which agrees with the well simulator temperature at present time. The year 2001 sample has been al- tered due to drilling in the field as in well AZ-5. 5. Conclusions Gas equilibria prove to be suitable in Los Azufres geothermal field in defining a concep- tual model for the natural state conditions at reservoir. Changes due to exploitation were ob- served in both zones of the field in the gas equi- libria by means of FT-HSH2 grid. Representative wells were studied using the FT-HSH2 method. For the south zone where reinjection is important, the FT-HSH2 grids for wells AZ-6, AZ-17 and AZ-33 show a decrease in temperature and an increase in y. This is due to the influx of fluids rich in gas as in the case of reinjecting fluids being water/air mixtures. The concentration of non condensa- ble gases has increased in the southern zone of the field because of both processes: a deep boiling producing two-phase fluids rich in gas and water/air reinjection. In the northern zone the wells AZ-5, AZ-13 and AZ-9 were studied. In wells AZ-5 and AZ- 13 an increase in temperature and a decrease in y values are seen. This result is due to the higher vapor fraction produced by both wells at present time compared to lower values produced before. In the past, the wells were liquid-dominated and CCG and silica geothermometers indicated high- er reservoir temperatures than those indicated by FT-HSH2 grid. As the grid provides the temper- ature for the last equilibrium, which occurs when the fluid enters the well, grid temperatures may not correspond to the fluid temperature in the «undisturbed» reservoir. Thus, the grid underes- timates temperatures of both wells at initial con- ditions. This fact is clearly seen in well AZ-9 where, in spite of exploitation, stable conditions have been inferred. As this well is liquid-domi- nated, the grid temperatures approach those ob- tained by the WELFLO simulator. These are lower than those estimated by liquid phase geother- mometers (CCG or silica), providing the temper- ature of the fluid entering the well. When the vapor fraction is higher, the FT- HSH2 grid provides better estimations of reser- voir temperature at Los Azufres wells, compa- rable to CCG results. Although FT-HSH2 method is very useful in studying data from dry steam wells where no liquid phase is available, additional evidence based on production data should be investigated in order to better under- stand the trends in the FT-HSH2 grid. Acknowledgements The authors thank the Comisión Federal de Electricidad, Residencia Los Azufres, Eng. M. A. Torres R. for providing data and allowing publication of this work. 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