Temperature–depth relationships based on log data from the Los Azufres geothermal field, Mexico

Geothermics 30 (2001) 111±132 www.elsevier.com/locate/geothermics

Temperature±depth relationships based on log data from the Los Azufres geothermal ®eld, Mexico Gerardo Garcia-Estrada a,*, Aida Lopez-Hernandez a, Rosa Maria Prol-Ledesma b a

Comision Federal de Electricidad, Alejandro Volta 655, Morelia 58290, Mich., Mexico b Instituto de Geo®sica, Universidad Nacional Autonoma de Mexico, Mexico Received 24 February 1999; accepted 22 June 2000

Abstract Equilibrium temperatures based on log data acquired during drilling stops in the Los Azufres geothermal ®eld were used to study the relationship between temperature, depth and conductive heat ¯ow that di€erentiate production from non-production areas. Temperature and thermal conductivity data from 62 geothermal wells were analyzed, displaying temperature±depth, gradient±depth, and ternary temperature±gradient±depth plots. In the ternary plot, the production wells of Los Azufres are located near the temperature vertex, where normalized temperatures are over 0.50 units, or where the temperature gradient is over 165 C/ km. In addition, the temperature data were used to estimate the depth at which 600 C could be reached (5±9 km) and the regional background conductive heat ¯ow ( 106 mW/m2). Estimates are also given for the conductive heat ¯ow associated with the conductive cooling of an intrusive body ( 295 mW/m2), and the conductive heat ¯ow component in low-permeability blocks inside the reservoir associated with convection in limiting open faults (from 69 to 667 mW/m2). The method applied in this study may be useful to interpret data from new geothermal areas still under exploration by comparing with the results obtained from Los Azufres. # 2001 CNR. Published by Elsevier Science Ltd. All rights reserved. Keywords: Stabilized temperatures; Thermal gradient; Temperature logs; Los Azufres; Mexico

* Corresponding author. Tel.: +52-4-322-7087; fax: +52-4-322-7060. E-mail address: [email protected] (G. Garcia-Estrada). 0375-6505/01/$20.00 # 2001 CNR. Published by Elsevier Science Ltd. All rights reserved. PII: S0375-6505(00)00039-0

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1. Introduction Los Azufres, the second largest geothermal ®eld in Mexico and the ®rst one in fractured media, is located in the central sector of the Trans Mexican Volcanic Belt (TMVB), 180 km west of Mexico City (Fig. 1). It had a maximum installed capacity of 98 MWe in 1995 (Gutierrez, 1997) and has an estimated potential of more than 200 MWe in the northern sector of the ®eld alone (Suarez, 1997). Los Azufres has

Fig. 1. Location and geologic map of Los Azufres geothermal ®eld.

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been used as a testing area for numerous exploratory techniques, because a comprehensive set of data is available to evaluate the results. This study is part of a project to analyse thermal data to estimate the undisturbed initial thermal state of the ®eld. The analysis of the relationship among equilibrium temperature, thermal gradient, well depth, and production zones at Los Azufres is based on the simultaneous graphic display of all available data. The objective is to characterize production areas in Los Azufres geothermal ®eld using these three parameters to interpret thermal data on other geothermal zones under exploration. Numerical modelling using TOUGH2 (Pruess, 1989) has been conducted to test the validity of the results presented here (Garcia, 2000). This model was constrained by the numerical estimates of thermal gradient and heat ¯ow given here for representative sectors of Los Azufres geothermal ®eld. 2. Geology Huitron et al. (1991) and Lopez (1991a,b) have described the geological setting of Los Azufres from a geothermal perspective, and Dobson and Mahood (1985), Pradal and Robin (1985) and Pradal (1990) from a volcanological point of view. The geothermal ®eld is located on a topographic prominence with a mean height of 3000 m above sea level (m a.s.l.). This topographic height is constituted by a fractured andesitic massif of Tertiary age (Fig. 1). The boreholes have a mean depth of 1500 m with minimum and maximum depths of 755 and 3544 m. The reservoir is hosted by andesitic rocks, underlain by sedimentary formations that outcrop outside the ®eld and are not reached by the geothermal wells. Fractured andesites are overlain by rhyodacitic and rhyolitic rocks, scarce pyroclastic deposits, rhyolitic domes, basaltic lava and cinder cones. Andesitic activity was dated to have occurred from 330,000 to 27,000 years before present (K-Ar) (Pradal, 1990), and basaltic rocks have ages from 0.15 Ma to present. Andesites have important textural variations, and secondary permeability changes depend on the distance to the main E±W faults that control the super®cial hydrothermal activity as well as the ¯uid production of geothermal wells at depth (Lopez, 1991a). Fluid production comes from 900 m depth in the northern sector (Maritaro) and 700 m in the south (Tejamaniles). This sector shows higher fracture density in the upper strata, which generates a shallow permeable zone. 3. Geophysics As part of the geothermal exploration program, Garcia (1995) conducted the most recent geophysical interpretation of all available data. Campos and GardunÄo (1995) also performed geophysical studies to characterize the caldera structure proposed by Pradal and Robin (1985). The geothermal ®eld is located near the northern and eastern limits of a gravity high (Fig. 2a) (Garcia, 1995). The lateral limits of these anomalies approximately

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Fig. 2. Contour maps of geophysical parameters measured in Los Azufres geothermal ®eld. Filled circles indicate well locations (see well numbers in Fig. 1): (a) residual of the Bouguer anomaly (contours in mGal, interval = 1 mGal); (b) apparent resistivity obtained with Schlumberger soundings for AB/ 2=2000 m (contours in ohm-m, interval 10 ohm-m). The hatched area shows the location of the known reservoir.

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coincide with the 50 ohm-m contour of apparent resistivity with AB/2=2000 m measured using a Schlumberger array (Fig. 2b). The identi®ed geothermal reservoir is contained within the 40 ohm-m apparent resistivity values. Campos and GardunÄo (1995) suggested that the geothermal ®eld might be the result of magma resurgence in the central sector of the caldera. However, Garcia (1997) considered that the heat source of the geothermal ®eld can not be related directly to the caldera-forming event because it is too old (more than 1.2 Ma before present, according to Pradal, 1990) to expect the thermal anomalies to remain, even assuming a purely conductive regime. The hydrothermal system must, therefore, be related to intrusive phenomena younger than 300,000 years. 4. Conceptual model of the internal structure of Los Azufres hydrothermal system The conceptual model of the Los Azufres hydrothermal system constructed with all available geoscienti®c data (Huitron et al., 1991) indicates that Los Azufres behaves as a typical high-enthalpy hydrothermal system in rough topography terrain with predominant secondary permeability. Drilling mud losses and a fast thermal recovery velocity identify the presence of highly permeable zones, related to active faulting. Within the reservoir, convection is the main heat transport phenomenon, while in the cap rock and below the reservoir, where low-permeability inhibits convection, conduction is predominant. As a typical hydrothermal system with predominant secondary permeability, temperature and the governing heat transfer phenomena change with position faster than in conduction-dominated areas, as a result of the presence of faults acting as water-¯ow channels. 5. Temperature data Temperature logs acquired during drilling stops at 62 wells drilled in the area were used for this study (a table with the equilibrium temperatures and dependant variables used in this paper is available from the authors). Temperatures are representative of pre-production data since 86% of the wells were drilled before large-scale production started and all the recent wells have been drilled away from the in¯uence area of previously drilled wells. Pressure logs that were acquired simultaneously with the temperature measurements in all the wells considered in this study show a hot hydrostatic pro®le from the piezometric surface to the bottom of the wells. Pressure control on temperature does not occur. Location of the wells is shown in Fig. 1. Three temperature logs are usually obtained during drilling stops, with elapsed times of 8, 12 and 24 h. At shallow depths temperature logs are obtained at a ®xed depth (300 m). In most bore-holes there are two or three temperature log series at di€erent depths, depending on reservoir engineering requirements, and it is possible to calculate bottomhole equilibrium temperatures for each series. An example is shown in Fig. 3. The resulting values are used to estimate a thermal gradient applying the ®nite-di€erences method; for convenience this value is assigned to the mean depth of the interval.

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Fig. 3. Temperature log series of well 22. Typical series correspond to measurements taken 8, 12 and 24 h after drilling was stopped. Bottom-hole equilibrium temperatures calculated with Horner method.

As can be seen in Fig. 1, most wells are located in the two separated areas, Maritaro and Tejamaniles. Wells outside these zones are progressively less attractive for geothermal energy production because temperature and permeability decrease. 5.1. Data reduction (equilibrium temperatures) Problems associated with temperature logs taken during drilling stops have been well studied (Jaeger, 1961), and still attract the attention of researchers because of their important economic implications. Problems are due to the thermal interfering e€ect of the drilling process on the natural temperature of the rock caused by mud circulation. In the case of large diameter production wells, the main risk is the permanent alteration of the temperature ®eld caused by the disturbance to the natural

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hydrologic equilibrium. The original formation temperatures are never recovered in successful production boreholes after well completion. Drilling temperature logs are, therefore, the best possibility to recover this information. Garcia (1986) conducted numerical tests of di€erent methods to estimate undisturbed formation temperatures, in order to understand their scope and limitations and to de®ne the application criteria for di€erent methods. On the basis of his results, the conventional Horner method (Fertl and Wichmann, 1977) was selected to estimate the equilibrium temperature, after a parameter sensitivity analysis and the testing of the method for the identi®cation of convective recovery phenomena (Garcia, 1997). Theoretical studies of the recovery phenomenon explain the hypotheses that yield the semi-logarithmic regression, for instance the assumption of a radial conductive recovery process (Lachenbruch and Brewer, 1959). 5.2. Qualitative graphic analysis and heat ¯ow estimates The use of numerical modeling programs to explain the presence of thermal anomalies and predict the behaviour of the system should be the ®nal goal of a thermal study; however, preliminary results during the feasibility stage can be obtained from a qualitative analysis. Assuming that anomalous phenomena a€ecting well temperature are random (not biased), a comprehensive data analysis should emphasize the regional conductive regime or phenomena less a€ected by local variations. Graphic display of the complete data set for a qualitative analysis included temperature±depth, temperature gradient±depth and the ternary temperature± depth±temperature gradient plots. Using available thermal conductivity data (Contreras et al., 1988) and gradient estimation by intervals, the conductive component of heat transfer (heat ¯ow density) was calculated for certain depths. A mean gradient was estimated by linear regression in some wells with a stable temperature gradient at depth, in which heat conduction may be dominant. This value was used to estimate the depth at which a typical temperature for magma could be attained and the deep regional heat ¯ow, which was calculated using the mean thermal conductivity. In the following sections a detailed description of the method is given and the results are discussed. 6. Temperature distribution A contour map calculated by interpolation of temperature data at 1500 m depth is presented in Fig. 4a. The highest temperatures observed in the ®eld are located in a N±S sector to the east of the 260 C contour. Temperature logs of wells located in this area (1, 9, 12, 28, 47 and 48) show convection e€ects occurring at temperatures higher than 300 C below 1500 m depth. There are no N±S faults connecting wells at reservoir depths; similar maximum temperatures in the N-S sector, therefore, suggest that ¯uids in the northern and the southern zone have a common source, regardless of the separation between shallow thermal anomalies shown in the temperature pro®le P-P0 (Fig. 4b). This common thermal energy source may be related to the

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magmatic body suggested by a preliminary interpretation of geophysical and thermal data (Garcia, 1995) combined with hydrogeologic information (CFE, 1991). According to Garcia (1995), the magmatic heat source is located to the east of the production zone, the andesitic massif acts as a hydrogeologic barrier for regional E±W

Fig. 4. Contours of stabilized temperatures in  C: (a) plane view of equilibrium temperatures at 1500 m depth, contour interval 20 C, ®lled circles indicate well locations (see well numbers in Fig. 1); (b) contours of stabilized temperatures based on well data projected over the pro®le P-P0 . Contours in  C, contour interval = 20 C. Straight lines correspond to projected fault traces. Local anomalies are related to convective phenomena.

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or SE±NW ¯uid ¯ow below the reservoir and, at reservoir depth, the ¯ow of ¯uids occurs from east to west along the E±W fault system (Lopez, 1991a). 7. Temperature behavior with depth Fig. 5 is a plot of equilibrium temperature ( C) versus depth (m) for all the available data from Los Azufres geothermal ®eld. Mean surface temperature of 12 C at 2500 m a.s.l. and air temperature gradient of 6 C/km were used to calculate mean temperatures of air at the surface as a function of elevation. Estimates show a maximum di€erence of 3 C between wells, which is not a signi®cant value for this study. Fig. 5 shows a solid line corresponding to the 30 C/km reference gradient, and a dashed line that represents the data trend calculated by linear regression (excluding data with depth=300 m), according to the following equation: t ˆ 83:92 log…z† ÿ 361:16

…1†

t ˆ temperature in  C z ˆ depth in m below the surface A boiling-point versus depth curve for pure water is also included in Fig. 5 as a reference. Some points are very near or slightly over the saturation temperature at the corresponding depth. 7.1. Outline of common behavior sectors in the temperature±depth plot Production data and information about permeable zones associated with intervals of drilling mud loss reported by Lopez (1991a) were used to delineate ®ve di€erent thermal sectors according to the behaviour of temperature with depth (Fig. 5). Convection e€ects are expected to a€ect stabilized temperature estimates, if these are within a neighbourhood of 50 m from drilling mud loss intervals. In some wells, data at di€erent depths appear in di€erent thermal sectors in the plot, due to local convective phenomena (wells 47 and 48, see Fig. 1 for well location), but in general each well has a dominant behaviour that allows us to include it in a single sector. 7.1.1. Sector of conductive wells outside the reservoir This sector corresponds to failed wells outside the convection zone that have the lowest temperatures (wells 10, 20, E1). They are located to the SW and NW of the production zone and have a dominant conductive heat transfer at depth. Temperature gradients do not decrease with depth. This shows that hydrogeologic changes (mostly a permeability decrease) occurring at a distance of a few kilometres from the production zones result in almost normal thermal conditions outside the reservoir.

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7.1.2. Sector of conductive and low-permeability wells inside the reservoir This sector includes wells within the hydrothermal system but very near to its external boundary (E2, 40, 50, 58) and wells in low-permeability blocks inside the reservoir (3, 23, 44, 47, 48, 54). These wells show moderate convection e€ects and the temperature can reach between 200 and 250 C at 2000 m depth; however, most of them are not productive (CFE, 1991; Lopez, 1991a).

Fig. 5. Equilibrium temperature versus depth for all available data. Sectors were traced to groups of wells with similar thermal and ¯uid production behavior. Solid line represents temperature variation with depth assuming a reference gradient of 30 C/km. Dashed line indicates the temperature variation with depth for the calculated linear regression given in Eq. (1). Boiling-point curve for pure water from the ground surface is shown as a reference.

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7.1.3. Sector of moderate permeability wells inside the reservoir This sector includes wells 7, 8, 27, 29, 49, 52, 53, 54. In some cases, drilling data showed relatively good temperature-permeability conditions but these wells have not encountered the best production zones and are not exploited. Some of them (7, 8, 52) are used as reinjection wells to dispose of the wastewater and to recharge the reservoir. They are the least permeable wells inside the reservoir limits and show temperatures slightly lower than those of successful wells at the same depth. 7.1.4. Sector of high-permeability production wells This sector comprises most production wells. They are characterized by irregular temperature-depth behaviour and high mean temperatures. Below the cap rock, a rapid temperature gradient decrease with depth is observed between 700 and 2000 m depth. This gradient decrease is produced by the presence of permeable zones that promote geothermal ¯uid convection and vertical uniformity of the temperature values. Below the most intense convection interval some wells (3, 8, 12, 25, 29, 47, 48, 54, 56, 57) show a gradient increase related to a decrease in permeability (CFE, 1991). 7.1.5. Sector of shallow high-permeability wells This sector corresponds to areas a€ected by intensive faulting at the surface in the Tejamaniles area or to limited portions of the Maritaro zone, where super®cial faults are not sealed by hydrothermal mineral deposition (Lopez, 1991a). These conditions result in an inecient cap rock that cannot sustain high pressures at depth. 8. Temperature gradient behavior with depth Fig. 6 shows a thermal gradient-average depth graph for all the available data. Temperature gradients were calculated applying the ®nite-di€erences method to the temperature-depth ratios, and were assigned to the average depth in the interval. The average calculated temperature gradient is 0.18 C/m (5.5 times the normal reference value of 30 C/km) but common gradient values in the reservoir are between 0.06 and 0.13 C/m. Initial gradients between the surface air temperature and the ®rst measurement at 300 m depth are representative of the low permeability of the cap rock. A maximum data spreading of temperature gradient is observed in the upper part of the wells, between 300 and 1000 m depth. Temperature gradient decreases with depth according to the following equation calculated by linear regression, omitting data points with zero or negative gradient (dashed line in Fig. 6): grad t ˆ ÿ0:09422 log…z† ‡ 0:7938

…2†

Conductive wells with low average temperature outside the reservoir (10, 20) and those with moderate average temperature in conductive blocks inside it (44) have an almost constant gradient between 0.03 and 0.09 C/m at most depths. However, they cannot be di€erentiated because of the lack of a temperature reference in the

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Fig. 6. Thermal gradient versus mean depth for all available data. Dashed line is the calculated linear regression given in Eq. (2).

gradient±depth graph. There are data from other wells in the same range; however, gradients in convective wells show a less regular gradient-depth relation and are not restricted to this temperature gradient interval. 9. Ternary diagram The processing of temperature data was completed with a simultaneous analysis of temperature-gradient-depth data in an attempt to introduce the average temperature information lacking in Fig. 6. This simultaneous analysis was implemented by the use of a ternary diagram similar to those used in geochemistry (Fig. 7). In order to plot data in a ternary diagram when data sets correspond to diverse units, normalization of data must be performed beforehand. Data calculation and normalization were performed as follows:

Fig. 7. Ternary diagram that presents relative mean values of: temperature, depth and thermal gradient at di€erent depths for all available data. Most production wells are located near the temperature vertex (T> 50) or under the line corresponding to a temperature gradient 5 times the reference value (33 C/km). Other reference curves are discussed in the text.

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Triads development. Three variables were calculated for each interval: the mean temperature, the mean depth, and the mean thermal gradient calculated as the ratio of temperature and depth variations. ÿ
ÿ
…3† gm ˆ Tj ÿ Ti = zj ÿ zi ÿ
…4† zm ˆ zj ‡ zi =2 ÿ
…5† Tm ˆ Tj ‡ Ti =2 where: = = = =

gm zm Tm Tk

mean gradient mean depth mean temperature stabilized temperature at depth zk

Normalization of variables. Data were normalized on the basis of extreme values for the same variable; depth between 0 and 3000 m; temperature between 12 and 360 C; and temperature gradient between 0 and 1 C/m. Tn ˆ 100 …Tm ÿ Tmin †=…Tmax ÿ Tmin †

…6†

Zn ˆ 100 …zm ÿ zmin †=…zmax ÿ zmin †

…7†

gn ˆ 100 …gm ÿ gmin †=…gmax ÿ gmin †

…8†

where: Tn Tmin Tmax zn Zmin zmax gn gmin gmax

= = = = = = = = =

normalized mean temperature 12 C 360 C normalized mean depth 0m 3000 m normalized temperature gradient 0 C/m 1 C

Normalization of triads. In order to plot data in a ternary diagram, the three variables in each triad are adjusted to give a total sum of 100 units. TN ˆ 100 Tn =Tn ‡ zn ‡ gn †

…9†

zN ˆ 100 zn =…Tn ‡ zn ‡ gn †

…10†

gN ˆ 100 gn =…Tn ‡ zn ‡ gn †

…11†

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where: TN zN gN

= new normalized temperature = new normalized depth = new normalized gradient

Only data triads with positive gradients were included. Data plotted in this ternary graph (Fig. 7) are located in a geometric position depending on the relative normalized values of the three variables involved at each point. As a reference to facilitate data interpretation in this plot, we included lines corresponding to conditions of temperature-depth-gradient calculated assuming a conductive regime with constant gradients from 0.033 C/m to nine times this value. Some other reference curves were also calculated: the lines that denote constant temperatures of 100, 200 and 300 C, and the depths at which those temperatures are reached for constant gradients. Higher gradients produce points progressively further from the depth vertex, meaning that a given temperature is reached at lower depths as the gradient increases. All the points correspond to temperature gradients over the reference value (33 C/ km), even those of failed wells. As depth increases data show a general trend from the gradient vertex to the temperature vertex, starting with gradients over nine times the reference value and progressively reaching lower gradient values at higher temperatures and greater depths. In Los Azufres, deep wells with the highest temperature values have gradients from 5 to 3 times the reference value. Thermal conditions favourable for exploitation correspond to points displaced towards the temperature vertex further than 50% of the temperature scale, and in some shallow wells at points located below 5 times the reference gradient line. Points outside the general trend move towards the depth vertex: they are medium to poorly successful wells or failed wells outside the reservoir (10, 20 and E1). Although wells 44 and E2 have low permeability, Casarrubias (1997) considered that they are located inside the reservoir and, therefore, show similar thermal conditions at their maximum depth. 10. Heat ¯ow density estimates At present, the only published estimates of heat ¯ow density in the Los Azufres area are based on indirect chemical methods: helium quotients (Polak et al., 1988) and silica geothermometry (Prol-Ledesma and Juarez, 1986). Conventional heat ¯ow measurements in gradient holes in Mexico (Smith, 1974; Smith et al., 1979; Ziagos et al., 1985) are too far from Los Azufres to be representative of the geothermal ®eld. Heat conduction is not the main heat transport method in a hydrothermal system as a whole, and with the exception of data outside the reservoir, even the conductive intervals within the ®eld seem to be a€ected by nearby convection. However, vertical temperature gradients give rise to a vertical heat conduction component. This value

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was estimated by intervals in Los Azufres, multiplying the temperature gradient of the interval by the thermal conductivity measured in cores where both parameters were available. In Los Azufres there is a very limited number of thermal conductivity measurements (Contreras et al., 1988) but the estimates are valid because of the lithologic homogeneity of the ®eld. Table 1 includes heat ¯ow density estimates; in a few cases conductivity data from a similar lithologic unit in a nearby well were used. Heat ¯ow estimates range between 69 and 668 mW/m2. Low conductive heat ¯ow within the reservoir is related to low gradients occurring at high average temperatures that are produced by convective phenomena (wells 3, 4). Near the outer limit of the reservoir (well 50), low heat ¯ow is the result of a real conductive condition at depth associated with a small gradient at somewhat lower than average temperature (below 200 C at 1500 m depth). In well 20, outside the reservoir limits, heat ¯ow estimates are around 100 mW/m2 associated with temperatures lower than 150 C. Heat ¯ow in this well is slightly higher than that observed in the upper part of well 3, which is located in a higher permeability sector inside the reservoir. Estimates for well 3 in a deep low-permeability zone yield heat ¯ow values above 173 mW/m2. Well 29 has low permeability and is located in the highest temperature zone; we believe that the 295 mW/m2 measured in this well, or similar values, are, therefore, representative of the heat ¯ow in conductive blocks inside the reservoir associated with the conductive cooling of a magma body. Very high heat ¯ow, of the order of 400 to 600 mW/m2, could be related to conductive blocks with boundaries a€ected by ¯uids of very di€erent temperatures. 10.1. Thermal gradient and heat ¯ow at the bottom of the wells We used some selected wells with a more stable temperature-depth relation near the bottom in order to study heat transfer below the reservoir, and to estimate a representative gradient for each well by linear regression, minimizing local variations. Combined evidence about the absence of drilling mud losses, increase of the temperature gradient, and low ¯uid production in some cases (CFE, 1991) suggest that permeability decreases below the reservoir and that the conductive heat ¯ow contribution increases. The results are shown in Table 2. Temperature gradients correspond to the bottom measurements in the indicated wells, where rocks are either andesites or basalticandesites. Heat ¯ow data included in Table 2 were calculated using a constant thermal conductivity of 1.750.35 W/m C, which is the mean value of the data reported by Contreras et al. (1988). The most reliable data for heat ¯ow estimation correspond to wells with a linear temperature-depth behaviour and very low drilling ¯uid losses, indicated by the superindex b in Table 2. For instance, wells 10 and 20 outside the reservoir have heat ¯ow between 114 and 117 mW/m2, a value slightly higher than the heat ¯ow estimation for the thermal province of the Trans-Mexican Volcanic Belt (100 mW/m2). Wells 3 and 44 are located in relatively low-permeability blocks inside the reservoir, and have heat ¯ow values between 124 and 106 mW/m2, respectively. Well E1 is

Well number

Minimum depth (m)

Maximum depth (m)

Mean depth (m)

Mean height m

Minimum temperature ( C)

Maximum temperature ( C)

Mean temperature ( C)

Temperature gradient ( C/km)

Thermal conductivity (W/m  C)

Heat ¯ow density (mW/m2)

3 3 4 5 8 19 20a 20a 22 25 26 26 29a 50a

300 1500 706 0 0 995 0 1000 300 550 589 994 302 889

1500 2440 1536 998 995 1096 1000 1810 1000 996 994 1240 793 1492

900 1970 1121 499 498 1046 500 1405 650 773 792 1117 548 1191

1888 818 1751 2409 2311 1803 1994 1089 2215 2124 2127 1801 2365 1545

132 200 220 12 12 223 12 82 81 148 123 171 90 146

200 285 257 218 238 249 82 133 234 240 171 277 228 182

166 243 239 115 125 236 47 108 158 194 147 224 159 164

57 90 45 206 227 257 70 63 219 206 119 431 281 60

1.68 1.92 1.55 1.17 2.34 1.97 1.58 1.71 2.17 1.75 2.2 1.55 1.05 1.52

95 173 69 242 532 507 111 108 474 361 261 668 295 91

a

Low permeability wells.

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Table 1 Estimates of thermal gradient and heat ¯ow density in selected depth intervals

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Table 2 Thermal gradients calculated by linear regression and heat ¯ow density estimates based on bottom-hole temperatures Well number

Thermal gradient ( C/km)

Times the reference gradienta

Heat ¯ow density (mW/m2)

1 3 4 9 10b 16 19 20b 21 22 23 25 27Ab 28 29b 42 43 44b 47 48 50 51 54 55 56 57 58b E1b E2

98 71 85 79 65 73 117 67 138 105 151 75 122 121 116 101 161 61 105 108 60 77 66 63 87 105 117 94 107

3.0 2.1 2.6 2.4 2.0 2.2 3.6 2.0 4.2 3.2 4.6 2.3 3.7 3.7 3.6 3.0 4.9 1.9 3.2 3.3 1.8 2.3 2.0 1.9 2.6 3.2 3.5 2.9 3.2

171 124 149 138 114 127 205 117 241 184 265 131 214 212 204 176 282 106 184 189 104 134 116 109 152 184 205 165 186

a b

Reference gradient=33  C/km. Low permeability wells.

located near the northwestern limit of the ®eld, in a sector where ¯uid discharges at the surface, and has a heat ¯ow of 165 mW/m2. Well 44 is of special interest because it is the deepest well in the ®eld (3544 m). It has no drilling mud loss intervals below 687 m and has more temperature data than any other well in Los Azufres. Well 44 is located in a block bounded by active faults that act as ¯uid ¯ow channels; these faults are crossed by other producing wells (wells 4 and 15). The heat ¯ow density estimation for well 44 (106 mW/m2) is reliable because this well presents a predominantly conductive behaviour. The extrapolation of the bottom-hole temperature in wells 3 and 44 (the most reliable conductive-dominated wells in the western sector of the reservoir) using the bottom temperature gradient indicates that a typical rhyolitic magma temperature

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of 600 C could be reached at depths roughly between 7000 and 9000 m. The same calculations performed with data corresponding to the bottom-hole of well 29 (the most reliable conductive-dominated well in the eastern sector of the ®eld) yield a depth of 5000 m. As these estimates appear low for the top of a magma chamber and data may be a€ected by errors due to remnant e€ects of convection, we consider that high temperatures could be associated with very high-temperature water between 5000 and 7000 m depth, or a magma body that could be located at depths as shallow as 9000 m. 11. Discussion and conclusions This study is based on the graphic display and the qualitative interpretation of bottom-hole temperatures at Los Azufres geothermal ®eld. Estimates of temperature gradient and heat ¯ow densities in the area were done for the ®rst time using borehole temperature data. The observed gradient changes are much larger than those expected if vertical heat ¯ow was constant and changes were related only to thermal conductivity variations. In the conductive case, we would expect at most a change by a factor of two with respect to the reference value because thermal conductivity ranges from 1.17 to 2.34 W/ C m (Contreras et al., 1988). Preliminary modelling results (Garcia, 2000) show that the gradient variations are not related to heat ¯ow changes due to the geometry of a magmatic heat source and must, therefore, be related to the distance to open faults and the temperatures of circulating ¯uids. In Los Azufres, low gradients from 0.05 to 0.08 C/m at depths between 500 and 2000 m may be associated with convection cells where intense water ¯ow exists. As they occur at high average temperatures (180±300 C), an up¯ow or a lateral out¯ow from the reservoir can be identi®ed. Very high temperature gradients (from 0.3 to 0.6 C/m) at low-to-moderate average temperatures (<150 C) and depths between 0 and 700 m are generally related to low-permeability zones in the cap rock overlying the reservoir. Temperature gradients from 0.1 to 0.2 C/m with high average temperatures (250±340 C) are observed in impermeable blocks inside or below the reservoir. High gradients (>0.2 C/m) at high temperatures (more than 180 C) at depths from 500 to 2000 m represent transient thermal phenomena associated with conductive blocks bounded by ¯uids at very di€erent temperature. Bottom-hole equilibrium temperatures are not suitable to evaluate the down¯ow from steam-heated water in a single well; this phenomenon could be observed in the complete set of temperature logs. At Los Azufres this down¯ow is uncommon because the pressure at deeper strata is higher than the pressure at shallow acid aquifers. Pressure and temperature logs acquired during drilling stops inside water or mud-®lled wells are not a€ected by this phenomenon. Constant temperature gradients of about 0.06 C/m are observed in low-temperature wells outside the reservoir. Some wells located in low-permeability blocks inside the reservoir but near its boundary have gradients of the same order but they occur only in limited depth intervals and at higher average temperatures (180±200 C). In the

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eastern border of the hydrothermal zone where the presence of a magmatic heat source is expected, a low-permeability well (29) has a gradient of 0.16 C/m and a bottom-hole temperature of 300 C at 2500 m depth. As a consequence of convection in active faults, high temperatures have a more irregular spatial distribution inside the reservoir than the low temperatures outside it, where temperature gradients are small and stable. This suggests that low temperature wells are less permeable, and low temperature ¯uid movement does not substantially disturb the conductive gradient. The equilibrium temperatures in low-permeability wells located inside the reservoir are low compared with those estimated at the same depth in wells near active faults. If Horner-extrapolated temperatures are good estimates of undisturbed values (even near faulted zones), then host-rock blocks must have a signi®cant temperature difference with respect to the ¯uids circulating through the faults. This indicates that there must be an important unsteady component of horizontal conductive heat ¯ow at the block size scale, even under pre-exploitation conditions. In order to explain the observed low conductive gradients in well 44, the conditions that could produce low-vertical conduction phenomena are investigated. In a non-fractured block bounded by active faults, the temperature of the ¯uid circulating along the fault planes could act as a boundary condition determining the conductive heat ¯ow towards the block. The internal temperature of the block would depend on the temperature di€erences between the boundaries, the conductivity of the rock matrix and the e€ective permeability. The background regional heat ¯ow was calculated by linear regression using data from the bottom of conductive wells inside (well 44) and outside the reservoir (wells 10 and 20). Estimated values are 106, 114 and 117 mW/m2, respectively. These values are slightly higher than the heat ¯ow density (around 100 mW/m2) representative of the Trans Mexican Volcanic Belt (Ziagos et al., 1985). Heat ¯ow calculated with bottom-hole data of other wells show values from 109 mW/m2 (well 55) to 282 mW/m2 (well 43), but as these wells were more a€ected by drilling mud losses (Lopez, 1991a) it is dicult to know the e€ect of convection on the estimated values. Heat ¯ow in well 29 (295 mW/m2) is considered the most reliable estimation of heat ¯ow associated with the conductive cooling of a magma source located to the east of the reservoir. The extrapolation of the bottom-hole temperature using the local thermal gradient of wells 44, 3 and 9 indicates that a 600 C temperature (representative of a rhyolitic magma) could be attained at depths of 9, 7 and 5 km depth, respectively. In low-permeability blocks inside the reservoir there is a local background conductive heat ¯ow that varies from 242 mW/m2 (well 5) to 295 mW/ m2 (well 29). However, in general, due to temperature di€erences between ¯uids circulating through faults bounding low-permeability blocks, the conductive heat ¯ow inside the reservoir varies from 69 (well 4) to 667 (well 26) mW/m2. In this case the conductive heat ¯ow is just a component of the mainly convective heat transfer. The use of a ternary graph to display the relationship among temperature, thermal gradient and depth of wells at Los Azufres geothermal ®eld is most useful in the qualitative analysis of thermal data from a comprehensive perspective. In the ternary diagram (Fig. 7), the less attractive area for energy production corresponds to the

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wells that plot close to the depth vertex, i.e. the deepest wells characterized by low temperature gradient and low temperature. This area is the best suited for the study of the regional conductive regime, as this is the combination that we expect in a heat transfer conductive regime. The wells that plot close to the temperature vertex are shallow wells with high temperature and low gradients. These are the expected conditions for a well located in an area characterized by intense convection phenomena; these are the best-suited wells for geothermal energy exploitation. The temperature gradient vertex includes shallow boreholes with high gradient, and low-to-moderate temperature. These conditions are found in wells reaching zones saturated with geothermal ¯uids that are separated from the surface by a cap rock with low temperatures near the surface. In some cases, temperature can be high if there are active faults that permit a rapid ascent of ¯uids from the top of the reservoir to the surface. This situation is observed by a relative displacement of the data to the temperature vertex with a small depth variation. During exploration, thermal characteristics similar to those of wells located in the border of the reservoir discourage the continuation of a feasibility study, regardless of the existence of important energy resources in the immediate vicinity. This result shows that it would be a good strategy to reinterpret the thermal data from apparently failed wells in other projects, from a hydrogeologic perspective. The analysis of the temperature data presented here may be useful for the interpretation of temperature data in other geothermal ®elds under exploration by using as a reference the relationships observed at Los Azufres. Acknowledgements The authors are indebted to Ruben Ostos from the Los Azufres geothermal ®eld for his assistance in the selection of temperature logs and the computation of equilibrium temperatures, and to the authorities of the Gerencia de Proyectos Geotermoelectricos (CFE) for their support to this project. Thanks are also extended to Howard Ross, Joe Moore and two anonymous reviewers for their comments and suggestions. References Campos, E.J.O., GardunÄo, M.V.H., 1995. Los Azufres silicic center (Mexico): inferences of caldera structural elements from gravity, aeromagnetic and geoelectric data. Journal of Volcanology and Geothermal Research 67, 123±152. Casarrubias, U.Z., 1997. Resultados de la perforacioÂn exploratoria en la porcioÂn occidental del campo geoteÂrmico de Los Azufres, MichoacaÂn, MeÂxico. Geotermia Revista Mexicana de GeoenergõÂa 12, 189±207. CFE (Comision Federal de Electricidad), 1991. CaracterõÂsticas y capacidades del yacimiento geoteÂrmico de Los Azufres, Mich., MeÂxico. Internal Report of Gerencia de Proyectos Geotermoelectricos OIY AZ 08-91, Comision Federal de Electricidad, Morelia, Mich., 250 pp. Contreras, E., DomõÂnguez, B., Iglesias, E., GarcõÂa, A., HuitroÂn, R., 1988. Compendio de resultados de mediciones petrofõÂsicas de nuÂcleos de perforacioÂn del campo geoteÂrmico Los Azufres. Geotermia Revista Mexicana de GeoenergõÂa 4, 9±105.

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