Vitrinite reflectance geothermometry and apparent heating duration in the Cerro Prieto geothermal field

Vitrinite reflectance measured in immersion oil (R_o) on kerogen extracted from hydrothermally altered mudstones in borehole M-84 at the Cerro Prieto geothermal field exhibit an increase in mean reflectance ( ) from 0.12 per cent at 0.24 km depth to 4.1 per cent at 1.7 km depth. Downhole temperatures measured over this interval increase from about 60° to 340°C. These data plotted against temperature fall along an exponential curve with a coefficient of determination of about 0.8. Other boreholes sampled in the field show similar relationships. A regression curve calculated for temperature and in borehole M-105 correctly predicts temperatures in other boreholes within the central portion of the geothermal system. The correlation between the reflectance values and logged temperature, together with consistent temperature estimates from fluid inclusion and oxygen isotope geothermometry, indicates that changes in are an accurate and sensitive recorder of the maximum temperature attained. Therefore, vitrinite reflectance can be used in this geothermal system to predict the undisturbed temperature in a geothermal borehole during drilling before it regains thermal equilibrium. Although existing theoretical functions which relate to temperature and duration of heating are inaccurate, empirical temperature- curves are still useful for geothermometry.A comparison of temperature- regression curves derived from nine boreholes within the Cerro Prieto system suggests that heating across the central portion of the field occurred penecontemporaneously, but varies near margins. Boreholes M-93 and M-94 appear to have cooled from their maximum temperatures, whereas M-3 and Prian-1 have only recently been heated.Comparison of the temperature- data from the Salton Sea, California, geothermal system indicates that the duration of heating has been longer there than at the Cerro Prieto field.     

Distribution of hydrothermal mineral zones in the Cerro Prieto geothermal field of Baja California, Mexico

Our ongoing studies of water-rock interaction at Cerro Prieto have now been extended to include samples from 40 wells. We have confirmed the regular sequence of progressive hydrothermal alteration zones previously described, and have mapped these alteration zones across the geothermal field. Our earlier work showed the relationships between hydrothermal mineralogy, temperature and permeability, in that alteration occurs at lower temperature in sandstone than in the less permeable shales. The effects of chemical parameters such as silica activity and differences in CO₂ fugacity have also become apparent when mineral assemblages are compared in sandstones from different wells at the same temperature. A rather complete picture of the shape of the reservoir and the nature of its boundaries is developing, and we have begun to identify patterns in the observed hydrothermal mineral zonations which are characteristic of different temperature gradients. We infer such different gradients to be indicative of different parts of the hydrothermal flow regime. In certain wells mineral zones are closely spaced, indicating steep thermal gradients, while in others they are much farther apart. We believe that patterns characteristic of recharge, discharge and upwelling zones as well as areas of primarily horizontal flow can be recognized.The geothermal circulation system at Cerro Prieto appears to be rather young and shows no indication of retrograde reactions due to cooling. The pattern of fluid flow does not seem to be significantly affected by faults, stratigraphic horizons or by the presence of a cap-rock. The mineral zones define a thermal dome which is apparently fed from the east and spreads westward.     

Simulation of reinjection at Cerro Prieto using an idealized two-reservoir model

Most of the papers published on the hydrodynamic and thermal effects of reinjection in geothermal fields assume a single reservoir with uniform transmissivity and storativity. However, there is evidence that the Cerro Prieto geothermal field is a two-reservoir system and that each reservoir has different hydraulic properties (Abril and Noble, 1979). Because two-reservoir fields have not been adequately studied, we will analyze the thermohydrological response of this kind of geothermal system to various alternative schemes of reinjection. We use parameters relevant to the Cerro Prieto system so that the results will be useful in planning future reinjection operations at this field.     

Geophysical well - log correlations along various cross-sections of the Cerro Prieto geothermal field

The Comisión Federal de Electricidad of Mexico and Lawrence Berkeley Laboratory of the United States are currently conducting joint investigations of the Cerro Prieto geothermal field, located approximately 35 km south of the city of Mexicali, Baja California, Mexico. Based on the geophysical well logs, transversal cross sections were constructed. This study consists mainly of correlating electrical markers observed in the logs. Temperature logs were also used to assist in the differentiation of what we call reservoirs A and B.In this article the stratigraphy observed in the cross-sections, the faults, and the identified producing layers will be described. Finally, the usefulness of this type of study for planning new wells, for selecting producing intervals, as well as for the development of the geologic model of the field will be presented.     

Lithological correlations of the cerro prieto wells, based on well - log interpretation

This paper presents the results of correlations of the geophysical well logs obtained from wells drilled at the Cerro Prieto geothermal field. From this correlation, a structural interpretation of the ‘receptacle’ holding the geothermal fluids has been obtained.Based on the temperature and electrical resistivity logs, the presence of two main geothermal reservoirs has been established. It was also found that most of the wells were completed such that production is obtained simultaneously from several sandy layers. In all probability this has created some mechanical problems in the wells in addition to a certain confusion in the interpretations based on fluid production data.An hypothesis is postulated about the deposition environment and the genesis of the reservoir, on the basis of which it is concluded that the reservoir is a paleochannel. It is also postulated that thinning of the basement rocks is the fundamental cause of the heat transmission to the reservoirs.Tables listing the correlating marker horizons for each well and maps showing the top of the two main reservoirs and the isopacks for the shallow one are presented. From these data have been inferred the areas showing the best possibilities for immediate geothermal development.     

Intersecting faults and sandstone stratigraphy at the cerro prieto geothermal field

The NW - SE trending Cerro Prieto fault zone is part of a major regional lineament that extends into Sonora, Mexico, and has characteristics of both a wrench fault and an oceanic transform fault. The zone includes a number of separate identified faults and apparently penetrates deep into the basement and crustal rocks in the area. The zone serves as a conduit for both large and rapid heat flow. Near well M-103, where the Michoacán fault obliquely intersects a shorter NE - SW trending fault (i.e., the Pátzcuaro fault), large circulation losses during drilling indicate greater permeability and hence increased natural convective fluid flow. Temperature contour maps for the southern portion of the field suggest that a shear fault zone also exists in the vicinity of wells M-48, M-91 and M-101. This shear zone aids in rapidly distributing geothermal fluid away from the Cerro Prieto fault zone, thus enhancing recharge to the western part of the reservoir.We have studied the distribution of lithologies and temperature within the field by comparing data from well cuttings, cores, well logs and geochemical analyses. Across the earliest developed portion of the field, in particular along a 1.25 km NE - SW section from well M-9 to M-10, interesting correlations emerge that indicate a relationship among lithology, microfracturing and temperature distribution. In the upper portion of the reservoir of this section, between 1200 and 1400 m, the percentage of sandstones ranges from 20 to 55. Well logs, calcite isotope maxima, and the Na - K - Ca geothermometer indicate temperatures of 225–275°C. The isothermal high in this vicinity corresponds to the lowest total percentage of sandstones. Scanning electron microphotographs of well cores and cuttings from sandstone and shale units reveal open microfractures, mineral dissolution and mineral precipitation along microfractures and in pores between sand grains. Our working hypothesis is that these sandy shale and siltstone facies are most amenable to increased microfracturing and, in turn, such microfracturing allows for higher temperature fluid to rise to shallower depths in this part of the reservoir.Our ongoing research is aimed at achieving a coherent geological model that provides a basis for estimating reservoir capacity, and that illustrates our understanding of fluid flow along major faults, laterally through fault shear zones, and within predominantly silty and shaley deltaic clastics that have been microfractured.     

Geology of the Cerro Prieto geothermal field

The Cerro Prieto geothermal field is located in the Mexicali Valley which is characterized morphologically by the presence of the elongated Cucapah range striking predominantly NW-SE. This range consists of Upper Cretaceous granite which has intruded and metamorphized the Cretaceous and/or Paleozoic sediments. Near the field is the rhyodacitic Cerro Prieto volcano which pierces through the Cenozoic deltaic sediments.We divided the deltaic sediments into two lithostratigraphic units: Unit A: Unconsolidated Quaternary deltaic sediments composed of clays, sands and gravels. Unit B: Consolidated Tertiary deltaic sediments composed of siltstone, shales and sandstones.Since the geothermal aquifer is intimately related to the structural behavior of the geological formations which are overlaid by unconsolidated deltaic sediments, it was necessary to apply different geophysical methods to understand the behavior of the consolidated sediments and the basement.Based upon geophysical surveys and the wells completed to date, a regional geologic model and several cross-sections of the Cerro Prieto field have been developed.Two fault systems have been defined. The principal one, which we called the Cerro Prieto system, has a predominantly NW-SE strike, parallel to important faults such as the San Andreas, San Jacinto, Cucapah, Imperial etc.Normal to the NW-SE system are faults with predominantly SW-NE strikes, which we designated the Volcano system. At depth, these two systems have apparently created a step-faulted horst and graben structure striking NW-SE with its eastern and western sides out.A number of cross-sections were made based on petrographic analyses of cuttings and core samples. These sections confirm the existence of two fault systems passing through the geothermal field, which complicates the structural interpretation of the field.The geologic sections based on the petrographic analysis of rock samples obtained from the drillings render very complicated a structural interpretation of the two fault systems which affect the geothermal field and which occur locally in great frequency.     

Parabolic troughs to increase the geothermal wells flow enthalpy

This work investigates the feasibility of using parabolic trough solar field to increase the enthalpy from geothermal wells’ flow in order to increase the steam tons; in addition, it is possible to prevent silica deposition in the geothermal process. The high levels of irradiance in Northwestern Mexico make it possible to integrate a solar-geothermal hybrid system that uses two energy resources to provide steam for the geothermal cycle, like the Cerro Prieto geothermal field. The plant consists of a geothermal well, a parabolic trough solar field in series, flash separator, steam turbine and condenser. Well “408” of Cerro Prieto IV has enthalpy of 1566 kJ/kg and its quality must be increased by 10 points, which requires a Δh of 194.4 kJ/kg. Under these considerations the parabolic troughs area required will be 9250 m², with a flow of 92.4 tons per hour (25.67 kg/s). The solar field orientation is a N–S parabolic trough concentrator. The silica content in the Cerro Prieto geothermal brine causes problems for scaling at the power facility, so scale controls must be considered.     

Evidence for thermal mining in low temperature geothermal areas in Iceland

This study deals with thermal mining in several geothermal systems in Iceland. A number of 2500- to 3000-m deep drillholes have been drilled into low temperature geothermal areas in the country. The conductive gradient outside active geothermal areas has also been mapped, and shows a systematic variation from lower than 50°C/km in the outer parts of the Tertiary basalts to over 100°C/km on the borders of the volcanic zones (rift zones). The difference between formation temperatures inside geothermal systems and the surrounding conductive gradient can be computed as a function of depth. This difference is termed ΔT in this paper. The ΔT-curves show that the upper parts of the geothermal systems are heated and the lower parts are cooled compared to the undisturbed conductive gradient. In many cases the cooling of the lower part is greater than the heating in the upper part, so that a net thermal mining has occurred. This thermal mining is calculated for several geothermal systems, and the systems are compared. The net thermal mining in the top 3000 m appears to be much greater in formations of Pleistocene and Pliocene age. It gradually decreases to zero for formations older than 6 million years. However, the net thermal mining is critically dependent on the maximum depth of water convection in these systems, which is unknown.     

Hydrothermal mineral zones in the geothermal reservoir of Cerro Prieto

Detailed petrologic studies completed to date on ditch cuttings and core from 23 wells in the Cerro Prieto field have led to recognition of regularly distributed prograde metamorphic mineral zones. The progressive changes in mineralogy exhibit a systematic relationship with reservoir temperature.The Cerro Prieto reservoir consists of a series of sandstones, siltstones, and shales composing part of the Colorado River delta. The western part of the field contains relatively coarser sediments apparently also derived from the delta and not from the basin margins as formerly thought. The most abundant detrital minerals in the sediments include quartz, feldspar, kaolinite, montmorillonite, illite, chlorite, mixed-layer clays, calcite, dolomite and iron hydroxides. Some of these minerals were also formed diagenetically.The following progressive stages of post-depositional alteration in response to increasing temperature have been observed: (1) diagenetic zone (low temperature), (2) illite-chlorite zone (above ~ 150°C), (3) calc-aluminum silicate zone (above ~ 230°C) and the biotite zone (above ~ 325°C). These zones are transitional to some degree and can be further subdivided based on the appearance or disappearance of various minerals.One immediate application of these studies is the ability, from a study of cuttings obtained during drilling of a well, to predict the temperatures which will be observed when the well is completed.     

Preliminary geochemical model of the Cerro Prieto geothermal field

The distribution of the Na-K-Ca index values in the geothermal aquifer of Cerro Prieto indicates a probable cold-water recharge in the northern and western parts of the present producing field. In the central part of the field, the distribution of Na-K-Ca values is very irregular, due to the effect of percolation or the vertical descending flow of colder waters caused by apparent over-exploitation.The large area with indices varying between 0·5 and 0·7 to the east, southeast, and south of the field confirm the presence of hotter geothermal waters than those observed in the present producing field. The limits of this hot aquifer have not yet been determined toward the east and southeast.The temperature distribution in the Cerro Prieto aquifer confirms the conclusions reached using the Na-K-Ca index. The maximum temperature calculated was in well M-53 (350°C) and the minimum was in M-9 (250°C), when wells M-6 and M-1A, which have lower temperatures, are not taken into account.The chloride distribution in the aquifer confirms the recharge of colder, less saline waters in the northern and western parts of the field. It was surprising to find that the chloride content in the aquifer was lower in the eastern part (M-53) than in the present field, even though the temperature is higher. Another interesting fact is the extremely low chloride content found in well M-101, with temperatures around 290°C.The distribution of potassium and silica in the aquifer was not as useful for the interpretation of fluid movement as originally expected. Discrepancies were found with respect to the interpretations based on the Na - K - Ca index and the chloride content.In regard to the changes in the Na - K - Ca index, in temperature, and in chloride content during the exploitation of the field, a gradual increase in Na - K - Ca index values was observed in wells with high and low enthalpy. In exceptional cases these values decrease or remain constant. This increase in the index has represented a 10 to 20°C temperature decrease with respect to the original values existing at the beginning of the exploitation of the field (1973). The chloride content in the aquifer has decreased at different rates, from 670 mg/l per year in M-26 to 371 mg/1 per year in well M-42, even though in some wells (M-20) it has increased and in others (M-5) it has remained almost constant.In order to decrease the rate of temperature and pressure reduction in the present producing field, it is recommended that production of low enthalpy wells be suspended and be replaced by new wells located in the periphery of the field, preferably to the southwest and northeast. Thus, the production area would at least be doubled, in an attempt to avoid the percolation or vertical flow, which is apparently causing the cooling of the reservoir.Another measure, which could help prevent a rapid drawdown, is to exploit the deeper hot aquifers that apparently exist in the center of the field. This would first have to be confirmed by an exploratory well or by deepening one of the present wells.     

Simulation and resistivity modeling of a geothermal reservoir with waters of different salinity

Apparent resistivities measured by means of repetitive dipole-dipole surveys show significant changes within the Cerro Prieto reservoir. The changes are attributed to production and natural recharge. To understand better the observed geophysical phenomena, we performed a simple reservoir simulation study combined with the appropriate DC resistivity calculations to determine the expected magnitude of apparent resistivity change. We consider production from a liquid dominated reservoir with dimensions and parameters of the Cerro Prieto ‘A’ reservoir and assume lateral and vertical recharge of colder and less saline waters. Based on rather schematic one- and two-dimensional reservoir simulations, we calculate changes in formation resistivity which we then transform into changes in apparent resistivity that would be observed at the surface. Simulated changes in apparent resistivities over the production zone show increases of 10 to 20% over a 3 year period at the current rate of fluid extraction. Changes of this magnitude are not only within our ability to discern using proper field techniques, but are consistent in magnitude with some of the observed effects. However, the patterns of apparent resistivity changes in the simulated dipole-dipole pseudosection only partially resemble the observed field data. This is explained by the fact that the actual fluid recharge into the ‘A’ reservoir is more complicated than assumed in our simple, schematic recharge models. DC resistivity monitoring appears capable of providing indirect information on fluid flow processes in a producing geothermal reservoir. Such information is extremely valuable for the development of quantitative predictions of future reservoir performance.     

Geochemistry of thermal waters in the Tengchong volcanic geothermal area, West Yunnan Province, China

The Tengchong volcanic geothermal area is one of the areas in China which has powerful geothermal energy potential. The chemical compositions of the thermal waters discharged in this area were studied to obtain information on boiling and mixing relationships and average reservoir temperatures. Then a conceptual model of the Tengchong volcanic geothermal area was formulated. Hydrothermal areas have reservoir temperatures ranging from 90 to 150°C; such temperatures can be found in up to 60% of the 58 hydrothermal areas. Five hydrothermal areas have high temperatures, with an average reservoir temperature of more than 150°C, and occupy less than 10% of the total. The Hot Sea geothermal field is one of the five high temperature hydrothermal areas where a more detailed investigation was made.     

Main aspects of geothermal energy in Mexico

With an installed geothermal electric capacity of 853 MW_e, Mexico is currently the third largest producer of geothermal power worldwide, after the USA and the Philippines. There are four geothermal fields now under exploitation: Cerro Prieto, Los Azufres, Los Humeros and Las Tres Vírgenes. Cerro Prieto is the second largest field in the world, with 720 MW_e and 138 production wells in operation; sedimentary (sandstone) rocks host its geothermal fluids. Los Azufres (88 MW_e), Los Humeros (35 MW_e) and Las Tres Vírgenes (10 MW_e) are volcanic fields, with fluids hosted by volcanic (andesites) and intrusive (granodiorite) rocks. Four additional units, 25 MW_e each, are under construction in Los Azufres and due to go into operation in April 2003. One small (300 kW) binary-cycle unit is operating in Maguarichi, a small village in an isolated area with no link to the national grid. The geothermal power installed in Mexico represents 2% of the total installed electric capacity, but the electricity generated from geothermal accounts for almost 3% of the national total.     

Study on the flow production characteristics of deep geothermal wells

This paper describes a study on the potential flow production characteristics of three non-producing, deep (average depth 4000 m) geothermal wells in the Cerro Prieto geothermal field. The expected production characteristics of these wells were computed in order to determine whether their inability to sustain flow was due to: (1) heat loss effects in the well; (2) the influence of casing diameters; (3) transient temperature effects during the first days of well discharge, and/or (4) the effects of secondary low-enthalpy inflows. For the study, the conservation equations of mass, momentum and energy for two-phase homogeneous flow were solved for the wellbore, since homogeneous flow provides the simplest technique for analyzing two-phase flows when the flow patterns are not well established. The formation temperature distribution was computed assuming radial transient heat conduction. The numerical model was validated by comparison with analytical solutions and with measured pressure and temperature profiles of well H-17 from the Los Humeros geothermal field, Mexico. It was found that the wells should have sustained production. The early heat losses were so large that the flow needed to be induced, and flow will be sustained only after a few days of induced discharge. For well M-202, the analysis suggests that the inflow of secondary colder fluids was responsible for stopping the flow in this well.     

The origin of the Cerro Prieto geothermal brine

The Cerro Prieto geothermal brine may have originated from mixing of Colorado River water with seawater evaporated to about six times its normal salinity. This mixture circulated deeply and was heated by magmatic processes. During deep circulation, Li, K, Ca, B, SiO₂ and rare alkalis were transferred from rock minerals to the water, and Mg, SO₄, and a minor quantity of Na were transferred to the rock. Similar alteration of seawater salt chemistry has been observed in coastal geothermal systems and produced in laboratory experiments. After heating and alteration the brine was further diluted to its present range of composition. Oxygen isotopes in the fluid are in equilibrium with reservoir calcite and have been affected by exploitation-induced boiling and dilution.     

Gases in steam from Cerro Prieto geothermal wells with a discussion of steam/gas ratio measurements

As part of a joint USGS-CFE geochemical study of Cerro Prieto, steam samples were collected for gas analyses in April, 1977. Analyses of the major gas components of the steam were made by wet chemistry (for H₂O,CO₂,H₂S and NH₃) and by gas chromatography (He,H₂,Ar,O₂,N₂ and hydrocarbons).The hydrocarbon gases in Cerro Prieto steam closely resemble hydrocarbons in steam from Larderello, Italy and The Geysers, California which, although they are vapor-dominated rather than hot-water geothermal systems, also have sedimentary aquifer rocks. These sedimentary geothermal hydrocarbons are characterized by the presence of branched C_4–6 compounds and a lack of unsaturated compounds other than benzene. Relatively large amounts of benzene may be characteristic of high-temperature geothermal systems. All hydrocarbons in these gases other than methane most probably originate from the thermal metamorphosis of organic matter contained in the sediments.     

The Cerro Prieto IV (Mexico) geothermal reservoir: Pre-exploitation thermodynamic conditions and main processes related to exploitation (2000–2005)

The Cerro Prieto IV (CP IV) reservoir, located in the northeastern part of the Cerro Prieto (Mexico) geothermal field, was studied in order to define its pre-exploitation conditions and initial (2000–2005) response to exploitation. Bottomhole thermodynamic conditions were estimated by modeling heat and fluid flows using the WELLSIM program and well production data. Produced fluid chemical and isotopic data were also analyzed to investigate characteristic patterns of behavior over time, which were then compared against simulation results to obtain a conceptual model of the CP IV reservoir. According to the proposed model, two zones in the reservoir – separated by Fault H and producing fluids of different characteristics – were identified under pre-exploitation conditions. Wells in the area to the east-southeast (south block) produce very high-enthalpy fluids (≥2000 kJ/kg), with very low chloride (≤7000 mg/kg) and high CO₂ (>6‰ molar) and δD (<−94‰). In contrast, wells toward the west-northwest (north block) show moderate-enthalpy fluids (1400–1800 kJ/kg), with high chloride (∼12,000 mg/kg) and relatively low CO₂ (<6‰ molar) and δD (<−94‰). Dilution caused by cooler water entry, boiling due to steam gain, both occurring in the north block, and steam condensation in the south block were identified as the three main reservoir processes associated with exploitation. Also, it was found that the dynamics of the CP IV reservoir is controlled by the Fault H system.     

Geochemical evidence of drawdown in the Cerro Prieto geothermal field

Some wells of the Cerro Prieto geothermal field have undergone changes in the chemistry of fluids produced which reflect reservoir processes. Pressure decreases due to production in the southeastern part of the field have produced both drawdown of lower chloride fluids from an overlying aquifer and boiling in the aquifer with excess steam reaching the wells. These reservoir changes are indicated by changes in fluid chloride concentrations, Na/K ratios and measured enthalpies and by comparisons of aquifer fluid temperatures and chloride concentrations calculated from enthalpy and chemical measurements. Fluid temperatures have not been greatly affected by this drawdown because heat contained in the rock was transferred to the fluid. When this heat is exhausted, fluid temperatures may drop rapidly.     

Production testing in miravalles geothermal field, Costa Rica

Wellbore deposition of calcium carbonate has occurred in the first wells drilled in the Miravalles geothermal field, Costa Rica. Well PGM-1 was flowtested for periods of 1, 5 and 6 months in 1980, 1981 and 1982, respectively. The well is 1300 m deep and produces from a liquid-dominated reservoir at about 240°C. The flowtests illustrate the time behavior of wells suffering from wellbore deposition. At early time the decrease in output flow rate and wellhead pressure is slow but at late time it becomes rapid; the curves are concave down. The ratio (flow rate/wellhead pressure) remains nearly constant with time for wells suffering calcium carbonate wellbore deposition, at least in the Miravalles geothermal field.