Chapter 013, Cerro Prieto Power Station, Baja California Norte, Mexico

Schlumberger and electromagnetic soundings, dipole mapping and audio-magnetotelluric surveys, gravity and magnetic measurements, frequency domain soundings, self-potential surveys, and subsurface temperature determinations. The geochemical program analyzed the thermal fluids of the surface and in the wells. The description and results of exploration methods used during the investigative stages of the Momotombo Geothermal Field are presented.

A conceptual model of the geothermal field was drawn from the information available at each exploration phase. The exploration methods have been evaluated with respect to their contributions to the understanding of the field and their utilization in planning further development.

Overview of Cerro Prieto studies

Geothermal drilling in Cerro Prieto. The number of characteristics of the different wells that have been drilled in the Cerro Prieto geothermal field to date enable one to summarize the basic factors in the applied technology, draw some conclusions, improve systems and procedures, and define some problems that have not yet been satisfactorily solved, although the existing solution is the best now available. For all practical purposes, the wells drilled in the three areas or blocks into which the Cerro Prieto field has been divided have been completed. The wells and their type, the type of constructed wells and the accumulative meters that have been drilled for such wells are summarized.

Exploration and development of the Cerro Prieto geothermal field. A multidisciplinary effort to locate, delineate, and characterize the geothermal system at Cerro Prieto , Baja California, Mexico, began about 25 years ago. It led to the identification of an important high-temperature, liquid-dominated geothermal system which went into production in An evaluation of the different studies carried out at Cerro Prieto has shown that: Directory of Open Access Journals Sweden.

Full Text Available In order to understand the geological and hydrogeological processes influencing the hydrogeochemical behavior of the Cerro Prieto Geothermal Field CP aquifer, Mexico, a characterization of the water samples collected from geothermal wells was carried out.

Different hydrochemical diagrams were used to evaluate brine evolution of the aquifer. To determine pressure conditions at depth, a calculation was performed using hydrostatic and lithostatic properties from CP, considering geological characteristics of the study area, and theoretical information about some basin environments. Groundwater shows hydrogeochemical and geological evidences of the diagenetic evolution that indicate overpressure conditions. Some physical, chemical, textural, and mineralogical properties reported in the lithological column from CP are explained understanding the evolutionary process of the sedimentary material that composes the aquifer.

Relationship between water chemistry and sediment mineralogy in the Cerro Prieto geothermal field: The chemical compositions of waters collected from the Cerro Prieto geothermal production wells and hydrothermal emanations are different. Compared to the Cerro Prieto well waters, the surficial waters generally contain significantly less potassium, slightly less calcium and chloride, and significantly more magnesium and sulfate. In comparison to the unaltered sediments, the changes in the mineralogy of the altered sediments appear to be controlled by the type of emanation well, spring, mud pot, geyser, fumarole, or cold pool.

However, an increase in quartz and potassium feldspar percentages seems to be characteristic of the majority of the sediments in contact with geothermal fluids. Preliminary attempts to model the chemical processes occurring in the Cerro Prieto geothermal field using chemical equilibrium calculations are reported. Then the theoretical mineral composition of altered sediments was predicted and compared to the mineralogy actually observed in the solid samples. The Cerro Prieto geothermal field is a liquid-dominated geothermal reservoir of complex geological and hydrological structure.

It is located at the southern end of the Salton-Mexicali trough which includes other geothermal anomalies as Heber and East Mesa. Although in , the initial power plant installed capacity was 75 MW of electrical power, this amount increased to MW in as field development continued. It is expected to have a generating capacity of MW by the end of , when two new plants will be completely in operation. Questions about field deliverability, reservoir life and ultimate recovery related to planned installations are being presently asked.

Numerical modeling studies can give very valuable answers to these questions, even at the early stages in the development of a field. An effort to simulate the Cerro Prieto geothermal reservoir has been undergoing for almost two years. The final project objective is tosimulate the behavior of the old field region when production from additional wells located in the undeveloped field zones will be used for feeding the new power plants. Geohydrology of the Cerro Prieto geothermal aquifer.

The most recent information on the Cerro Prieto geothermal aquifer is summarized, with special emphasis on the initial production zone where the wells completed in the Alpha aquifer are located. These wells produce steam for power plant units 1 and 2. The location of the area studied is shown. The Alpha and Beta aquifers differ in their mineralogy and cementing mineral composition, temperatures, and piezometric levels. The difference in piezometric levels indicates that there is no local communication between the two aquifers. This situation has been verified by a well interference test, using well E-1 as a producer in the Beta aquifer and well M as the observation well in the Alpha aquifer.

No interference between them was observed. Information on the geology, geohydrology, and geochemistry of Cerro Prieto is presented. Hydrothermal alteration of sediments associated with surface emissions from the Cerro Prieto geothermal field. A study of the mineralogical changes associated with these hydrothermal vents was initiated with the aim of developing possible exploration tools for geothermal resources.

The Cerro Prieto reservoir has already been explored by extensive deep drilling so that relationships between surface manifestations and deeper hydrothermal processes could be established directly. Approximately samples of surface sediments were collected both inside and outside of the vents. The mineralogy of the altered sediments studied appears to be controlled by the type of emission. A comparison between the changes in mineralogy due to low temperature hydrothermal activity in the reservoir, seen in samples from boreholes, and mineralogical changes in the surface emission samples shows similar general trends below C: These mineral assemblages seem to be characteristic products of the discharge from high intensity geothermal fields.

Extension of the Cerro Prieto field and zones in the Mexicali Valley with geothermal possibilities in the future. This study concerns the possible extension of the Cerro Prieto field and identification of other zones in the Mexicali Valley with geothermal development potential by assessing the structural geologic conditions in relation to the regional tectonic framework and the integration of geologic and geophysical surveys carried out at Cerro Prieto.

This study is based on data obtained from the wells drilled to date and the available geological and geophysical information. With this information, a geologic model of the field is developed as a general description of the geometry of what might be the geothermal reservoir of the Cerro Prieto field. In areas with geothermal potential within the Mexicali Valley, the location of irrigation wells with anomalous temperatures was taken as a point of departure for subsequent studies. Based on this initial information, gravity and magnetic surveys were made, followed by seismic reflection and refraction surveys and the drilling of m-deep multiple-use wells.

Based on the results of the final integration of these studies with the geology of the region, it is suggested that the following areas should be explored further: Application of oil- field well log interpretation techniques to the Cerro Prieto Geothermal Field. An example is presented of the application of oil- field techniques to the Cerro Prieto Field , Mexico. The lithology in this field sand-shale lithology is relatively similar to oil- field systems.

The suites of logs for individual wells were far from complete. This was partly because of adverse borehole conditions but mostly because of unavailability of high-temperature tools. Temperature data about the wells were sketchy, and the logs had been run under pre-cooled mud condition. A system of interpretation consisting of a combination of graphic and numerical studies was used to study the logs. From graphical studies, evidence of hydrothermal alteration may be established from the trend analysis of SP self potential and ILD deep induction log.

Furthermore, the cross plot techniques using data from density and neutron logs may help in establishing compaction as well as rock density profile with depth. From this approach, values of formation temperature and mud filtrate resistivity effective at the time of logging were established. Exploration plays an important role in tapping underground natural resources-whether water, oil, natural gas or minerals.

Exploratory data allow us to learn reservoir conditions, increasing probable reserves and reservoir life span. The division sent technical personnel to structurally map the northern and eastern portions of Laguna Salada. The paper offers a general outline of the main zones undergoing exploratory studies-studies perhaps culminating in siting exploratory wells to locate more geothermal resources and ultimately producing them using binary power plants. CFE also wants to site injection wells west of the current production zone, and this is covered, as well.

All activities are meant to increase the productive lifespan of the geothermal reservoir. En las zonas aledanas al campo geotermico de Cerro Prieto , BC, y en general en el Valle de Mexicali, la exploracion estaba practicamente detenida habiendose reactivado a raiz de que la Subgerencia de Estudios de la Gerencia de Proyectos Geotermoelectricos de la Comision Federal de Electricidad CFE envio personal para realizar mapeos estructurales en las porciones norte y oriente de la Laguna Salada.

Este trabajo presenta un panorama general de las areas prioritarias para realizar estudios exploratorios y poder programar, con mas bases, pozos exploratorios enfocados a localizar mas recursos geotermicos, inclusive para generar energia por medio. During the development of Cerro Prieto geothermal field , BC, exploitation has increased gradually, causing a continuous drop in pressure to almost bars in the central and eastern parts of the field. This has occurred despite the high natural recharge induced by the reservoir exploitation and helped by the high permeability of the reservoir and the wide availability of natural recharge of low-temperature water in the vicinity.

The strata above the production zones have significantly lower temperatures than these zones, but due to the particular characteristics of the reservoir, do not have pressure drops. As the pressure of producing strata declines, the hydraulic pressure differential between them and the overlying strata increases. Thus in recent years the phenomenon of cross flow occurs with greater frequency and severity. In this paper, this phenomenon is analyzed, detailing the specific mechanisms favoring it and identifying the stage drilling or workover in which it commonly occurs.

Rigorous supervision during these stages is crucial to identifying cross flow and to taking necessary measures to save the well. Cross flow cases are presented at different stages in the history of a well: Esto ha ocurrido a pesar de la enorme recarga natural inducida por la explotacion, favorecida por la alta permeabilidad del yacimiento y la gran disponibilidad de recarga natural de agua de baja temperatura en los alrededores del mismo.

Los estratos ubicados encima de las zonas productoras presentan temperaturas significativamente menores que estos, pero debido a las caracteristicas particulares del yacimiento, no han presentado abatimiento en su presion. Minimal thermodynamic conditions in the reservoir to produce steam at the Cerro Prieto geothermal field , BC; Condiciones termodinamicas minimas del yacimiento para producir vapor en el campo geotermico de Cerro Prieto , B.

Minimal thermodynamic conditions in the Cerro Prieto geothermal reservoir for steam production are defined, taking into account the minimal acceptable steam production at the surface, considering a rank of mixed-enthalpies for different well-depths, and allowing proper assessments for the impacts of the changes in fluid reservoir pressure and enthalpy.

Factors able to influence steam production are discussed. They have to be considered when deciding whether or not to drill or repair a well in a particular area of the reservoir. These evaluations become much more relevant by considering the huge thermodynamic changes that have occurred at the Cerro Prieto geothermal reservoir from its development, starting in , which has lead to abandoning some steam producing areas in the field.

Se discuten los factores que pueden afectar la produccion de vapor, los cuales se deben tomar en cuenta para determinar la conveniencia o no de perforar o reparar un pozo en determinada zona del yacimiento. Estas evaluaciones adquieren gran relevancia al considerar los enormes cambios termodinamicos que ha presentado el yacimiento geotermico de Cerro Prieto , como resultado de su explotacion iniciada en , lo que ha llevado a abandonar algunas zonas del campo para la produccion de vapor. Cerro Prieto , entalpia, evaluacion de yacimientos, politicas de explotacion, presion, produccion de vapor.

Cerro Prieto is the oldest and largest Mexican geothermal field in operation and has been producing electricity since The large amount of geothermal fluids extracted to supply steam to the power plants has resulted in considerable deformation in and around the field. The deformation includes land subsidence and related ground fissuring and faulting. These phenomena have produced severe damages to infrastructure such as roads, irrigation canals and other facilities.

The DInSAR results were compared with published data from precise leveling surveys and and detailed geological information in order to improve the understanding of temporal and spatial distributions of anthropogenic subsidence in the Mexicali Valley. The leveling and DInSAR data were modeled to characterize the observed deformation in terms of fluid extraction. The results confirm that the tectonic faults control the spatial extent of the observed subsidence.

These faults likely act as groundwater flow barriers for aquifers and reservoirs. The shape of the subsiding area coincides with the Cerro Prieto pull-apart basin. In addition, the spatial pattern of the subsidence as well as changes in rate are highly correlated with the development of the Cerro Prieto Geothermal Field.

Corrosion in pipelines and well casings at the Cerro Prieto geothermal field , BC; Corrosion en tuberias de linea y de revestimiento de pozos del campo geotermico de Cerro Prieto , BC. Miranda Herrera, Carlos A. In the area called Poligono Hidalgo, inside the Cerro Prieto IV zone in the Cerro Prieto geothermal field , BC, corrosion has occurred in the last few years on well casings and pipelines used for geothermal fluids.

These wells have thrown corroded material from their respective casings. With these data we pinpoint corrosive conditions in this field area and determine which pipes are adequate to case wells in similar chemical, production conditions to minimize adverse effects and extend the life of the well, allowing more efficient exploitation of the deepest production zones in the reservoir. Se presentan resultados de pruebas de corrosion con tuberias tipo API L y ASTM A grado B al ser sometidas al condensado de los pozos y , los cuales han estado arrojando material de sus respectivas tuberias de revestimiento.

Con estos datos se pretende conocer las condiciones corrosivas de esa zona del campo y determinar cual seria la tuberia ideal a utilizar en pozos con condiciones quimicas de produccion semejantes a fin de minimizar este efecto adverso y prolongar la vida de los pozos, a la vez que se permita la explotacion eficiente de las zonas productoras mas profundas del yacimiento. Geohydrologic conditions of the shallow aquifer in the Cerro Prieto geothermal field zone; Condiciones geohidrologicas del acuifero somero en la zona del campo geotermico de Cerro Prieto. Based on geohydrologic information reported by Comision Nacional del Agua, Comision Federal de Electricidad and several academic institutions, we have performed an analysis to establish in general terms, the past and present behavior of the shallow aquifer in the Cerro Prieto geothermal zone CPGZ.

The main features of the aquifer, derived from the interpretation of the results obtained with several disciplines, are as follows: The basin is filled with unconsolidated delta deposits interbedded with alluvial sediments from the Sierra Cucapa, which forms an impervious boundary. Because the lack of information about the hydraulic properties of the sediments, just possible ranges of values were obtained for the hydraulic conductivity and storativity corresponding to three deposit environments: De la interpretacion e integracion de los resultados obtenidos de las diferentes disciplinas utilizadas en el desarrollo del estudio, se derivan las siguientes conclusiones: La secuencia litologica en el area de estudio esta constituida por sedimentos no consolidados de origen deltaico interdigitados con sedimentos aluviales provenientes de la Sierra Cucapa, que sirve como frontera impermeable.

La informacion sobre las propiedades hidraulicas de los materiales en el area de interes es limitada, por lo cual solo se determinan los. Cost model for geothermal wells applied to the Cerro Prieto geothermal field case, BC Abstract; Modelo de costeo de pozos geotermicos aplicado para el caso del campo geotermico de Cerro Prieto , BC. Vaca Serrano, Jaime M. A project for drilling geothermal wells to produce electrical energy can be defined as a sequence of plans to get steam or geothermal fluids to satisfy a previously known demand, and, under the best possible conditions, to obtain payment.

This paper presents a cost model for nine wells drilled at the Cerro Prieto geothermal field in and to supply steam to the power plants operating in the field. The cost model is based on the well cost, the initial steam production, the annual decline of steam, the drilling schedule and the break-even point for each well.

The model shows the cost of steam by the ton and the sale price needed to determine the discount rate and the investment return time. Este trabajo presenta un modelo de costeo para nueve pozos en el campo geotermico de Cerro Prieto , que fueron perforados entre y como parte del suministro de vapor para las plantas generadoras que operan en este campo.

El modelo de costeo se basa en el costo por pozo, la produccion inicial de vapor, la declinacion anual de vapor, los intereses de las obras de perforacion y el punto de equilibrio para cada pozo. Los resultados permiten conocer el costo de la tonelada de vapor y el precio de venta para determinar la tasa de descuento y el tiempo de retorno de la inversion. Report of the seismic activity at the Cerro Prieto B. It is constituted by five digital stations with velocity sensors of three components and a natural period of one second.

The network worked continuously until December 2, During this period, we located events in the field area and its neighborhood, with coda-length magnitude 1 Cerro Prieto Dos and Cerro Prieto Tres. The recorded activity is less than in Zone A, focal depths are between five and seven km, and the coda- length magnitude is 1 field , at three and four km from CPOE station, coda- length magnitude 1 field in the outskirts of the Cerro Prieto volcano and to the NW of it.

Trabajo en forma ininterrumpida hasta el 2 de diciembre de Durante ese periodo se localizaron. Configuration of the mudstones, gray- and coffee-colored shale lithologic units, zones of silica and epidote, and their relation to the tectonics of the Cerro Prieto geothermal field. Based on well cuttings, five lithological units have been recognized within the area of what is now the Cerro Prieto geothermal field.

These five units are described. Differences in origin, mineralogy, grading, color, compaction, etc. Criteria to determine the depth of the production interval in wells of the Cerro Prieto geothermal field , Mexico; Criterios para determinar la profundidad del intervalo productor en pozos del campo geotermico de Cerro Prieto , Mexico.

Ways to select the depth of the production interval or to complete wells in the Cerro Prieto geothermal field have changed during the development of the field. From when drilling began to the middle of , a total of wells were drilled. The paper compares the approaches used in the past with those of the last ten years.

The Cerro Prieto system has been classified as being of liquid-dominated and high-temperature. Today, after 33 years of commercial exploitation, it has experienced a series of thermal and geochemical fluid changes making it necessary to modify the ways to select the depth of the well production intervals, according to the observed behavior of the reservoir. The new criteria include the thermal approach, the geological approach, the geochemical approach and a comparative approach with neighboring wells.

If most of these criteria are interpreted correctly, the success of a well is ensured. De , cuando se perforaron los primeros pozos, hasta mediados del se han perforado un total de pozos. En el presente articulo se hara una breve revision de cuales han sido los criterios usados en el pasado y los que se han venido empleando en los ultimos diez anos. El yacimiento de Cerro Prieto ha sido clasificado como de liquido dominante, de alta temperatura, pero actualmente, despues de 33 anos de explotacion comercial, ha sufrido una serie de cambios termicos y geoquimicos en sus fluidos, por lo que ha sido necesario modificar los criterios para seleccionar la profundidad del intervalo productor de los pozos de acuerdo al comportamiento observado en el yacimiento.

Los criterios actuales se dividen en cuatro: Criterio geoquimico y 4. Criterio comparativo de los pozos vecinos. Cuando la mayoria de estos. Update of the basement model of the Cerro Prieto , B. With the aim to actualize the basement model of the Cerro Prieto Geothermal Field , a gravity interpretation has been held. We modeled in 2. Well data concerning lithology and density changes were used to fit the gravity models. Results of this work confirm that the geometry of the basement of Cerro Prieto , corresponds to a structural sequence. From west to east it starts as a big depression, continues with a strong uplift of the basement in the middle sector and finally deepens eastward in steps.

The basement model proposed in the present work, defines a similar trend to that established by Fonseca y Razo , but there are differences in basement depth in some areas. In the present model we interpret basement depths between and m deeper than in previous models to the south and northwest of the actual exploitation zone. Utilizando la informacion litologica obtenida a traves de numerosas perforaciones profundas, se hicieron coincidir, dentro de los mofelos gravimetricos, las principales variaciones litologicas con cambios en la densidad.

Los resultados de este trabajo reiteran que la geometria del basamento del Campo de Cerro Prieto , vista de W a E, corresponde con una secuencia estructural que se inicia con una gran depresion en el W, continua con un fuerte levantamiento y finalmente se extiende con una tendencia a profundizarse hacia el E de forma escalonada. El modelo de basamento derivado del presente trabajo, define una tendencia muy similar al modelo de basamento establecido Fonseca y Razo, , pero difiere en cuanto a la profundidad en algunos sectores del campo.

En el actual trabajo se interpreta una profundidad entre y m mayor hacia el sur y noroeste de la. Fourteen new wells have been drilled there since The wells in CP IV zone produce two-phase fluids at wellhead with heterogeneous steam fraction characteristics: This work studies the gas compositions of produced fluids to obtain reservoir parameters such as temperature and steam fraction and identify different sources of fluids in the wells. Data are aligned in a FT-HSH3 trend, suggesting that the well discharges consist of a mixture in different proportions of the two end members.

One seems to be a liquid with a temperature of over degrees Celsius with negative or negligible excess steam. The other seems to be a two-phase fluid with a temperature of about degrees Celsius and an excess steam fraction of about 0. According to the data for single wells and depending on the production conditions of the wells, reservoir fluid mixtures could occur in different proportions of liquid and steam. Data for that included wells drilled after suggest the presence of a steam phase in the reservoir.

The steam could be generated with the boiling of deep reservoir fluid from a pressure drop. The mixing trend obtained for the natural state was also seen for data but lower temperatures from to degrees Celsius were obtained compared with those for natural conditions. The entry of lower. Steam silencers have been placed on the power units regulation system of the Cerro Prieto geothermal field , but not where the steam-pipes vent high-pressure steam rupture disks. Moreover, the power-unit silencers are large and their abilities to reduce noise depend on an external, uncontrollable factor: Thus a compact, economic and relatively easy to construct metallic silencer was designed to be used at several points of dry-steam venting to minimize the noise.

The design was based on orifice-plate equations and a prototype was constructed and tested. It proved capable of reducing noise by 30 decibels for atmospheric discharges of primary steam at operating pressures. The size of the silencer can be adjusted to the particular needs of each vent case. Ademas, los silenciadores de las plantas son muy robustos y su capacidad de reduccion del ruido depende de un factor externo no controlable, como es el tamano del poro de la roca volcanica con el que estan construidos.

Por lo tanto, se diseno un silenciador metalico compacto, economico y relativamente facil de fabricar, para utilizarse en diversos puntos de descarga de vapor seco a fin de minimizar el ruido. El diseno se realizo con base en las ecuaciones de la placa de orificio. Se construyo un prototipo cuyas pruebas demostraron que es capaz de reducir el ruido hasta en 30 decibeles, al descargar desde la presion de operacion de vapor primario hacia la presion atmosferica.

Las dimensiones del silenciador pueden ajustarse a las necesidades particulares de cada caso de desfogue. Overview of Cerro Prieto studies. The studies performed on the Cerro Prieto geothermal field , Mexico, since the late 's are summarized. Emphasis is given to those activities leading to the identification of the sources of heat and mass, the fluid flow paths, and the phenomena occurring in the field in its natural state and under exploitation.

Hydraulic model of the steam-lines network of the Cerro Prieto , B. The steam-line network of the Cerro Prieto geothermal field is composed of wells, and of the wells are integrated and connected by pipes. Thirteen power units, with an installed electrical capacity of MW, are fed by that network. The network length is km, including pipes of several diameters with branches and interconnections. The extension and complexity of the steam-line system make it difficult to analyze the transport and supply of steam to the power plants.

For that it was necessary to have a tool capable of analyzing the system and the performance of the network as a whole, as well as the direction and flow volumes in each part of the system. In this paper, a hydraulic model of the Cerro Prieto steam-line network is presented. The model can determine the performance of the whole network by quantifying the pressure drops, flows and heat losses of the components.

The model analyses the consequences of changes in operating conditions, steam production, maintenance activities and design such as the integration of new wells. It is used to model systems and pipe networks for steam- and condensate-transport. Por medio de esta red se alimentan 13 unidades generadoras de electricidad con una capacidad total instalada de MWe. La red tiene una longitud aproximada de kilometros y esta compuesta por tuberias de diferentes diametros, ramales, interconexiones, etc.

La complejidad y extension del sistema de vaporductos hace muy dificil el analisis del transporte y suministro de vapor a las plantas generadoras. Lo anterior creo la necesidad de contar con una herramienta que ayudara en el analisis del sistema con el fin de. Geological and production analyses focused on exploration of the eastern part of the Cerro Prieto geothermal field , BC; Analisis geologico-productivo enfocado a la exploracion de la parte oriental del campo geotermico de Cerro Prieto , BC.

The eastern part of the Cerro Prieto geothermal field CGCP , known as Poligono Nuevo Leon, is an area with proven geothermal resources, as confirmed by seven directional wells located toward the east and by vertical well M located inside the polygon. Well M was drilled in and has produced about 4 million tons of steam to date. In the last few years, the steam has come only after increasing the number of production wells located in the eastern zone of CGCP Rodriguez, , where pressure, enthalpy and temperature conditions are better than in other parts of the field.

However in the long term it will be necessary to incorporate Poligono Nuevo Leon into the productive area to expand the productive life of CGCP. This paper includes a geological analysis, plus models for steam production, temperature and enthalpy for Poligono Nuevo Leon. El suministro de vapor en los ultimos anos se ha logrado cubrir aumentando el numero de pozos en operacion localizados en la zona oriente del CGCP Rodriguez, , ya que es aqui donde hay condiciones de presion, entalpia y temperatura del yacimiento que son mejores que en otras areas del campo. Update of the conceptual geological model for the geothermal reservoir in Cerro Prieto , BC; Actualizacion del modelo geologico conceptual del yacimiento geotermico de Cerro Prieto , BC.

An updated, conceptual geologic model is presented for the geothermal reservoir in Cerro Prieto geothermal field. Called the Cerro Prieto basin, it includes listric faults, predominately northwest-southwest trending, stepped generally to the northeast. The zone of cortical weakness, formed during the Tertiary, allowed an intrusion of basic rock associated with the magnetic anomaly know as Nuevo Leon.

The intrusive rock has been fed by new magmatic intrusions originated by the present tectonic extension of the Gulf of California. The oldest rocks identified in the area are gneiss and biotite-schists of Permian-Jurassic age and tonalities of Jurassic-Cretaceous age in contact with Cretaceous granites, all representing the regional basement. The lithologic column in the subsurface of the Cerro Prieto basin in formed by a basement of Cretaceous granites; an argillaceous package resting on the basement composed of gray shales with interleaves of sandstone, Tertiary brown-shales and mudstone, with an average thickness of m; clastic sediments of the Quaternary age deposited mainly by the Colorado River and alluvial fans of the Cucapa Range, comprised of gravel, sands and clays with an average thickness of m, covering the shales.

The fluids feeding the geothermal reservoir heat as they pass though the zone where the basic intrusive is located the heat source and migrate through the listric faults toward the permeable layers of sandstone located within the gray shales. Studies for recovering injection capacity in wells of the Cerro Prieto , BC, geothermal field ; Estudios para recuperar la capacidad de aceptacion en pozos inyectores del campo geotermico de Cerro Prieto , BC.

As in geothermal fields around the world, at Cerro Prieto geothermal field , Baja California, former exploratory and production wells are used to inject residual brine. Since the s, studies and activities have been carried out to find ways to recharge the reservoir and dispose of brine without harming the environment or underground aquifers. These include infiltration and cold-and-hot injection.

Some of the studies are presented here, including analyses of litho-facies; core samples; pressure, temperature and spinner logs; well tests and injection rates-plus some studies on the shallow aquifer. All have been useful in fulfilling requirements made by environmental authorities. Because injection rates constantly decrease due to formation damage, it is proposed an additional technique be used to reduce such damages and prolong the lifetime of cold-and-hot injection wells-while ensuring the environment and shallow aquifers are not affected.

Desde la decada de los 80 se han realizado diversos estudios y acciones en ese campo geotermico para recargar al yacimiento y para disponer del fluido residual, sin ocasionar danos al ambiente ni a los cuerpos hidricos del subsuelo, que van desde la infiltracion hasta la inyeccion en frio y en caliente.

Este articulo presenta los diferentes estudios realizados con ese objetivo en el campo, incluyendo el analisis de litofacies, de nucleos de formacion, de registros de presion, temperatura y spinner, las pruebas en pozos y analisis de tasas de aceptacion, asi como los efectuados en el acuifero superficial. Todos ellos han sido de utilidad para atender los requerimientos de las autoridades ambientales.

Finalmente, y en virtud de que las tasas de aceptacion de los pozos. The wells of this zone produce two-phase fluids, with different characteristics as far as their steam fraction content; in the central part and towards the NW the fluids are of dominant liquid type whereas towards the E and the S, the fluid contains a relatively higher steam fraction.

The results of gaseous equilibrium FT- HSH3 showed that the fluids in the deposit are at temperatures between Celsius degrees and Celsius degrees and contain steam fractions between 0. The data found for the natural state are aligned in a tendency defined in the diagram FT- HSH3, that suggest the wells receive different proportions from preexisting steam in the deposit, which are mixed with the liquid phase to produce the observed discharges. The present data, besides showing the presence of deposit steam, also indicates the entrance of lower temperature fluid in the central part of sector CP IV.

Los pozos de esta zona producen fluidos bifasicos, con diferentes caracteristicas en cuanto a su contenido de fraccion de vapor: Los resultados de equilibrio gaseoso FT-HSH3 mostraron que los fluidos en el yacimiento se encuentran a temperaturas de entre grados Celsius y grados Celsius y contienen fracciones de vapor de entre -. Los datos hallados para el estado natural se alinean en una tendencia definida en el diagrama FT-HSH3, que sugiere que los pozos reciben diferentes proporciones de vapor pre-existente en el yacimiento, que se mezclan con fase liquida para producir las descargas que se observan.

Los datos actuales, ademas de mostrar la presencia de vapor de yacimiento, tambien indican la entrada de fluidos de. A full scale, wellhead Biphase turbine was manufactured and installed with the balance of plant at Well of the Cerro Prieto geothermal resource in Baja, California. The Biphase turbine was first synchronized with the electrical grid of Comision Federal de Electricidad on August 20, The Biphase power plant was operated from that time until May 23, , a period of 2 years and 9 months.

A total of 77, kWh were delivered to the grid. The power plant was subsequently placed in a standby condition pending replacement of the rotor with a newly designed, higher power rotor and replacement of the bearings and seals. The maximum measured power output of the Biphase turbine, kWe at psig wellhead pressure, agreed closely with the predicted output, kWe. The design relations used to predict performance and design the turbine were verified by these tests.

The performance and durability of the Biphase turbine support the conclusion of the Economics and Application Report previously published, Appendix A. The newly designed rotor the Dual Pressure Rotor was analyzed for the above power condition. The Dual Pressure Rotor would increase the power output to kWe by incorporating two pressure letdown stages in the Biphase rotor, eliminating the requirement for a backpressure steam turbine.

The power plant availability was low due to deposition of solids from the well on the Biphase rotor and balance of plant problems. A great deal of plant down time resulted from the requirement to develop methods to handle the solids and from testing the apparatus in the Biphase turbine. Finally an online, washing method using the high pressure two-phase flow was developed which completely eliminated the solids problem. The availability of the Biphase turbine itself was Comments on some of the drilling and completion problems in Cerro Prieto geothermal wells.

From to the present, 85 wells with a total drilling length exceeding , m have been constructed at Cerro Prieto , a modest figure compared to an oil field. This activity took place in five stages, each characterized by changes and modifications required by various drilling and well-completion problems. Initially, the technical procedures followed were similar to those used in the oil industry. However, several problems emerged as a result of the relatively high temperatures found in the geothermal reservoir.

The various problems that have been encountered can be considered to be related to drilling fluids, cements and cementing operations, lithology, geothermal fluid characteristics, and casings and their accessories. As the importance of high temperatures and the characteristics of the geothermal reservoir fluids were better understood, the criteria were modified to optimize well-completion operations, and satisfactory results have been achieved to date.

Well MA has been the most productive in the Cerro Prieto field , producing around 40 million tons of fluid. The well went on line in February and was in continuous operation until June Monthly measurements for over 30 years of operating conditions and chemical and isotopic analyses of the fluids allow identification of the most significant processes occurring in the zone where the well is located. Three dominant recharge types have been identified, each lasting about 10 years. They are a a recharge of fluids of lower temperature than the production fluids, even though the chemical and isotopic compositions are similar; b a recharge of lower-temperature fluids with diluted chemical and isotopic compositions; and c a progressive recharge of reinjected water with a higher chloride concentration and heavy isotopic composition.

The production and reinjection rates for over 30 years of production history in the MA well zone allow for speculation of a reservoir pressure recovery, which is confirmed with a calculation using production data from well MA. The evolution of the reservoir pressure in the zone contrasts with the rest of the reservoir, where a constant pressure drop is observed, particularly toward the central and eastern parts of the field.

Fue integrado a produccion en febrero de y hasta junio de ha permanecido en produccion en forma continua. Las mediciones mensuales de sus condiciones operativas y los analisis quimicos e isotopicos del agua producida permiten identificar los procesos mas significativos que han ocurrido a lo largo de esos mas de 30 anos en la zona donde se localiza el pozo. Se han identificado tres tipos de recarga dominantes que se presentan durante un periodo cercano a 10 anos cada uno, los cuales son: Analysis of the automation and control of the well production conditions in the Cerro Prieto geothermal field , Mexico; Analisis de la automatizacion y del control de las condiciones de produccion de los pozos del campo geotermico de Cerro Prieto , BC, Mexico.

Through 31 years of development of the Cerro Prieto geothermal field , the number of wells in operation and the distances between them has been increasing. Now there are wells producing a mixture of water and steam at different ratios, depending on the reservoir characteristics pressure, temperature, depth, etc. Therefore, Comision Federal de Electricidad CFE has started a pilot plan to handle the automation and control of the operating conditions of production wells, and to install a data production acquisition system.

The initial results of the system are in this paper. Ahora hay mas de pozos produciendo una cantidad de mezcla de agua y vapor a diferentes rangos, dependiendo de las caracteristicas del yacimiento presion, temperatura, profundidad, etc. Como resultado la Comision Federal de Electricidad CFE ha dado inicio a un plan de pruebas para llevar a cabo la automatizacion y control de las condiciones de operacion de los pozos productores, e instalar un sistema con equipos para recoleccion de datos del campo, cuyos primeros resultados se presentan en este articulo.

The goal is to trace the origin of reservoir fluids and to place paleoclimate and structural-volcanic constraints in the region. Radiogenic in situ 4 He production age modeling at LTV, combined with the presence of radiogenic carbon 1. CP geothermal water is composed of infiltrated Colorado River water with a minor impact by halite dissolution, whereas a vapor-dominated sample is composed of Colorado River water and vapor from deeper levels.

Exploration and development of Cerro Prieto. A brief retrospective of the exploration and field model development at Cerro Prieto are presented. Representative field models are presented for each of the work phases. A model integrating information from these studies was developed.

Even though geothermal energy is a renewable energy source that is seen as cost-effective and environmentally friendly, emissions from geothermal plants can impact air, soil, and water in the vicinity of geothermal power plants. The objective of this study was to evaluate whether the emissions generated by the geothermal complex have increased the soil mercury concentration in the surrounding areas. Additionally, four soil depth profiles were performed in the vicinity of the complex. The mercury concentration ranged from 0. Our results show that the activities of the geothermal complex have led to an accumulation of mercury in the soil of the surrounding area.

More studies are needed to determine the risk to human health and the ecosystems in the study area. Hydrothermal alteration zones and present reservoir conditions: Geological factors are as essential for locating new wells as they are for defining the production zones of these wells. It has been observed that the continuous variation of reservoir thermodynamic conditions temperature, pressure and enthalpy is due mainly to the exploitation of geothermal resources.

The presence of new thermodynamic conditions recorded at the reservoir has led to the re-location of production wells originally located during the drilling campaign of to In turn, this is related to the current, superior, thermodynamic reservoir conditions. Based on this, a new geologic approach is proposed to define possible production zones for new wells, relating the ZAH zones to current thermodynamic reservoir conditions.

Por otra parte, tambien se ha constatado que la continua variacion de las condiciones termodinamicas del yacimiento temperatura, presion y entalpia es originada en buena medida por la explotacion del recurso geotermico. La ocurrencia de nuevas condiciones. Heat losses estimation associated with the physical state of the thermal insulation of pipes vaporductos network in Cerro Prieto geothermal field ; Estimacion de perdidas de calor asociadas al estado fisico del aislamiento termico de las tuberias de la red de vaporductos del campo geotermico Cerro Prieto.

The Cerro Prieto Geothermal Field CPGF steam transportation network is constituted by km of pipes with diameters ranging from 8 to 48 inches, which transport the steam of producing wells to 13 power plants which have a total installed capacity of MWe. Originally, the pipes are thermally insulated with a mineral wool or fiber glass layer and an external aluminum or iron cover.

Due to the insulation material has been exposed to weather conditions during the field operation it shows nowadays different grades of wear-out, or even it is lacking in some parts of the network, causing higher heat losses from the pipes to the environment. In this work, the magnitude of the heat losses related with the present condition of the thermal insulation throughout the pipeline network is assessed.

This involved determining the longitude and diameter, as well as the insulation condition of each single pipeline section, and the calculation of the heat transfer coefficients for the different thermal insulation conditions defined for this study. Originalmente, estas tuberias son aisladas termicamente con una capa de 2 pulgadas de material aislante a base de lana mineral o fibra de vidrio, y una proteccion exterior de aluminio o hierro. Debido principalmente al impacto de las condiciones meteorologicas durante el tiempo de operacion del campo, en algunas porciones de la red el aislante presenta actualmente distintos grados de deterioro, o incluso se encuentra ausente, lo cual se traduce en una mayor perdida de calor desde las tuberias hacia el medio ambiente.

En el presente trabajo se evalua la magnitud de las perdidas de calor asociadas al estado fisico del aislamiento termico de las. After 35 years of exploitation of the CP-I Cerro Prieto I sector of the Cerro Prieto geothermal field , its thermal conditions have been declined substantially; the number of production wells is reduced and exploitation has been abandoned in some zones.

However with a binary cycle, it is still possible to generate electricity from the remaining reservoir energy. To do so, one needs to know a detailed reservoir history of the sector, identifying the best potential strata to be assessed. In this paper, the CP-I reservoir evolution is presented and its exploitation and main reservoir characteristics -which play important roles- are identified. The CP-I zones offering the best conditions for binary cycle exploitation are identified.

Exploiting partially abandoned reservoir sections is crucial for the future of the field. If such sections can be made to produce, this will help reduce the progressive steam-production decline in the occurring since production began. The production decline had been mitigated by a large area available for drilling replacement production wells, but such an area is limited now, due to the vast number of operating wells.

Sin embargo, aun es posible aprovechar la energia que contiene este sector del yacimiento para la generacion de electricidad mediante ciclo binario. Para ello primeramente se debe conocer en detalle cual ha sido la evolucion del yacimiento en este sector, identificando los estratos de mejor potencial para evaluarlos posteriormente. En este estudio se presenta la evolucion del yacimiento de CP-I como respuesta a su explotacion, identificando las principales caracteristicas del yacimiento que determinan su comportamiento.

Determination of heat losses in the Cerro Prieto , Baja California, geothermal field steam transportation network based on the thermal insulation condition of the steam pipelines; Determinacion de perdidas de calor en la red de transporte de vapor del campo geotermico de Cerro Prieto , Baja California, con base en el estado fisico del aislamiento termico de vaporductos. In Cerro Prieto Geothermal Field CPGF , the steam from producing wells is transported to power plants through a large and complex system of pipes thermally insulated with a 2 inches thick mineral wool or a fiber glass layer and an external aluminum or iron cover.

The insulation material has been exposed to weather conditions during the field operation and has suffered density and thickness changes. In some cases the insulation has been lost completely, increasing heat transfer from the pipes to the environment. This paper analyzes the impact of the conditions of thermal insulation on heat losses in the CPGF steam-pipeline network.

The heat losses are calculated by applying an iterative method to determine the surface temperature based on a heat balance calculated from the three basic mechanisms of heat transfer: Finally, using length and diameter data corresponding to the condition of the thermal insulation of each pipeline-and field operation data, the overall heat losses are quantified for steam lines throughout the pipeline network in the field.

The results allow us to evaluate the magnitude of the heat losses in comparison with the overall energy losses occurring during steam transport from wells to the power plants. Debido a la exposicion a las condiciones meteorologicas a traves del tiempo de operacion del campo, el aislamiento ha experimentado cambios en su densidad y espesor y en ocasiones se ha perdido por completo, lo cual repercute en una mayor transferencia de calor de las tuberias hacia el medio ambiente. Evolution of the Cerro Prieto geothermal system as interpreted from vitrinite reflectance under isothermal conditions.

Temperature estimates from reflectance data in the Cerro Prieto system correlate with modern temperature logs and temperature estimates from fluid inclusion and oxygen isotope geothermometry indicating that the temperature in the central portion of the Cerro Prieto System is now at its historical maximum.

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Isoreflectance lines formed by contouring vitrinite reflectance data for a given isothermal surface define an imaginary surface that indicates an apparent duration of heating in the system. The distribution of hydrothermal minerals, mineral assemblages and fluid inclusion data were taken from drill cuttings from the production zone of wells all over the Cerro Prieto geothermal field. The production zone has been termed the silica-epidote mineralized zone SEMZ , and is located in the deep part of the gray shale where thick layers of sandstone are found.

Common mineral assemblages show three temperature ranges in the SEMZ: Las muestras de recortes y nucleos de perforacion estudiados provienen de la zona de produccion a la que se le ha denominado Zona Mineralizada de Silice-Epidota ZMSE , que se encuentra en la parte profunda de la lutita gris con importantes horizontes de areniscas. En esta zona las asociaciones parageneticas mas comunes han mostrado tres intervalos de temperatura para la ZMSE: Evaluation of geologic characteristics at Cerro Prieto.

Computerized well-log analysis of Cerro Prieto has led to the identification of a relatively large and irregular low-density volume extending from near the surface in the vicinity of Laguna Volcano to greater depths toward the northeast. This low-density volume is located about a plane of symmetry of a self-potential anomaly and a group of northeast trending active faults. The presence of a volume of relatively high-density rock has been recognized at shallow depths in the initially developed part of the resource.

It is believed to be due to minerals deposited by cooled, rising geothermal brine. Storativity calculated from well logs at the south end of the western part of the field shows acceptable comparison with storativity calculated from well tests. The amount of fluid produced from the field during the period is greater than the amount in situ calculated from the completed interval thicknesses. Because this part of the field is still producing today, fluid must be recharging this part of the field.

The data showed a 'basement' high in the production area which was interpreted to be the result of a shallow zone of densified sediments Goldstein, In , Elders and his group at the University of California at Riverside UCR began thorough petrological and light isotope studies of well cuttings and cores. A number of regularly distributed metamorphic mineral zones were recognized in the field. The progressive changes in mineralogy indicated a systematic relationship with reservoir temperatures Elders et al.

The isotope studies indicated that the 8'80 for calcite in sandstones could be used to estimate stable reservoir temperatures and that the same parameter in shales could indicate the extent and distribution of subsurface flow Olson, Other results of this work indicated that the Cerro Prieto hydrothermal system is relatively open to fluid flow.

The analysis of cuttings and cores continued at UCR over the entire 5-year cooperative program, adding vitrinite, fission-track annealing and geophysical well log studies to the petrologic and light isotope work begun earlier Elders et al. Mainly on the basis of the results of these studies, Elders et al, in press developed a model for the natural preproduction flow regime at Cerro Prieto Fig.

A heat source for the hydrothermal system was suggested to lie to the northeast in an area where wells drilled through mafic and silicic dikes. From that source an inclined plume of hot water ascends toward the southwest, boiling in places and precipitating minerals which tend to seal the rock pores. The plume discharges upward and horizontally to the southwest creating surface manifestations and shallow zones of temperature reversals. The model also shows cold water recharge from the northeast which may also seal the reservoir pores as these waters are heated.

Southwest - northeast section across the Cerro Prieto field showing the flow regime proposed by Elders et al. In a study of the paleomagnetism of the rhyodacitic rocks of the Cerro Prieto volcano was made. Magnetic polarity and paleopole position data suggested that the volcanism was initiated about , years ago.

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The youngest volcanics reaching the surface cooled about 10, years ago de Boer, The analysis of geophysical well logs was conducted during the entire length of the program as new wells were completed. Noble and Abril described the problems found in interpreting logs obtained from Cerro Prieto wells. Later, and along the same line Ershaghi et al. Using mainly wireline well logs and surface geophysical data Lyons and van de Kamp developed a geologic model of the area that was significantly more detailed than any of the earlier models.

They proposed three groups of faults of different ages and presented a depositional model for the reservoir that showed a transition from a deltaic environment in the east, to a marine environment in the west. These authors gave well log evidence that disputed the basement-horst structural model; they showed that the gravity high over the reservoir is due to the high densities of the indurated shales of Unit B.

They also showed that the reservoir is not overlain by a laterally continuous low-permeability layer, that the reservoir sandstones present secondary porosity and that the production intervals in the wells generally straddle or underlie the top of the high-density and high-resistivity shales. Lyons and van de Kamp seem to have been the first to suggest that the positive magnetic anomaly, approx.

The degree and distribution of hydrothermal alteration, and M. L i p p m a n n the origin and distribution of dissolution secondary porosity in the field were discussed by Vonder H a a r , Teihnan and Cordon briefly described how the geologic model of Cerro Prieto evolved as new exploration data became available.

It showed two types of faults: On the basis of new data, this map was somewhat modified during a second workshop Zelwer, Overview o f Cerro Prieto Studies Recently a micropaleontological analysis of 30 core samples was completed Ingle, The distributed patterns of ostracodes and foraminifera, together with the lithofacies studies of Lyons and van de Kamp , indicated that the sediments at Cerro Prieto represent a complex of alluvial, deltaic, estuarine and shallow marine environments.

These sediments were deposited along the front of the Colorado River delta as it prograded across the Salton Trough during the Pliocene and Pleistocene.

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The existence of reworked Cretaceous specimens confirmed that a significant part of the deltaic sediments in the area derived from the Colorado Plateau. After an extensive analysis of geophysical and lithological well logs, Halfman et al. By superimposing on the model downhole temperature and well completion data it was possible to establish the movement of the geothermal fluid in the field, prior to its exploitation Fig.

The hot fluid appears to enter the system from the southeast, from the area where igneous dikes are believed to have intruded into the valley fill Lyons and van de Kamp, ; Elders et al.

Overview of Cerro Prieto studies

Then, the fluid moving westward through permeable layers ascends gradually to shallower depths by flowing upward through faults and sandy gaps in shaly layers acting as local cap rocks. Eventually, in the western part of the field, part of the geothermal fluid leaks to the surface and the rest mixes with colder groundwaters. Halfman's model seems to indicate that the Michoachn fault Fig. This study has identified three faults called H, L, and S on Fig. Reflection seismic data Majer and McEvilly, in preparation , the location of boiling zones in the reservoir Nehring and D'Amore, in press and the result of numerical modeling studies Lippmann and Bodvarsson, have substantiated the existence of these faults.

The model has also shown, confirming the conclusions of others, that there is no laterally continuous cap rock at Cerro Prieto which prevents the leakage of geothermal fluids from the reservoir. In the first LBL detailed seismological study was conducted at Cerro Prieto; two others were made in and Majer and McEvilly, , , in preparation.

In addition, in late a downhole geophone was installed in a m-deep well near well M The purpose of this work was to determine the relation of wave propagation characteristics and microearthquake activity to the production zone, recharge areas, heat source and general lithology and structure of the field. The events registered in indicated strike-slip movement along n o r t h w e s t - s o u t h e a s t trending faults. The results of later surveys did not permit establishing the type of faulting associated to the monitored events.

The seismic events were clustered near the center of the production area, at depths from 2 to 5 km, on a fairly well defined north - south plane extending from well M to the power plant. On the other hand, the events were distributed in a rather diffuse pattern concentrated on the western edge of the field.

The diffuse pattern of the events and the A ': Location of the main faults controlling the subsurface flow of geothermal fluids in the Cerro Prieto field from Halfman et al. Also shown is the position of cross-section A - A ' given in Fig. During the to period four precision gravity surveys were conducted at Cerro Prieto to study the possibility of geothermally induced subsidence Grannell et al.

Comparison of results from year to year showed subsidence clearly associated with the June earthquake. Also, significant changes were observed at several stations in the western older part of the field. The pattern of these latter changes is elliptical in a northwest-southeast direction parallel to the structural grain and faulting in the region.

L i p p m a n n correlating precision gravity and leveling data, Zelwer and Grannell in preparation concluded that changes in gravity and surface elevation correlate well, and were primarily produced by compaction that occurred in response to the June earthquake.

However, Grannell et al. Zelwer and Grannell in preparation also indicated that a density increase is taking place in the reservoir zone because of mineral precipitation or the entrance of cooler, denser, waters. Further analysis of the data will be required to clarify the importance of fluid extraction on the subsidence observed at Cerro Prieto.

A twodimensional vertical resistivity model along a n o r t h e a s t - s o u t h w e s t line across the field line E - E ' was developed Wilt et al. It showed that the production region is characterized by high resistivity 4 ohm-m relative to the surrounding region 1 to 2 ohm-m.

The higher resistivity is assumed to be related to the densification of the shales in Unit B. Top of A B horizon Two-dimensional resistivity model for a southwest northeast line crossing the Cerro Prieto field Line E E ' ; Wilt and Goldstein, The studies improved the resistivity model for the eastern part of the field Wilt and Goldstein, It revealed that the 4 o h m - m body associated with the producing zone dips eastward at 30 to 50 degrees to a depth greater than 2 km Fig. A narrow, steeply dipping 1. Repetitive high-precision d i p o l e - d i p o l e resistivity measurements taken during the period to over line E - E ' showed a consistent pattern of apparent resistivity changes.

A zone of increasing resistivity is related to the production region; above and flanking this region are zones of decreasing resistivities. There is also an unresolved component that might relate to ascending hot fluids at the eastern edge of the producing zone Wilt and Goldstein, in press.

Overview o f Cerro Prieto Studies The changes in apparent resistivities detected at Cerro Prieto from the d i p o l e - d i p o l e monitoring surveys were studied by Pruess et al. Based on schematic one- and two-dimensional reservoir simulations, assuming the existence of waters of different salinity and temperature, changes in formation resistivity were computed which were then transformed into changes in apparent resistivity that would be observed at the surface.

Over a period of three years starting in , three magnetotelluric MT studies were made in the area of the field. The initial purpose of the MT soundings was to provide additional and deeper subsurface resistivity data to supplement the d i p o l e - dipole and Schlumberger surveys c o n d u c t e d in the region. S u b s u r f a c e resistivity models developed jointly f r o m the d i p o l e - d i p o l e resistivity and MT data sets helped define the deep resistivity structure of the field. The models also showed: Later MT studies were conducted to help delineate a possible boundary on the south side of the thermal area.

Recently Goldstein et al. Their modeling studies, coupled with the analysis of cuttings of mafic dikes recovered from well NL-I, and viewed in conjunction with other geological and geophysical data, gave further support to the hypothesis that Cerro Prieto is located in a pullapart basin like others in the Salton Trough and the Gulf of California Lomnitz et al. These authors estimate that the top of the main magnetic source body is approx.

On the basis of preliminary thermal modeling results, Elders et al. The model age for such an event is 40, to 50, years, which is in the range of the age for the volcanic activity at Cerro Prieto de Boer, The depth is shallower than the 9 to l0 km depth of the present magma inferred by Goldstein et al. Above the intrusive, Elders postulated the existence of a sheeted dike complex which would correspond to the dikes found in some of the deep wells in the eastern part of the field.

Measurement of horizontal ground surface movements in the northern part of the Mexicali Valley and around Cerro Prieto began in Massey, a, b, Inward movement toward the center of the production area was observed during the to study period. The movement, probably not exceeding 6 or 7 cm, occurred mainly between and and was confined to the production area. A consistent change in the length of the survey lines crossing the well field, similar to the general shortening detected in and , was not observed over the to period. Massey indicated that the ground surface movements in the production area were due to fluid extraction.

However, an increase in the rate of deformation resulting from the continuous exploitation of the field was not evident for the longer time period. Outside the wellfield ground deformation, excluding earthquake related movements, was small. Their analysis showed that production-induced deformations at the field could not be observed or were masked by earthquake-related changes. L i p p m a n n In cores from Cerro Prieto were studied in the laboratory to determine the permeability, thermal conductivity and deformation properties of reservoir rocks.

Later, cores were tested under simulated in situ conditions to measure porosity reduction and establish short- and long-term compaction triggered by reservoir drawdown Abou-Sayed et al. It was found that the rocks behaved as expected with respect to instantaneous compaction and that they present a tendency to compact further with time creep. The results obtained when extrapolated to a reservoir life of 20 to 30 years showed that reductions of several per cent in porosity and several tens of per cent in permeability are possible. Instantaneous compactionrelated subsidence might be small; however, creep-related subsidence might be significant at Cerro Prieto Schatz, in preparation.

A review of the studies carried out both by U. The isotopic studies indicated local recharge from the area immediately to the west of the field and leakage of shallower waters into the reservoir. The tritium and " C contents in the fluids could only suggest an average age between 50 and 10, years Truesdell et al. The work by Mazor and Mafi6n indicated regularities and correlations between different ions in the Cerro Prieto fluids, indicating concentration-dilution processes, such as steam loss or mixing with condensed steam and possibly fresher waters from shallower aquifers.

The data also suggested that the deep brine has a chloride content of about 10, ppm, that no significant reactions occur during the ascent of fluids through the wells and that temperature zonation existed in the reservoir. The study of the composition of the gases produced from the wells began in Nehring and Fausto, By analyzing the spatial distribution of different gases in the field and using gas geothermometers, Nehring and D'Amore in press interpreted that in a large boiling zone existed in the center of the production area.

Changes between and seem to indicate that some colder groundwater is leaking into the reservoir, and that the boiling zone is not expanding and possibly receding because some of the wells have been taken out of production. The study showed that this mixture circulated deeply and was extensively altered compositionally by high-temperature reactions involving reservoir rocks. The surface manifestations were studied by Valette and Esquer-Patifio Five types of surface emissions, each of different chemical characteristics, were identified along the western edge of the production area.

A similarity in isotopic composition and ratios of major elements was observed between the hot springs and the producing wells, suggesting that the geothermal reservoir and the springs are closely related. This relationship is also indicated by the chemical changes observed in the hot springs after fluid production began in the field Valette and Esquer-Patifio, The hydrothermal alteration of the sediments associated with the surface manifestations appears to be controlled by the type of emanation.

However, most of the sediments in contact with the geothermal fluids show an increase in quartz and potassium feldspar. Preliminary attempts at modeling the chemical Overview o f Cerro Prieto Studies processes and studying the reactions between sediments and fluids of the surface manifestations using chemical equilibrium calculations were made by Valette-Silver et al. Nehring and Valette-Silver in preparation studied the differences in gas composition between well and surface manifestation samples. Mazor and Truesdell in press used radiogenic and atmospheric noble gases as tracers to develop a dynamic model for the field.

They suggested that the geothermal fluids are dominated by meteoric waters which have penetrated to more than m depth and then mixed with radiogenic helium and argon formed in the reservoir rocks. Afterwards small amounts of steam are lost by continuous removal as it forms a Raleigh process and mixing occurs with shallower colder waters. The radon and ammonia concentrations in the produced fluids were analyzed by Semprini and Kruger , in press.

The data show that a correlation exists between the radon content and the specific volume of the reservoir fluids. They studied the variations in time and space of the radon concentration to determine changes in the field due to exploitation. Supported by other reservoir data, their analysis indicates an expansion of the two-phase zone in the northeastern part of the field and a decrease in the southeast. Their model indicates that this reservoir is bounded below by low-permeability rocks and above, and to the sides, by an interface with cooler water; there is no continuous permeability barrier around or immediately above the reservoir, and permeability within the reservoir is predominantly intergranular.

They also show that in the natural state, reservoir cooling takes place mainly by mixing of cooler water with the hot water, rather than by boiling. Production has caused displacement of hot water by cooler water rather than by vapor. Local boiling occurs near most wells in response to pressure decreases, but no extended vapor zone has formed Fig. Schematic section across the western part of the Cerro Prieto field, showing movement of hot water gray arrows and cold water white arrows toward the producing wells, the chemical and thermal fronts, and zones of nearwell boiling from Grant et al.

Recently, on the basis of stable water isotopes, tritium values and chemical composition data, Makdisi et al. The chemical equilibrium between Cerro Prieto geothermal fluid and detrital and alteration minerals of the reservoir were studied by Truesdell and Henley in preparation. The fluid appears to be in equilibrium with aquifer minerals. They indicated that the source of the carbon may be of magmatic or sedimentary origin, and that the CO2 from some surface manifestations is heavily influenced by decomposition of organic matter.

Des Marais et al. Parallel measurements were made on gases from laboratory pyrolysis of coal obtained from samples of one of the wells, in an attempt to simulate the production of the geothermal hydrocarbons. This suggested that the wells are sampling hydrocarbons produced over a range of depths and temperatures in the sediments. The only long-term well-interference and build-up tests conducted by DOE-sponsored groups at Cerro Prieto were described by Schroeder et al. The communication between these aquifers was confirmed by the geological model of H a l f m a n et al. In a two-rate flow test was carried out using a downhole tool modified by Sandia Laboratories for high-temperature wells.

Castafieda and H o m e described the use of pressure gradient techniques to locate feed zones and internal flows in Cerro Prieto wells. Using known wellhead data, Goyal et al. The data show that during the period to the pressures and temperatures decreased by approx. The studies confirmed the fact that computed downhole pressures are very sensitive to measured wellhead conditions and to changes in inside well diameter.

Heat and mass production data for the period to were analyzed by Goyal et al. The average enthalpy of the produced fluids has varied over the years. The increases in elathalpy were usually the result of bringing higher-enthalpy wells on line. The decrease in the average enthalpy was thought to be due to the mixing of relatively colder water with the geothermal reservoir fluids.

O v e r v i e w o f Cerro Prieto Studies Goyal et al. They also analyzed the changes in production characteristics in different wells and the interference effects between wells, establishing the existence of flow barriers and cold boundaries in the reservoirs. Preliminary reservoir modeling studies of the field's behavior were carried out by Lippmann and Goyal and Lippmann et al. Westwood and Castanier used a lumpedparameter model to gain insight into the physical processes in the reservoir and its response to production.

On the basis of the temperature data given by Castillo et al. A mineable heat of Taking into consideration the efficiency for electrical conversion Brook et al. This value is similar to the one given by Molinar et al. Observed chemical and thermal changes in the reservoir were used by Grant and O'Sullivan in preparation to study the recharge characteristics of the old western production field.

Based on pressure changes the permeability of the layer overlying the reservoir was calculated. It was found that it was considerably less permeable than the producing aquifer but not sufficiently impermeable to exclude inflow of cooler waters from above. The authors concluded that the a reservoir in the old field is best considered as a leaky aquifer.

They estimated that one-quarter to one-half of its recharge derives from cooler rock immediately above it. A two-dimensional distributed-parameter model was used by Lippmann and Bodvarsson to study the natural flow of heat and mass through the Cerro Prieto system and the impact of fluid production on the behavior of the field. The results indicated that the Halfman et al. It is shown that the field in its natural state is recharged from the east with hot approx.

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• Geothermal Power Plants, 3rd Edition by Ronald DiPippo.

Some boiling occurs as the hot water ascends through a sandy gap in the shaly layers. In support of CFE reinjection tests, the chemistry of silica in Cerro Prieto brines was studied theoretically and in the laboratory Iglesias and Weres, ; Weres and Tsao, It was established that part of the dissolved silica quickly polymerizes to form suspended colloidal silica. Raising the pH of the brine and stirring produces a rapid and complete flocculation and settling; these results were confirmed by field tests Hurtado et al.

Weres and his colleagues also developed a simple preinjection treatment for the Cerro Prieto brines, analyzed their equilibrium chemistry and studied the rate of deposition of silica scale from synthetic brines. The response of the Cerro Prieto field to reinjection was simulated numerically by Tsang et al. These studies confirmed that the breakthrough of the injected M. L i p p m a n n cold water into the production zone is strongly dependent on the location and completion of the injection wells, indicating the need of a carefully planned and monitored production - injection operation to optimize the extraction of the heat stored in the reservoir rocks.