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5194THE CERRO PRIETO GEOTHERMAL FIELD: PRESENT AND FUTURE '1''

From 1949 to 1950 the Comisi6n Federal deElectricidad (CFE) sent a group of engineers andtechnicians to visit the geothermal field in

Larderello, Italy. At that time, Mexico was inter-

ested in the possibil'ity of using geothermal energyfrom the numerous geothermal manifestations through-out the country. As a result of that mission,

an inventory of geothermal manifestations was begun.Special emphasis was placed on the Pathd area, inthe state of Hidalgo, 120 km north of Mexico City,

because of its geographic location. Surficialgeological studies were carried out in this area;gradient wells were drilled, and chemical analysesof thermal manifestations were performed. Based

on this information, the first exploratory wellwas drilled between 1955 and 1956. The successof this first exploration well marked the beginning

of the development of geothermal technology inMexico. In 1959, the first 3500-kWe experimentalunit was installed. This unit was dismantled after14 years when it was operating at a capacity ofiless than 500 kWe.

Based on the results. obtained in Path6, theexploration of the Ixtldn de los Hervores geother-

mal field, in the state of Michoacdn, was startedin 1956. Two wells were drilled, one of whichhas been producing hot water and steam for the

past 21 years. Geothermal explorations were thenundertaken in Los Negritos, where some wells werealso drilled with positive results. At the begin-ning of 1969, exploration work started in theCerro Prieto geothermal area, with surficialgeological surveys and some chemical analyses ofthermal manifestations. This enabled us to pin-

point the first well, from which we obtained awater-steam mixture. Two more wells were drilledat a later date, one at 700-m and another at 450-mdepth, and very promising temperatures were observed.As a result of this, we decided to undertake detailedgeological, geochemical, and geophysical studies,starting with a seismological refraction survey

in 1962. The results of the survey made the presentdevelopment of the Cerro Prieto field possible.Based on this study, we located the first deepwell ( M-3), reaching.down to a depth of 2550 m andpenetrating the granitic basement.

Production was achieved at a depth between700 and 900 m, and we observed a lower temperaturewhen we went below this depth. Well M-4, about25 km northeast of well M-3, was drilled later, inan attempt to cross the well through an importantfault detected by the seismological study. Well

M-4 reached a depth of 2000 m, with a bottom-holetemperature of 360C. Wel,1 M-5, which is 2.5 km

south of M-3, marked the discovery of the fieldand established the basis for future developmentat Cerro Prieto due to its excellent productioncharacteristics.

In 1967, it was decided to drill 14 wells

in order to build the first 75,000-kW power plant.

Simultaneously, detailed geological and geochemical

studies were carried out, the most outstanding of

which was the one performed by Washington StateUniversity using stable-isotope analysis to definethe origin and age of water supplying the CerroPrieto field.

A decision was reached to install the first

two units with a total capacity of 75,000 kW,after drilling the 14 aforementioned wells ( 1967-

1969 ), and after carrying out production testsin different wells. These units were put intocommercial operation in 1973, and have continuedworking satisfactorily since then, with a plant

efficiency that has gradually reached 90%. The

operation of this plant saves CFE one millionbarrels of oil per year.

Simultaneously with construction and operationof the first power station, detailed geophysicalstudies continued to be carried out to the southand east of the present field. It was concludedthat the geological structure closest to the fieldis made up of a series of parallel faults, someof which are visible in the Sierra Cucupah, anda horst that coincides with the presently exploitedarea and with main downthrown structures toward theeast of Cerro Prieto. Based on this, we decidedto drill a well (M-53) outside the present exploi-tation area. This well, at a depth of 2000 m,

proved to have a production of about 100 ton/hrof steam and a bottom-hole temperature of 3400C.

This and well M-51, situated to the south of thepresent fields opened up a new era in the expansion

possibilities of Cerro Prieto.

The result was the construction of a new75,000-kW station ( Units 3 and 4 ), fed by 14 wells,

lying east and south of the present field. Thesewells have better characteristics than those supply-ing Units 1 and 2, since the majority of them havea bottom-hole temperature of between 3000 and 3500C

and produce between 80 and 140 ton/hr of steam.( This is the case for well M-51.)

The 14 abovementioned wells are completedand the steam lines are being installed and readiedfor connection with the second 75,000-kW station,which will go on-line commercially at the beginningof next year ( 1979). Based on experiences acquiredin 1977, it was decided that in 1978 we would drill26 wells in the eastern part of the present field,to supply a third power station, which will becalled Cerro Prieto II. As an interesting footnotewe would like to point out that most of the wellsalready drilled have a 7 5/8-in.-diam. productioncasing, ( J-55 and K-55 ).

Due to the problems ocurring in these well

casings, we undertook a study and decided to changethe construction characteristics of wells for theCerro Prieto II station. We decided to use9 5/8-:in.-diam. production casings (N-80) , insteadof 7 5/8-in.-diam. ( J-55 and K-55 ). With this

new diameter, we hope to obtain production thatwill fluctuate between 170 and 180 ton/hr of

separated steam; this is feasible, since we have

observed production of 140 ton/hr with 7 5/8-in.-diam. in well M-51. This will represent an impor-

tant savings for the future Cerro Prieto II plant.If it is possible to obtain an average well capa-

city of 15 M'We, we will need to drill fewer wells,

even though t.hey might have to be deeper. On theeastern part of the field, the depth of the produc-ing areas varies between 2000 and 2500 m.

Construction of the Cerro Prieto II plantis scheduled to begin in 1980. We hope to startthe first st•ige of the commercial operation in1983 and the second stage in 1985. The totalcapacity of this plant will be 220,000 kW. In

general terms, this is the most relevant data

regarding the present field of Cerro Prieto.

As for other areas of geothermal interest,we have begun geologic, gravimetric, magnetic,and electrical resistivity studies, as well as

thermal gradient studies south and north of thepresent field. In the southern portion, about17 km from tlie present field, we have found anextremely interesting resistivity anomaly, whichalso coincides with an area of earthquake swarms.We are awaiting the results of seismological

studies begun in September 1978. These studiescover the present field and its southern extension(where the aforementioned anomaly is situated )

and seek to define the structure and locate a deepexploration well. If it proves to be productive,this would definitely broaden the possibilitiesof the Cerro Prieto field.

Thermal gradient wells were also drilled in

the northern part of the field, in an area calledTule Check, to complement the geophysical studiesalready carried out. These included resistivity,gravimetric, magnetic and geochemical studies ofsurface manifestations. In addition, we haveplanned seismic studies, which will enable us tolocate an exploratory well in this area in 1979.On the other hand, in order to pinpoint additionalgeothermal areas, studies will be undertaken ina region called Mesa de Andrade, where surficialgeological surveys are already under way. Thisarea is situated approximately 40 or 50 km southeastof the Cerro Prieto geothermal field; available

information, which is still very scarce, showsthat irrigation wells drilled in this region haveabnormal temperatures of around 500C at depths

of 100 to 150 m. We have scheduled geophysicalstudies for this area ( resistivity, gravimetric,

magnetic, thermal gradient and seismic studies )

in order to determine the location of an exploratorywell, which will probably be drilled next year,depending on the result of these studies.

In Baja California all construction programsfor geothermal plants have been contingent on theincrease in demand for energy. The electricalsystem in the state is an isolated one, and

operates with two power stations: one located

in the city of Rosarito, south of Tijuana

( thermoelectric), and the other operating in CerroPrieto ( geothermal). In order to build powerstations whose capacities will go beyond the needsof the northern zone, we are contemplating

connecting the existing system to the nationaldistribution network, which is approximately700 km away at its closest point. If the govern-ment approves this connection, the building pro-grams for geothermal plants would change, and

instead of planning for a 400,000-kW capacity for

1984-1985, we could consider installing 700,000 kW.We must point out that the proven capacity in thearea surrounding the Cerro Prieto field is about750,000 kW.

In very broad terms, these are our future

programs; they are especially important to CerroPrieto and Baja California, since this is prac-tically the only easy-to-operate and clean naturalenergy resource available in the region. Pollutionproblems are minor compared with other types ofpower plants. The problem of waste water hasseveral solutions, among them recovery of chemicalproducts. drainage into the Laguna Salada oranother area close to it, or reinjection.

The outlook for the central part of thecountry is also very positive. Once we developthe geothermal fields under study at this time,we hope, as Ing. Carrion said, to have a minimuminstalled capacity of 4000 MWe throughout ourcountry by the year 2000.

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