A Study Of The Structural Control Of Fluid Flow Within The Cerro Prieto Geothermal Field, Baja California, Mexico

Abstract

Noble, John E., Lippmann, Marcelo J., and Witherspoon, Paul A., Lawrence Berkeley Laboratory, University of California, Berkeley, California 94720. Manon M., Alfredo, Comision Federal Electricidad, Coordinadora General Ejecutiva de Cerro Prieto, Mexicali B.C. Mexico. Abstract The Lawrence Berkeley Laboratory and the Comision Federal de Electricidad of Mexico are conducting a joint investigation of the Cerro Prieto Geothermal Field, located approximately 35km south of Mexicali, Baja California, Mexico, in the Sea of Cortez-Salton Trough. Recent analyses of various geophysical/electrical logs, temperature logs, production and geochemical data and the subsequently developed preliminary model of the structure of the geothermal system and the distribution of geothermal fluids are presented. Techniques routinely applied to petroleum exploration were successfully used in the development of a preliminary model of this water-dominated system. Our study indicates the upwelling of geothermal fluids along an east bounding fault from a deep, as yet unexplored source. The fluids dissipate into various sand horizons at various depths. The resulting stratigraphic and fluid flow model is of importance in planning additional developments of the Cerro Prieto planning additional developments of the Cerro Prieto Geothermal Field. Introduction The Imperial-Mexicali Valley is recognized as having a potential for large scale production of electrical power from geothermal resources to help satisfy the energy requirements of Southern California and the Mexican states of Sonora and Baja California, North. Estimates of the potential of the Imperial Valley, alone, range from 5000 to 10000 Mwe. Exploration has defined various geothermal prospects in the Imperial Valley, and consequently several areas are currently being evaluated, e.g.: Brawley, Heber, East Mesa, etc. (Figure 1). In the Mexicali Valley, exploration for geothermal resources has concentrated south-southeast of the Cerro Prieto Volcano, over a 400 square km area approximately 35 km south of Mexicali, Baja California. This area is typified by numerous hot springs, mud volcanos, and fumaroles. The neighboring Sierra de los Cucapas, which lie to the west of Cerro Prieto, are the predominate structural feature of the area. In 1959 exploration for geothermal resources began in this area with photo and field geologic surveys In the same year three shallow exploratory wells (M-1A, M-2, M-2A) were drilled to depths of 523 m, 755 m, and 403 m, respectively. Geophysical investigations were commenced in 1961 with gravimetric and resistivity surveys. These surveys were completed in 1963. In 1964 four exploration wells (M-3, M-4, M-5, M-6) were drilled to depths of 2563 m, 2006 m, 1303 m, and 2040 m, respectively (Figure 3). Well M-3 penetrated Cretaceous Cucapas granodiorite basement at 2532 m. Well M-5 was a discovery well, and from 1966 to 1968, 14 production wells were drilled. The production from these wells, supplemented by production from four of the twelve wells drilled since 1972, supply separated steam to a 75 Mwe power plant. The total production is currently 750 tonne/hr separated steam (at 100 psig) and 2000 tonne/hr separated water. The separated water is piped to a brine waste pond; while in transit to the pond, the water flashes and vents to the pond as steam. Approximately 30 Mwe are lost to the waste pond, and development plans call for future pond, and development plans call for future installation of low-pressure turbines to utilize this energy source. The Cerro Prieto power plant commenced operation in April, 1973. It was the first operational geothermal power plant in Latin America and the first liquiddominated geothermal system to produce commercial amounts of electricity in the Americas. The plant is currently being expanded to 150 Mwe, utilizing production from new and existing standby wells. Development production from new and existing standby wells. Development and exploration drilling are continuing, and a target of 400 Mwe has been set for 1982. Comision Federal de Electricidad's (CFE) systematic exploration and development of the Cerro Prieto geothermal resource has resulted in a vast accumulation of data. These data provide a rare opportunity to conduct a case study of the exploitation of a geothermal system. In 1975, a working relationship was established between CFE and the Lawrence Berkeley Laboratory (LBL) to conduct such a study. In addition, LBL would become a repository for Cerro Prieto data in order to make such data available to the geothermal community.