Well Completion and Development at Cerro Prieto

Abstract

To date, 55 wells have been drilled in the Cerro Prieto geothermal field for a total of more than 80,000 linear meters, a moderate figure compared with that for oil wells. These wells were constructed over a period of almost 18 years, which could be divided into six stages. During this time modifications to specifications were made, which tended to optimize the results obtained in preceding stages or simply to correct faults and problems caused, in large part, by the high temperatures encountered--a distinctive characteristic of geothermal wells.
In 1964 the drilling of the first four deep exploratory wells was done with oil-well techniques for which the API standards establish l600C (3200F) as a limit. At Cerro Prieto temperature at the production zones frequently exceeds 3000C (5720F), creating some of the problems discussed below and compelling significant changes in the operations. These changes were made mainly in the casing specifications, to obtain a satisfactory exploitation of the geothermal reservoir.
These notes present general comments on the completion of the different wells drilled in each stage and the methods used to heat up and develop the wells until the start of their formal exploitation. Some maintenance and repair aspects are also included since there is a direct relation between the construction characteristics, damage, and repair of the wells. To conclude, changes with time of temperature and pressure, and production volumes are discussed, indicating the evolution of the reservoir.