| Abstract |
Wellhead power plants have played an important role in the development of the Miravalles field in Costa Rica. Wellhead units were being considered during the well-drilling phase to provide on-site power and to allow well testing under realistic flow conditions. Within one year of the startup of the 55 MW single-flash Unit 1 in 1994, the first of three wellhead units was installed and put into operation. This 5 MW unit, originally designated Wellhead Unit 1, took advantage of excess steam that was available prior to the installation of the second 55 MW unit. This paper covers the history of wellhead units at Miravalles. It focuses on the motivation for and the performance assessment of Wellhead Unit 1 throughout its operating lifetime from its original location in the north sector of the field about 800 m north-northeast of Units 1 and 2 to its current location in the southeastern sector at well PGM-29, about 2.3 km east-southeast of Unit 5. The unit now carries the designation, PGM-29 Wellhead Unit. A brief overview of the current Miravalles operation including a layout of the field showing the locations of power units, wells, satellite separator stations, and gathering and reinjection system pipelines is included in a companion paper in these Proceedings; Moya and DiPippo (2010). This paper provides detailed information on the subject wellhead unit beginning with the motivation for the unit, its field position, and the means of geofluid supply, along with some performance data including electrical generation, as well as capacity and load factors, for each location where the unit has operated. The unique design aspects of the plant are presented and discussed. The performance of the wellhead unit is given in terms of its thermodynamic design and its actual operating conditions. The processes followed by the steam in the plant are shown in state-point diagrams; state-point property tables are given for design and typical operating conditions; actual performance data are analyzed in terms of power generation, parasitic power requirements, utilization efficiency, specific steam consumption, and specific brine consumption. The results of the present-day operation are viewed in terms of the knowledge gained from production and reinjection at the southern part of the field that has been used mainly as the injection zone for the other power plants. The operation is markedly different for the two different locations, since in the first location the unit was fed steam directly from a satellite separator, whereas in the current location it is connected to one well that is equipped with a cyclone separator. Thus both steam and brine flows are now directly associated with its operation. Although it is difficult to isolate the effects of one small 5 MW unit among four other larger units that now total 163 MW, several possible impacts including acid rain, air pollution, and pollution of ground waters are examined. |