Reservoir Pressure Behavior During Nine Years of Exploitation in the Miravalles Geothermal Field, Costa Rica

Abstract

The Miravalles geothermal field have a liquid dominated two phase reservoir, with temperatures of 220-250 ?C. The field started to operate in March 1994 producing 55 MWe and todate, it produces 142 MWe, using three condensing units and one back-pressure unit.
This paper discusses the analysis made on the decline in reservoir pressure using three different methods. Data were obtained from the monitoring of downhole pressure in observation wells, measurement of the hydraulic levels and analysis of the static pressure profiles in productive and inactive wells. These data were then correlated with the amount of mass extracted and injected in the Miravalles wells, to establish the reservoir behavior during the first nine years of exploitation.
The results of the data analysis showed a wide range of pressure decline, with an average decline of around 2-3 bar/year in the production areas and 0.5-2 bar/year in the injection and peripheral zones. The changes in the mass extraction and injection in the reservoir during plant shutdowns have produced significant variations in the reservoir pressure behavior, indicating an important connection between the different zones of the reservoir. The general reservoir pressure response indicated a pseudo-stabilization in pressure decline in the last three years, expanding considerably in the affected areas and with the maximum decline of not more than 3 bar/year.