Reservoir Pressure Behavior during Fourteen Years of Exploitation in the Miravalles Geothermal Field, Costa Rica (1994-2008)

Abstract

The Miravalles geothermal field has a liquid dominated two phase reservoir with temperatures of 220-250ºC. Miravalles started to operate in March 1994 with a 55 MWe power plant; nowadays, it produces a maximum of 155 MWe, using three condensing units (136 MWe), one back-pressure unit (5 MWe) and a binary plant (14 MWe). This production represents approximately a 12% of the total energy consumption of the country.
This report includes an analysis of the data related to reservoir pressure decline obtained by three different methods. Data were obtained from pressure monitoring in observation wells, measurement of hydraulic levels and from the static pressure profiles in productive and inactive wells. In order to do an analysis of the reservoir behavior during the fourteen years of exploitation, pressure data were correlated with information about extracted and injected mass.
Data analysis shows a wide range of pressure decreases, the drops varies from 1-2 bar/year in the production areas and from 0.5-1.2 bar/year in the injection and peripheral zones. These values are lower than those obtained during first 10 years of exploitation. Some factors related to this behavior are: the changes in the mass extracted and injected in the reservoir during the power plants stops, the variation produced by the management of the wells to avoid an over exploitation of the reservoir, and finally the increase in the number of wells that produce two-phase fluids. The pressure behavior confirmed a good connection between the different permeable zones found in the reservoir.