A conceptual reservoir model and numerical simulation studies for the Miravalles geothermal field, Costa Rica

Abstract

Commercial exploitation of the Miravalles geothermal field started in 1994 with eleven production wells and six reinjection wells. In this report a general description of the field is given. A conceptual model is defined, based on estimated formation temperatures and initial pressures in twenty-five wells. In the conceptual reservoir model a 260°C upflow zone is proposed in the north and an outflow zone in the south. The geothermal fluid moves laterally to the south at -100 to -300 m a.s.l. The wellfield is clearly bounded to the west by cold temperatures and low pressures. The character of the eastern boundary is unknown due to a lack of data in that region. Lumped modelling simulation suggests that the Miravalles field will behave as a closed reservoir system for the next years, resulting in rapid drawdown with time. This model may be pessimistic as only net production is considered in the study. A 3-D natural state model simulates reasonably well the temperature distribution in the wellfield, by using a large recharge rate of 180 kg/s of 270°C fluid and very low thermal conductivity of model boundaries. Further modelling studies must address critical questions such as future development of a two-phase reservoir zone, the destiny of reinjected fluids and the nature of the outflow zone south of the wellfield.