Volumetric, lumped and 2-D numerical models of the Momotombo reservoir, Nicaragua

Abstract

The production capacity of the Momotombo geothermal field has declined over time when production wells were damaged by scaling and suffered from cooling. The reservoir is characterized by a deep, vertical upflow zone of 280-320°C feeding a shallow horizontal reservoir at 300-500 m depth and 180-240°C temperature. Geothermal fluid was observed on the surface in hot springs and fumaroles. The chloride concentration in most of the production wells has been decreasing because of the incursion of cold water into the reservoir, presumably entering the reservoir by the hot spring conduits. The proven and possible production capacity for 25 years was estimated using the Monte Carlo volumetric method at 20 MW and 50 MW, respectively. The total production data and pressure drawdown were simulated satisfactorily by a one- and two-tank open lumped model. The two-tank open model yielded a reservoir volume in the 4 km3 range, the same as the estimated proven reservoir volume in the Monte Carlo method. The permeability of the lumped model ranged from 75 to 230 mD. A 2D numerical model was developed and calibrated using the code iTOUGH2, allowing for the study of the chloride concentration changes over time. The thin 2D slice could be heated up by 3.3 kg/s of 330°C water. The highest permeability of 340 mD complies well with the inner 230 mD permeability of the lumped model. The results of the simulation indicated that chloride concentration is a good parameter for future studies. Decreases in chloride concentration imply intrusion of low temperature and fresh groundwater into the reservoir. Intrusion of low-temperature groundwater into the reservoir and extensive boiling in the shallow reservoir due to pressure drawdown induced cooling in the reservoir and these are regarded here as the main reasons for the operational problems encountered in the wellfield management.