Review of Generating Capacity Estimates for the Momotombo Geothermal Reservoir in Nicaragua

Abstract

A proven 2 km2, 1300 m thick and initially 240-320 °C Momotombo reservoir has reached a stabilized 32 MWe generation after declining from a short peak of 60 MWe to 9 MWe in 1991 to 1999. The field operation has been troubled with scaling and lost enthalpies, mainly due to a 100-150 °C recharge from a constant pressure boundary. These difficulties have now been mitigated by mechanical workovers, calcite scaling inhibition, acid stimulation, commissioning of a binary unit and drilling of deep wells. For environmental concerns, Ormat also implemented full reinjection to avoid pollution of the Managua Lake done previously. Earlier volumetric models predicted generating potentials in excess of 100 MWe. They presumed 2-3 times thicker and larger reservoir than seen in downhole data collected later. A numerical model, calibrated in 1994, suggested up to 150 MWe generating capacity, largely from a hypothetical production zone at 1600 to 2600 m. As this model lacked the constant pressure boundary, it failed to predict enthalpy losses due to cold water invasion after 1991. Furthermore, the deep reservoir theory still needs to be confirmed. A later model, calibrated in 2005, includes the constant pressure boundary. It suggests a moderate generating capacity of 50 MWe. As the current proven reservoir seems to be constrained by heat more than mass reserves, attempts to increase generation should be directed to greater depths, into a possible resource. Statistically, deeper layers have average well success of 2 MWe and well output may decline by 5 % annually.