The Momotombo Reservoir Performance upon 27 Years of Exploitation

Abstract

The Momotombo geothermal reservoir has been developed for more than twenty years since 1983 when the first unit of 35 MWe commissioned. And the second unit was installed in 1989 by increasing steam production rate. During this period, production wells show marked changes in flow rates, fluid chemistry and specific enthalpies of produced fluids. These changes are mainly attributed to reservoir pressure decline because of excessive fluid production. By 1999, when the power plant output dropped to 9 MWe an international tender was issued for the rehabilitation of the project under a 15 year Concession. Ormat won the tender and undertook to drill additional wells, implementing a full reinjection and installing a bottoming OEC unit. Since then the plant is producing 30 MWe supported by an intensive well maintenance program. Ormat’s investment stands at about US$ 45 Million, producing electricity at less than US$5.22 Cents/kWh, making the Momotombo plant the lowest cost electricity producer in Nicaragua.
As for today, the proven Momotombo reservoir seems to be constrained by heat more than mass reserves; therefore the current 30 MWe seems to be the maximum power output that the reservoir can handle for a reasonable period of time. 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.