Mataloko geothermal field is located in Eastern Nusa Tenggara, Indonesia. Its geothermal reservoir is distinguishable into two main zones: 1) shallow vapor-dominated; and 2) deep liquid-dominated. Unlike in many geothermal reservoirs where a liquid-dominated reservoir underlies a steam cap, the shallow vapor-dominated zone in Mataloko is unique due to its presence within the low permeability cap rock region. Geoscience and well data interpretation suggests that the highly permeable Wae Luja fault provides the permeability for the geothermal fluid from the deep liquid-dominated zone to flow upwards through the cap rock thus forming a shallow steam pocket. In 2000-2006, four exploration wells and two development wells were drilled in the field. Despite all six wells just penetrating the shallow vapor-dominated zone, a 2.5 MWe single-flash geothermal power plant was commissioned in 2010 to utilize the discharge steam. However, the power plant has not been producing since 2015 due to rapid decline of the production from the shallow vapor-dominated zone. This study analyzes the technical and economic feasibility of a hypothetical Mataloko field development scenario by developing both reservoir zones in-stages, assuming that no power plant has been installed yet in the field. This aims to demonstrate an example of a geothermal reservoir development involving a unique vapor-dominated and liquid-dominated zones distinction as it is in Mataloko. In this hypothetical scenario, a development capacity of 22 MWe is proposed by considering the results of resource assessment using the volumetric method for each reservoir zone. The development consists of 2 MWe development of the shallow vapor-dominated zone and 20 MWe development of the deep liquid-dominated zone using two scenarios: Scenario 1 (2+20 MWe) and Scenario 2 (2+10+10 MWe). The 2 MWe power generation from the shallow vapor-dominated zone will be supported by the existing wells. Therefore, the subsequent development drilling will target the deep liquid-dominated reservoir. To support the 20 MWe production from the deep liquid-dominated reservoir, six deep production wells and three deep reinjection wells are required to be drilled. Furthermore, based on the economic analysis, Scenario 2 is more feasible, requiring 102.9 million USD capital expenditure and yielding a project internal rate of return (IRR) of 19%, a project net present value (NPV) of 18.6 million USD, and a payback period of 6 years.

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