Characterizing the Two-Phase Geothermal System of the Ulumbu Field Through Updated Data and Numerical Reservoir Modeling to Support Capacity Expansion Planning

Abstract

The Ulumbu geothermal field in Flores, Indonesia, exhibits a two-phase system characterized by steam-dominated conditions at shallow depths and liquid-dominated conditions at greater depths. An existing facility currently generates 4 × 2.5 MW, with a planned expansion of 2 × 20 MW as outlined in the 2021–2030 RUPTL. However, uncertainties concerning subsurface structure, heterogeneous permeability, and long-term reservoir sustainability necessitate a comprehensive re-evaluation. Earlier studies relied on exploratory conceptual models with limited geoscientific data. The studies introduced the first natural state model using TOUGH2, confirming the presence of a steam cap overlying a liquid-dominated zone.

This study presents an updated and integrated conceptual and numerical model of the Ulumbu geothermal system based on recent geoscientific, geophysical, and well data. Numerical simulation was conducted using the Volsung simulator to improve natural state representation and inform future development. A potential upflow zone is identified beneath the Lungar area, directly connected to the heat source and underlying a steam-supplying two-phase zone. The model achieves calibration with well data, reproducing key reservoir characteristics: a ~600 m thick steam cap (saturation 0.6–0.7, ~260°C), a ~200 m thick boiling zone (~290°C), and a deeper liquid-dominated reservoir (~300°C) below –1000 m asl. While this study successfully reconstructs the natural state of the system, future production simulations are still required to confirm long-term deliverability and the feasibility of 2 × 20 MW expansion.