Model of a post-caldera volcano-hosted and vapor-dominated system through a numerical simulation study of the Bedugul geothermal field, Bali Island, Indonesia

• Published in: Geothermics 2024-01, Vol.116, p.102848, Article 102848
• Main Authors: Pratama, Heru Berian, Koike, Katsuaki, Pratama, Angga Bakti, Ashat, Ali
• Format: Article
• Language: English
• Subjects: Conceptual model; Counterflow; Dual-porosity model; Fracture permeability; TOUGH2; Two-phase reservoir
• ISSN: 0375-6505; 1879-3576
• DOI: 10.1016/j.geothermics.2023.102848

•Dual-porosity model and TOUGH2 EOS1 are used to simulate natural state condition.•Vapor-dominated zone in post-caldera volcano-hosted geothermal system is specified.•Geothermal system model is refined using numerical reservoir simulation results.•Steam gradient pressure in steam zone is 47bar at saturation temperature of 260°C.•Steam-water counterflow controls heat transfer in the vapor-dominated system. To significantly increase the use of geothermal resources, describing geothermal systems in post-calderas is essential because this type of system is infrequent and has not yet been thoroughly researched. The Bedugul geothermal field (BGF), Bali Island, Indonesia, is a representative system with a complex structure composed of a vapor-dominated zone formed above a deep liquid reservoir. This study examines the BGF system and updates a conceptual model, particularly the fluid flow and heat transfer in the reservoir. A dual-porosity model and TOUGH2 EOS1 equation of state for water and steam module are used to simulate natural state conditions, including calderas with a domain area of 148.8 km2 and a thickness of 3.3 km. The BGF model was calibrated using the available geosciences data, temperature core holes and deep explorations wells and then evaluated statistically. A reliable natural state model with decent accuracy was obtained by optimizing the parameter calibration of the physical and structural properties of the reservoir model; the anisotropic and heterogeneous fracture permeability distribution and specific boundary conditions are crucial to the calculation accuracy. The BGF model reveals hot hydrostatic liquid, vapor-dominated, and transition zones and an overpressured liquid domain that is specific to a vapor-dominated geothermal system overlying a liquid reservoir. Occurrence of counterflow is also a noted feature in the vapor-dominated zone. Because of the high pressure in the steam zone, around 47 bar at a near-saturation temperature of 260 °C, the BGF is characterized as a premature field of a vapor-dominated system.