Danau Ranau geothermal field is located in Lampung and South Sumatra province, Indonesia. It is classified as a high terrain volcanic hydrothermal system controlled by the Sumatran fault as the permeability zone. The study aims to characterize the Danau Ranau reservoir, update the conceptual model, and assess the geothermal power capacity coupled with Monte Carlo simulation. Therefore, the numerical reservoir simulation is performed using TOUGH2 software to achieve a natural state model based on geoscience interpretation data. The numerical reservoir models’ output matches the geological, geochemical, and geophysical interpretation, thus becoming important information (3D pressure and temperature, heat and mass flow, location of heat source, boundary condition, reservoir geometry) in the updated conceptual model. The probabilistic heat stored method results based on the natural state model’s output show that Danau Ranau geothermal field can generate up to 30 MW.

The development of geothermal fields in Indonesia is carried out under the provisions of the applicable laws and regulations, namely Geothermal Law No. 21 of 2014 and Environmental Protection and Management Law No. 32 of 2009. Based on Law No. 21 of 2014, geothermal business activities consist of preliminary surveys, exploration, and exploitation. After the exploration phase (a detailed geological, geochemical, and geophysical study), a pre-feasibility study is needed to determine the feasibility of exploration drilling. The pre-feasibility study is done to manage project risks. Danau Ranau geothermal field is one of the 166 medium enthalpy geothermal prospects in Indonesia. Activities carried out in this field were geoscience studies. This research was conducted to determine how feasible the Danau Ranau medium enthalpy geothermal field was developed for direct and indirect utilization by looking at the acceptance of technical, financial, environmental, and risk studies. The study can be a tool for the developer to decide whether to continue the next development stage. The result shows that applying the approach for Danau Ranau Geothermal Field is feasible to develop with consideration. Plant of Development by cascade model with leveling utility of energy for Danau Ranau Geothermal Field shows optimal value both in the operational and financial sides. The first level is technically designed for electricity with ORC conversion technology and the cascade level for indirect use by Lindal Diagram and community empowerment in economic and social. In financial analysis, the cascade model has optimum profitability value expressed in NPV and equity IRR.