Investigating the Causes of Production Decline in ULB-02 Well, Ulumbu Geothermal Field: A Combined Decline Curve Analysis and Reservoir Scaling Simulation Approach

Abstract

 This study examines the declining steam production at the Ulumbu Geothermal Power Plant in East Nusa Tenggara, Indonesia, focusing on the ULB-02 well from 2015 to 2023. Initially, a 5% annual decline in steam output was expected; however, recent data indicates an accelerated decline, likely due to scaling within the wellbore. Scaling reduces steam flow and well productivity, contributing to a significant decrease in wellhead pressure (WHP) and steam flow rate. Decline curve analysis (DCA) shows an exponential decrease in steam output, with projections indicating a reduction to about 12.5 kg/s by 2030, corresponding to a drop in power generation capacity to around 5 MW. To assess the impact of scaling, the study uses TOUGHREACT simulations to model mineral deposition and its effect on wellbore permeability. The results confirm that scaling is a primary factor in the well's declining performance. The study highlights the importance of reservoir management and scaling mitigation strategies to ensure the long-term sustainability of the Ulumbu plant. Recommendations include monitoring wellhead pressure, addressing scaling issues, and implementing periodic interventions to optimize well performance and prevent further output decline. These findings are relevant to other geothermal fields with similar challenges.