Iqbal Kurniawan, Sutopo & Heru Berian Pratama
Geothermal Master Program, Institut Teknologi Bandung, West Java, 40132 Indonesia
Email: iqbalkurniawan123@yahoo.com

Abstract

Ulumbu Geothermal Field is located in Manggarai District, East Nusa Tenggara, Indonesia. This field is owned by PT. PLN (Persero) and has a total installed capacity of 10 MWe. Integrated reservoir study by using reservoir simulation will be powerful as PT. PLN (Persero) is planning to increase the capacity by adding two more additional units having capacity 2×20 MWe in 2020 and 2024. The study is needed to increase the reservoir characteristics understanding, to calculate the potential generation, and to do reservoir performance forecasting by using various field development scenarios. The model was successfully built by carrying some steps of calibrations. Reserve calculation was carried out using experimental design method and it shows a potential generation of 95 MWe. The reservoir performance forecasting indicates that a total generation of 50 MWe with production wells from steam cap and make-up wells from two phase zone is categorized as the best scenario.

Keywords: Reserve calculation, reservoir performance forecasting, reservoir simulation, Ulumbu.

Introduction
Ulumbu geothermal field is located in Manggarai District, East Nusa Tenggara, Indonesia. It is owned by PT. PLN (Persero) with concession area of ± 10 km2. The field has been producing electricity from four units installed having a total capacity of 10 MWe. This field is a part of a government program called Flores Geothermal Island as the release of ministerial decree No. 2268 K/30/MEM/2017 on June 19th by the Ministry of Energy and Mineral Resources of Indonesia. The aims of the program are mainly to optimize the use of geothermal energy in Flores Island for both indirect and direct utilization in 2025.

PT. PLN plans to increase the electricity production from Ulumbu by adding two more units having total capacity of 2×20 MW, they are targetted to operate in 2020 and 2024 based on Adiprana [1]. In accordance with the plan, further evaluation of the reservoir needs to be carried out. An integrated reservoir study is needed to evaluate the power generation capacity of the reservoir, to increase the understanding of the reservoir characteristics, and to determine an optimum development scenario. In O’Sullivan et al., [2], the application of a computer model in planning and managing a geothermal field has been a common practice as more than 100 geothermal fields has been modeled all over the world. Given that this field has a considerable number of data, the integrated reservoir study by using reservoir simulation can be carried out. The numerical model will be used as a basis in deciding an optimum development scenario of Ulumbu geothermal field.

The objectives of this study cover many studies such as building a calibrated numerical model, calculating reservoir potential generation by using experimental design and selecting the best development scenario based on technical consideration.

Conceptual Model Review
Geological review by Utami [3] shows that the rocks in this area consist of two ages, quaternary and tertiary rock. The tertiary volcanic rocks comprising mainly lavas, breccias, tuff, and calcareous sediments act as the basement, meanwhile, the quaternary rocks spread to an elevation of approximately 1,600 masl.

From geochemistry review that was based on Mahon et al. [4], the water chemistry and gas suggests that the major upflow zone is beneath Kokor or towards Lungar. That fact might also indicate that the main outflow zone is located west of Ulumbu geothermal field.

In ESDM [5] magnetotelluric survey analysis confirms that the upflow zone is located below Poco Rii and Poco Leok depression as indicated by a doming high resistivity body beneath the area. Furthermore, the magnetotelluric survey was also used to delineate the area of the reservoir and the thickness of low resistivity anomaly.

Based on Grant [6], the temperature and pressure measurements for all wells indicates a liquid zone beneath the steam column. The lower section of the three wells is probably a liquid-dominated reservoir as the evidence show that there are downflows within the liquid column and also there is liquid discharged from ULB-01.

Conceptual model was constructed from integrated geological, geochemical, geophysical, and wells data. Figure 1 shows the conceptual model of the field by using slice plan NE-SW. This conceptual model was used as a guideline in building the reservoir model.

Figure 1 Conceptual model of Ulumbu modified from ESDM [5]

Conclusion

The calibrated numerical model of Ulumbu geothermal field has been built successfully based on several actual data. It was then used to calculate the potential generation of the field and 95 MWe was concluded from the calculation. Based on the reservoir performance forecast and some consideration from the technical point of view, the best development scenario will be the field development with a total generation of 50 MW by liquid-dominated zone as the target for make up wells drilling.