Nenny Miryani Saptadji, Heru Berian Pratama
Geothermal Engineering Study Program, Institut Teknologi Bandung, Indonesia
Jl. Ganesha 10, Bandung, 40132, Indonesia
E-mail: nenny.saptadji@geothermal.itb.ac.id

Abstract

The main issue in the management of the two-phase liquid-dominated geothermal field is a rapid pressure decline in the reservoir which affects steam supply to the power plant. The changes of reservoir characteristic due to mass and heat extraction from the reservoir can be learned through a numerical simulation under numerous strategies production-injection. The model developed on liquid-dominated geothermal fields are assessed with various scenarios of production strategies (steam cap, brine reservoir, and combination) and injection strategies (deep, shallow, centered and dispersed), for generation of 110 MW and 220 MW using separated steam cycle. A full factorial scenario of 32 production-injection strategies has been successfully carried out. The results show a relatively low decrease in reservoir pressure and steam flow rate for a production strategy combined with the steam cap and brine reservoir, with deep and dispersed injection strategy. From the simulation, it can be concluded that the strategy for sustainable production is a production of 25% from steam cap and 75% from brine reservoir, dispersed and deep reinjection with 9 make-up wells from the steam cap. The implementation of production-injection strategy needs to be planned right from the beginning of exploitation, therefore, the strategy can adapt to changes in reservoir characteristics.

Keywords: Reservoir liquid-dominated, Steam cap and Sustainable production.

Introduction
Liquid-dominated hydrothermal reservoir is most widely developed for power plant generation. As known, the sustainability of steam production from this type of reservoir has been a major concern since the massive mass production from the reservoir can significantly decrease reservoir pressure over time. The pressure drop simultaneously can induce reservoir to boil. According to Grant , et.al in [1] and [2], there are two possibilities that can occur in the liquid-dominated reservoir after production. The steam is mixed uniformly and the fluid dryness around the production well increases. The second possibility is that the steam zone and water zone will be separated due to gravity, therefore steam cap is formed at top of reservoir. With a good vertical permeability, the reservoir boiling causes the steam which has lower density than liquid phase, moves up and is formed at top reservoir. Both of this process occurs in most areas or in the whole reservoir. This phenomenon occurred in several geothermal fields with high power plant capacity, such as Wairakei – New Zealand [1,2,3,4,5], Tongonan – Philippines [6,7,8,9], Awibengkok – Indonesia [10,11,12,13], and Wayang Windu – Indonesia [14,15].

Common practice from several fields, such as Wayang Windu and Awibengkok, steam cap zone is more favorable to produce, because it is shallower therefore from drilling point of view, the drilling cost, can be reduced. Number of injection wells and separators may be reduced since less liquid produced. This potential cost reduction from a geothermal developer’s point of view, it makes mass and heat exploitation from a steam cap somehow more interesting.

The number of research in numerical simulation developed for liquid-dominated reservoir is fewer than those for single-phase (liquid) reservoir or vapor-dominated reservoir. Liquid-dominated reservoir can be said to be more challenging to model since one has to construct a model of vapor-dominated reservoir underlying liquid-dominated reservoir. Some results from the study [16] encounter a difficulty in modeling these types of reservoir. The model was run under various production-injection strategies. By observing pressure and temperature drop, vapor saturation and mass flow in reservoir model as function of time, this model can predict the performance of the reservoir with various production-injection strategies to exploitation time. The objective of this study is the optimum scenario of production-injection and make up well strategy that can be learned in order to manage the mass and heat production from the reservoir for a sustainable geothermal field management.

In this study, there are three production strategies introduced, as follows; fluid produced from steam cap only, fluid produced from brine reservoir only located below steam cap, and fluid produced from both steam cap and brine reservoir with several fractions of steam production. As for the injection strategies, there are two strategies, as follows; fluid injection into both deep shallow reservoir, and peripheral injection strategy which are either centered or dispersed.

Methodology
The synthetic numerical model was developed based on characteristic of typical two-phase liquid-dominated reservoir. Using 4 (four) different strategies both production and injection for generation of 110 MW and 220 MW, therefore the model is run for 32 strategies by full factorial. The further study was to forecast the reservoir performance for 30 years. The make up well scenario is to produce them either from brine reservoir only, steam cap only, or both of them to sustain steam the steam supply for power generation of 220 MW.

Acknowledgment
This research was supported by P3MI (Research, Community Service and Innovation Program) for the Reservoir Engineering Research Group at FTTM ITB. The TOUGH2 simulator was supported by BAGUS – SATREPS.

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