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Review of NQ Academy Webinar #8: Lessons Learned from Geothermal Exploration in Muara Laboh

NQ News

Monday, May 3, 2021

NQ Academy Webinar #8 has been held on April 24, 2021. The webinar was hosted by Dr. Yunus Daud, Founder of NewQuest Geotechnology, and presenting Ir. Novi Ganefianto, Vice President of Exploration & Subsurface at Supreme Energy. The event was discussing an interesting topic about "Lessons Learned from Geothermal Exploration in Muara Laboh”. The 8th NQ Academy Webinar was well organized and attended by more than 150 participants.

Muara Laboh geothermal working area is located in Solok Selatan Regency, around 135 km in the southeast part of Padang. The concession area is covering 43,200 Ha with an elevation varying from 450 to 2000 m. The geothermal occurrence in this area is characterized by the presence of fumaroles, mud pools, and hot springs.

PLTP Muara Laboh is managed by PT Supreme Energy Muara Laboh (SEML), the first Geothermal Power plant in West Sumatra province. PLTP Muara Laboh Unit-1 (85 MW) has been operating since December 2019, which means ten years since government gives the concession permit. In addition, PLTP Muara Laboh Unit-2 with a capacity of 65 MW is targeted for COD in 2024.

The geothermal system of Muara Laboh is influenced by the Great Sumatran Fault. At the exploration stage, MT measurements were conducted at 138 points. Then a conceptual model was made figuring that the prospect area is on the Idung Mancung fumarole area. So, the exploration was directed around the fumarole to prove the resource capacity.

The exploration was carried out by conducting exploration drilling on 6 wells (big hole) with a depth up to 3000 m, carried out in 2014-2015. Based on the numerical model with the volumetric assumption method, a potential of 60 MW is obtained. The exploration wells have temperatures range of 215 - 300 ^oC with neutral pH and low NCG.

After finishing the exploration stage, the next stage is development stage. One of the main question in this stage is how many MW the geothermal power plant will be developed? Is it maximum development or conservative development? The decision will depend on resources capacity, economic aspect, commitment to PLN, and also Bankers.

The next stage is how to place the production and reinjection wells. The highest temperature was located in the north so that the placement of production wells was focused in that area. The reinjection wells were located in the western part which is actually outside the Muara Laboh WKP area. The Muara Laboh geothermal power plant unit 1 is using dual flash technology generating 80 MW of electricity.

How about the drilling strategy? One thing that should be considered is how we understand the conceptual model. The minimum liquid temperature required for a commercial well for Muara Laboh based on the powerplant design is 240 C, SEML team conducts the mapping so that they know where the limitations are. The team also conduct mapping of two-phase zones and permeability from numerical simulations that match pressure, temperature, and production test data. The development drilling was carried out by conducting drilling on 13 big hole wells using 2 rigs to speed up the time.

In the drilling campaign, a guide for decision making is necessary, because the prediction and the reality can be different. The development drilling on Muara Laboh was successfully completed in June 2018, a long time before the completion of powerplant construction in November 2019. There was a surprise, the well in the southwest has a very high temperature, which is 50^oC higher than the exploration well in the north area. However, this was inconvenient with the steam gathering system that has been decided previously, which use temperature of 240^oC, and also it has a potential for greater silica scaling. Thus, a separate steam gathering system will be designed for the wells in this area as a backup for the previously designed production wells.

How to manage production-reinjection well for sustainability of the geothermal systems? Start from the philosophy for long term reservoir management is to manage the reservoir as steam factory. What often happens are problems related to scaling, cooling, lack of pressure support, and others? The key success factors are stay on top surveillance programs, doing reliable well diagnostics, and periodic update of reservoir model to generate “fresh” view in attempt to identify opportunities.

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