Updated Tulehu Conceptual Model Due to Exploratory Drilling: an Example of Geothermal Field with Tertiary Volcanic Activity

Abstract

The Tulehu geothermal field is located in Maluku province, Ambon Island. Following several surface surveys, six thermal gradient holes (40‐150m deep) had been drilled in total in 2008 and in 2009. The highest borehole bottom temperature (122.6 ⁰C) was recorded in the well W2.1. The first exploratory well TLU‐01, located near by the well W2.1, was drilled to total depth 932 m in 2011. Following this exploratory drilling, another drilling campaign was conducted in June 2017 to April 2018, where four wells were successfully drilled to 1800 – 1900 m depth and two of which were tested. Drilling campaign in 2011 brings a hypothetical conceptual model that became the basis for the development plan and the drilling campaign in 2017. It was found that the drilling result in 2017 gave indication of prospect area shifting and new hypothesis of the heat source. The prospect is shifted to the southeast direction toward the expected system heat source and the area shrinks currently from 7-10 km2 to 0.7-1.9 km2 , thus, need to have additional Surveys. Part of the previous prospect area, which is mainly delineated by MT survey, on the northwestern side was confirmed to be inferred as paleo-geothermal reservoir. The temperature log from the well which was directed to the northwest is relatively low. Whereas, the fluid inclusion analysis from the cutting sample shows a very high homogenization temperature of fluid inclusion (387 ⁰C) which corresponds to high resistivity value of approximately 160 – 200 Ωm based on the MT model. On the other hand, the most successful well having around 1.5 MWe capacity, shows very close temperature value resulted from PTS flowing survey and fluid inclusion analysis from the cutting sample, i.e. 209 ⁰C and 223 ⁰C, respectively. The well is located in the southeastern side of the prospect area. It seems that the geothermal system in Tulehu field might experience an evolution, with a direction from the northwest to the southeast, which inferred to be controlled by the volcanism activity evolution in the area.