Drilling slimhole well is commonly used for conducting geothermal exploration drilling, as it has the potential to reduce the drilling cost while still fulfilling the exploration objectives but was rarely used in Indonesia for the last two decades. With the recent increase in geothermal exploration activity in Indonesia, slimhole drilling is once again considered as a promising alternative for exploration, especially in the fields with less convincing surface manifestations. However, with only one slimhole drilling campaign in the 2000-2020 period, the availability of drilling rig capable of drilling slimhole well in Indonesia is currently very low. The readily available drilling rigs such as conventional rotary rig and coring rig commonly used for mineral exploration might require extensive modifications, while multipurpose rigs are not available in Indonesia. This complicates the drilling planning process, as the cost for modifications or importing the rig can affect the total exploration cost. The purpose of this paper is to assess the options of conducting the slimhole drilling for geothermal exploration in Indonesia, whether by using coring rig, conventional rotary rig, multipurpose rig, or the combination of them. Various aspects such as estimated well cost, capability to satisfy the subsurface objective, estimate market acceptance, and operational risk and challenges were evaluated. Finally, a Multi-Criteria Decision Analysis was used to demonstrate the decision-making process that can be used to determine the most optimum method to conduct slimhole drilling in Indonesia.

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