Formation evaluation program in drilling operation, particularly geothermal drilling, is very much determined by the purpose and objective of drilling itself. During early exploration phase, the formation evaluation program is carried out by collecting as much subsurface information as possible. This was applied because of available subsurface data to support the development geothermal project are very limited. On the other hand, the program is very influenced and directly involve to the drilling operation that might risks the drilling operation, in term of well stability and well cost perspective. Thus, the effective and efficient techniques are highly required in the making of proper formation evaluation program. The main purpose of formation evaluation is to get a better understanding of subsurface geology and developed geothermal system so the best future strategy of field development can be determined. By doing this evaluation while drilling, it will give an advantage to the drilling operation itself. Knowledge of the real-time drilled formation, in term of its behaviour, characteristic, and composition, will help the improvement of drilling performance and increase the well success ratio. This paper will present the available technical guidelines that can be applied to evaluate the formation during drilling operation, completed by the pros and cons. Those techniques including drill cutting examination and analysis, drilling data interpretation, rock coring analysis, both conventional and sidewall coring, and well-logging, including PTS logging, acoustic and electric borehole imaging, and also other logging activity. All those techniques expected to enhance the better understanding of subsurface geology and improve the drilling operation

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