When overbalanced drilling is done in depleted reservoir, lost circulation occurs and cause formation damaged. Underbalanced drilling (UBD) is proposed to be done to prevent lost circulation and increases production due to minimizes formation damaged. Besides, as formation being drilled underbalanced, it is possible to produce hydrocarbons from permeable zones, therefore earlier benefit could be acquired by company as producing phase began.When a well is drilled underbalanced, formation fluids flow into the wellbore from any permeable formation in the open-hole section. It is possible to use the volumes of produced hydrocarbons, from a well drilled underbalanced, to provide an indication of the productivity of pay zones that have been penetrated (GRI, 1997).The study in this paper is started from reservoir simulation to estimate the current reserve and plan for re-entry and new drilling well. Furthermore, drilling fluid selection and pressure window are analyzed based on reservoir characteristics and engineering approach. The last, the total hydrocarbons gained during underbalanced drilling in reservoir section will be predicted by Vogel equation.

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