1. Severe lost circulation during overbalanced drilling condition is reported during drilling depleted carbonate fields in Iran, for this reason using the underbalanced drilling technique with proper wellbore stability analysis is recommended for drilling in these fields. 2. An elastoplastic model combined with a finite explicite code was used for mechanical wellbore stability analysis of underbalanced drilling technique in two depleted Iranian fields. Based on the results and compared with field data using elastoplastic models gives good predictions for wellbore stability in these fields. 3. A criterion based on size of yielded zone or NYZA (Normalized Yielded Zone Area), was used to assess stability condition. Based on the simulation results and for keeping UBD condition in most of the time the critical value of NYZA was adjusted to 1.20 instead of unity in Iranian carbonate fields. Choosing this critical value an ECD of 5.06-5.30 Lb/Gal was proposed for drilling the well in field A.


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