Under-Balanced Drilling (UBD) practice is an optimized drilling approach, aiding in more reservoir recovery, too. Considering long-period production from some of Iranian oilfields and their correspondent pressure decline, it seems more necessary to replace UBD with the Conventional Overbalanced Drilling (OBD). Utilizing UBD in one of Iranian low-pressure fields named here "S" field however improves Productivity Index (PI) of the wells drilled besides minimizing drilling challenges and increasing Rate-of-Penetration (ROP) but extremely increasing risk of wellbore instability. This paper presents wellbore stability assessment of three layers of Mishrif reservoir using common reservoir data, correlated in-situ stress regimes and rock mechanics parameters based on its lithology & regional data. By employing a computer modeling software, stability limitations of the borehole as optimized Equivalent Circulating Density (ECD) were specified through the Finite Difference Method (FDM) and Mohr-Coulomb criteria. By applying the results of this modeling, about 6000 ft of 5 wells in "S" field was drilled with good hole condition and negligible instability problems while mentioned benefits of UBD was preserved.

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