Formation damage and heavy circulation losses represent critical factors for drilling, testing and completion of depleted reservoir. In mature fields like Belayim oil field in Egypt operating under depletion zones that exists between high pressurized shale causes a serious drilling problems such as differential sticking, drilling fluid loss to the formation and hole packed off due to shale movements. A complete study was made prior to drilling the horizontal section 6" diameter in zone III of well 112-78H, while maintaining under-balanced conditions in the wellbore. The sandstone pressure of zone III was estimated to be 3000 - 3500 psi which was suitable for UBD application. Zone III is an oil bearing sandstone reservoir lying at approximately 2350 mVD, it's approximately 3m thick. The primary objectives of applying the UBD technique in Belayim field were to:- minimize formation damage caused by fluid invasion in the near wellbore zone eliminate conventional drilling related problems such as differential pipe sticking and drilling fluid loss to the formation improve drilling performance. All the afore said objectives were achieved while drilling the 6" hole section of 112-78H. Due to geological uncertainty, two successful sidetrack were made one in 8 ½" hole (overbalance) and other one in 6" hole (underbalance).

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