Conventional overbalance drilling in naturally fractured reservoirs causes the invasion of mud and fine rock fragments into the fractures, which result to decrease the productivity of hydrocarbon from these reservoirs. In some cases, the sever problem of mud invasion into the fractured prevents flowing of HC's into wellbore after initial well completion especially in the depleted fractured Iranian reservoirs.

Underbalanced drilling (UBD), with a reduced exerted pressure down in the wellbore avoids the build up small solids being deposited on the sides of the hole, it also prevents the invasion of mud and fine rock fragments into the fractured formation which, together, help to improve the productivity of hydrocarbon from the reservoir. UBD method avoids many of the problems associated with the use of conventional drilling such as stuck pipe. Also, the rate of penetration increase by 2 to 5 folds by using UBD method, which resulted in low cost of drilling.

The main objective of this study is to investigate the use of UBD method in the fractured reservoirs in southern Iranian oil field in terms of drilling equipment's, drilling operation, drilling fluid, formation characteristics, and economic evaluation as compared with conventional overbalance drilling method.

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