Planning UBD in Hroizontal Well to Minimise Formation Damage : 9783659713194 : 22 May 2015 : Underbalanced drilling (UBD) is defined as a condition which is generated when effective downhole circulation pressure of the drilling fluid system in touch with reservoir rock is less than presenting formation pressure. Underbalanced drilling (UBD) has been utilized as a mean to eliminate or significantly reduce the problem associated drilling like formation damage which is often this problem greatly reduce oil and gas productivity in open hole horizontal well section. The significant of minimizing formation damage results from the fact that wellbore pressure is intentionally maintained lower than effective formation pore pressure during drilling operation. In order to achieve this condition several drilling techniques are used such as aerated drilling, foam drilling which are based on low drilling density. In appropriate design or inability to maintain well in underbalanced condition during o