Abstract
Underbalanced drilling (UBD) is defined as a condition which 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 often this problem greatly reduce oil and gas productivity in open hole horizontal well section. The advantage of reduced formation damage results from the fact that the wellbore pressure is intentionally maintained lower than formation pressure during drilling operation. In order to achieve this condition several drilling techniques are used such as aerated drilling, foam drilling which based on low drilling density. In appropriate design or inability to maintain well in underbalanced condition during operation will be potential source of sever invasion damage which linked to several mechanism for damage like spontaneous imbibition, glazing, and macroporosity. Therefore, it is significant to select reservoir candidate carefully, to achieve high profitability. Furthermore, during UBD formation fluid invade into to wellbore therefore the size and design of surface equipment like separator and reserve tanks are required to handle the produced well fluid. Therefore, the ability to accurately estimate the influx volume and rate of produced fluid is critically significant to size and design surface equipment. In this project a mathematical model has been designed based on rigorous reservoir borehole-cross flow which used to determine the influx volume during UBD and investigate how aerated and foam drilling fluid acts during UBD.
Highlights
International Association of Drilling contractors (IADC) has defined underbalanced drilling (UBD) as a technique in which equivalent circulation density (ECD) of drilling fluid is intentionally maintained less than effective formation rock pressure (Pf) in openhole section at any depth
Bennion and Thomas [1] have stated the possibility of existing formation damage in condition where original saturation of hydrocarbon or water in the reservoir is lower than irreducible saturation of the phase that used in drilling
Several potential damages could appear during Underbalanced drilling (UBD) such as spontaneous imbibition
Summary
International Association of Drilling contractors (IADC) has defined underbalanced drilling (UBD) as a technique in which equivalent circulation density (ECD) of drilling fluid is intentionally maintained less than effective formation rock pressure (Pf) in openhole section at any depth. Bennion and Thomas [1] have stated the possibility of existing formation damage in condition where original saturation of hydrocarbon or water in the reservoir is lower than irreducible saturation of the phase that used in drilling. This lead to spontaneous imbibition is caused by capillary pressure and wettability properties. The key principle of underbalanced drilling is keeping downhole pressure less than formation pore pressure This sometimes occur naturally when unweight fluids are used and the reservoir is geostatically over pressured for its depth [1]. Tendency of formation fluid to flow into borehole (Figure 1)
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