The value of underbalanced drilling (UBD) and completion technology lies in several distinct areas. These include improvement in drilling performance by, for example, reducing losses or differential sticking, or increasing rate of penetration or even depletion of troublesome high-pressure zonesi . UBD is also used to improve production rates and increase ultimate recovery. In these cases, UBD has been applied to avoid formation impairment and thus to enhance well productivity. Recently, some operators have recognized that value can also be created through gathering and interpreting reservoir data while drilling underbalanced. This information is used to improve reservoir knowledge and consequently enhance reservoir management.
In low-cost drilling environments, such as land operations in the Middle East and USA, UBD drilling-enabling savings are often marginal and the cost of UBD operations becomes a blocker for wider implementation. The extra cost of UBD acts to discourage a company from fully evaluating the technology's capabilitiesii , even following technically successful trials. In addition, historically the Well Engineering community has championed UBD programs often with scant involvement from the subsurface Petroleum Engineering (PE) disciplines.
Petroleum Development Oman (PDO) operates in such a low-cost drilling environment. PDO commenced UBD campaigns in 2002 with the primary goal of enhancing production and has succeeded in sustaining UBD operations by making significant progress both in reducing costs and increasing the value of information derived from UBD to the Petroleum Engineering organization. This has created the opportunity to more fully evaluate the technical and economic benefits that UBD can deliver and the possibility to deliver cost-neutral UBD wells.
This paper describes some of the challenges faced when initially introducing UBD and the step-by-step approach followed by PDO in Oman and Shell Exploration & Production Co. (SEPCo) in the USA to widen its implementation to other fields. The paper underlines the need for integrated well engineering and petroleum engineering teams in UBD and confirms the pivotal role of sub-surface engineers in interpreting and using the additional and complementary reservoir data that becomes available during UBD. The paper also analyses more closely the actual cost of UBD and provides a balanced perspective on the longer-term UBD value equation.