The oil from the Ngimbang Carbonate reservoir in Sepanjang Island, East Java, Indonesia is composed mostly of heavy black oil with a pour point of 120°F and a wax content of 24.89%. Its specific and API gravity are 0.8574 and 33, respectively, and the bottomhole temperature is 223 °F.

Underbalanced drilling (UBD) operations in this high-pour-point oil marginal reservoir, for two wells, SED-3 and SED-4, were planned to achieve the main goals of improving reservoir productivity, avoiding reservoir damage, enabling fracture identification, and making testing while drilling possible. Secondary benefits expected by employing underbalanced drilling techniques included avoiding drilling fluid losses, increasing the rate of penetration, and minimizing drilling problems.

This paper presents the details as to how the challenges of drilling in UBD mode in a reservoir with high-pour-point and waxy oil were addressed in the planning stage and how the UBD program was implemented in the field. The circulating system for the UBD operation was designed with the capability to heat the fluid system to above the pour point to avoid solidification of the fluid. The drilling fluid was to be heated to 160°F and kept at this temperature to prevent damage to the drilling elastomers. Agitators were installed on all the tanks to prevent wax formation. The effectiveness of these measures and the lessons learned from the UBD operations on these two wells are presented. The paper provides an overview of the achieved well objectives and show that UBD in a heavy oil marginal reservoir is beneficial.

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