Field "A" in Indonesia will be drilled dual lateral using underbalanced drilling technique. The reservoir has been highly depleted, low reservoir pressure about 1130 psi and high bottom hole temperature about 350°F. This field produce natural gas and condensate which contain 80-100 ppm H2S, 15% CO2 and water. Underbalanced drilling will be conducted to minimize the potential problems by utilize Coiled Tubing and nitirified mud.

Coiled Tubing Drilling (CTD) has reached its popularity in last two decades. In principle, CTD provides hydraulic horsepower which is transfered to the bottomhole. In operation, CT has advantages and disadvantages compared to jointed pipe. CT Unit (CTU) are complete package consists of surface and subsurface equipments.

Underbalanced drilling has several advantages: to minimize problems, such as lost circulation, pipe sticking, formation damage, etc. From several drilling fluid, only two types are available in this field and nitrified mud is choosen for several reasons and based on qualitative method regarding factors: wellbore diameter, water influx, gas production, and Extended Reach Drilling (ERD). Gas injection methods are also reviewed.

Combination of CTD and underbalanced drilling, which is dual lateral drilling, delivers maximum productivity improvement compared to single lateral. This is obtained from well performance study or upflow performance study from existing wells. This study is conducted to estimate productivity improvement for various lateral configurations, liner diameter, and well position.

This paper discusses the multiphase flow modelling of underbalanced drilling in Field "A" to determine operational drilling parameters, to select effective drilling fluid and to select CT string and BHA. The main concerns are the challenging conditions in Field "A": 1). High temperature, 2). Corrosive environtment, and 3) Requirement of effective drilling fluid. The outcome of this paper is to provide analysis of upflow performance study, operational window parameters, optimal drilling fluid, and CT string and BHA.

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