During recent years, interest in underbalanced drilling (UBD) has grown rapidly. As a drilling technique it has gained acceptance because it provides a method of minimizing formation damage, preventing lost circulation risks and increasing penetration rates. One of the most important tasks in UBD design is to estimate how much oil can be produced during the drilling operation. This task is very difficult because of the complex nature of the multiphase flow in the UBD system, especially in the annulus between the drillpipe, collars and the wellbore where water, gas, cuttings and fluid influx from the penetrated formations are present. To accomplish this task, the bottomhole pressure must be calculated. However, the bottomhole pressure, fluid influx flow rates, and fluid properties along the wellbore are interdependent parameters and can only be derived through a combination of iterative and finite differential methods. It is therefore necessary to use a computer program to carry out the work.

This paper presents a method to predict production capacity by coupling drilling parameters and inflow performance parameters such as surface choke pressure, gas injection rates, liquid flow rates and fluid production rates for UBD. A software tool named UBDRILL has been developed which can be used to perform thorough investigations of UBD hydraulic calculations and the estimation of production capacity.

The proposed method and developed software are validated by application examples of UBD in granite basement reservoirs in Vietnam, SE Asia.

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