Underbalanced drilling (UBD) has been recognized as an effective technique for reducing formation damage and finding new discoveries of oil and gas reserves in Southwestern China. However, the excess production of formation water in UBD has hindered the application of this technology in several other regions. The rate of formation water production increases with the gas injection rate that decreases bottom hole pressure. Adding water to the injected nitrogen stream can moderate bottom hole pressure and thus mitigate the water production problem. Optimizing nitrogen and water injection rates to depress formation water production requires accurate prediction of water influx from the formations under various drilling conditions. Currently there is no rigorous method that can be used to perform such a task with adequate accuracy. This paper helps fills the gap. The NODAL analysis technique traditionaly employed in oil and gas production forecast was introduced to UBD in this work to predict water influx rate and optimize nitrogen gas and liquid injection rates. This paper presents analytical models and solution procedure that are necessary for performing NODAL analysis in nitrogen-UBD design. Inflow performance relationship (IPR) is established on the basis of horizontal well productivity models. The outflow performance relationship (OPR) is constructed using a close-form analytical solution for gas-water-solid 3-phase flow in annular space. It was found that the NODAL analysis technique was able to predict water influx rate with drilling time and thus cumulative water production under the nitrogen-UBD conditions in Sichuan, China. This paper provides drilling engineers a practical method for controlling formation water production in UBD operstions.
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