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The Arun natural gas field is located onshore in the North Sumatra Basin, Aceh Province, Sumatra, Indonesia and is operated by Mobil Oil Indonesia Inc. under a Production Sharing Contract agreement with Pertamina.The ability to predict Arun water production is essential for planning and scheduling future field facibties and forecasting future Arun gas deliverability. Arun water production is primarily due to condensation of water vapor which is produced with the gas. Therefore, the prediction of water production requires the ability to describe the equilibrium water vapor content of the reservoir gas as a function of reservoir pressure. The increasing water vapor content will help to maintain reservoir pressure, and will increase the ultimate recovery of hydrocarbons from the reservoir.Three calculation methods were used to predict this equilibrium water vapor content. Values predicted by these three methods were very close and are confirmed by field production data. As reservoir pressure declines the water vapor content will increase. The future water production was predicted based on gas demand and the estimated water vapor content of the Arun gas. Further studies have been carried out to design and to schedule the additional field facilities which are required to handle this water production.