Gas lift plays a major role in the oil production operations of mature depleted fields of various E&P companies worldwide. Likewise, in Oil India Limited, India about 48% of producing wells of Upper Assam Basin are on gas-lift assistance. In this project, we have started and completed a gas-lift optimization campaign in Hapjan field, one of the brown fields of Oil India Limited. This project includes complete problem troubleshooting and optimization study of all the 23 gas lifted wells of the mature field. All gas lifted wells of the Hapjan field are intermittent lift by design and are operating with surface intermitters. As such, all available diagnostic tools developed and available for intermittent gas-lift well optimization are aggressively used during this campaign. The used tools are as mentioned below:Two-pen pressure recordings.Gas volume measurements.Acoustic surveys.Downhole pressure/temperature and long cycle surveys.Revamping of compressor station. In the present optimization campaign, entire candidate wells of Hapjan field were included and the endeavor has resulted in crude oil production gain of 76 m3/day, which is 11.8% growth of the total production of the field. In addition, based on the results of the comprehensive study of vertical regime of the wells, 3 wells have been taken up for recompletion. Moreover, the gas volume in the injection gas network have been increased from 280000 Scmd to 340000 Scmd with the commissioning of a new compressor, and by revamping of the all other compressor units of the station. This has further supported to reallocate the required gas quantity in each individual well as per the design volume. The technical experience and knowledge gathered from this gas lift optimization campaign certainly confirms that optimization of gas lifted wells is a vital tool to boost up production in mature depleted fields in Oil India Limited as well as in other exploration and production (E&P) operators worldwide.