The Handil field is one of the mature fields in the Mahakam Delta, East Kalimantan, Indonesia. This field is widely known as an oil producer for more than 40 years with peak production of 180,000 BOPD in 1977 and has been challenging in terms of oil production decline ever since. Today, this field delivers ~16,000 BOPD from 115 active wells with more than 90% of oil production coming from gas lifted wells. Therefore, evaluating gas lift performance is very crucial to maintain hydrocarbon production of the field. As a gas lift well is produced, it is common to find gas lift unloader damage, sealing element problems, or even leaks at the tubing due to aging of equipment that degrades the gas lift performance. This paper explains the use of well testing data on investigating the performance of gas lift by estimating gas lift injection depth. The best fit vertical lift correlation should be chosen to represent actual pressure profile of the wells inside the tubing and annulus casing pressure. Estimated injection point is derived from gas lift unloader valve opening status or meeting point between tubing and casing pressure profile. The calculation was done using computational simulation and was applied for every flowing gas lifted well in an integrated module. Based on the simulation, wells that were found to encounter behavior anomalies requested to perform P-T (pressure temperature) + spinner surveys to confirm leak points prior to leak isolations. Based on 3 proven leak cases, it is confirmed that estimated gas lift injection point from simulation versus production logging survey are in line. In 2019, we had 6 gas lift well cases that were confirmed to have a leak and continued with a leak isolation program. After these wells were put back into production, it gave cumulative oil production of up to almost 100,000 Bbls oil. The high success rates of this method verifies the applicability of this effective approach to maintain gas lift performance and is easy to replicate for others PSC companies.