Semoga field, which produces from Baturaja (carbonate) formation, has experienced a decline in production and has a high water cut (WC). The current average water cut in Kaji-Semoga field is 86%, while several producing wells have above 95% WC and even 100%. In 2007, Lemigas conducted a laboratory study to determine the wettability of Baturaja formation (BRF), the result of which was that BRF indicated very oil wet characteristics. However, after being immersed in surfactant, core samples became less oil wet. This test shows that trapped oil can be recovered using tertiary recovery methods. One option for confirming the laboratory test result is to use surfactant huff & puff method in a pilot project. Surfactant is a surface active agent compound which has the ability to reduce the surface tension between the rocks, produced water and crude oil so that wettability may change. The surfactant type to be used on BRF formation is non-ionic. The pilot huff and puff project was conducted on wells X-1 & X-2. High WC wells with low WC at initial (early production) and high liquid production were selected to minimize risk and to yield more oil. This project was evaluated using pressure build-up test (transient analysis) and production performance monitoring before and after the surfactant stimulation job. An economic analysis was also conducted As a result of employing surfactant stimulation, Semoga Field yielded NPV @ 3 month 24,013 USD with a pay-out time (POT) of 11 days. Oil gain would be more significant if the technique was implemented on high production wells.

Continuous gas lift system is currently widely used as artificial lift in Kaji-Semoga Field, in fact at about 46% of total producing wells. The average depth of gas lift wells in Kaji-Semoga is 3,200 ft, utilizing 2 to 5 conventional gas lift valves in a single production string. Common problems experienced when optimizing gas lift wells in Kaji Semoga field are instability of flow due to fluctuation of gas lift injection rate and pressure, limited gas injection volumetric rate, and limited compressor discharge pressure that leads to limited casing head pressure at well head, especially for remote wells with high tubing pressure at injection point.A new injection valve type, venturi orifice gas lift valve with breaking-out gas device, has been studied and proposed as a solution to the aforementioned problems. This type of valve has been installed as gas lift injection valve at some pilot wells by using slick-line unit. The aim of venturi orifice is to reduce pressure difference between casing (upstream) and tubing (downstream) at injection point and to deliver a greater amount of gas lift injection at the same casing head pressure (compared to traditional orifice valve). Meanwhile, the aim of the breaking-out gas device is to break the injected gas into very small bubbles and homogenize with the liquid so that flow stability can be achieved.Selected candidates for pilot wells are the ones with high productivity index (PI), high flowing pressure gradient (above 0.18 psi/ft) and limited gas lift manifold pressure. Well modeling and simulation have been conducted for these selected wells using production optimization software to predict gas lift well performance after installation of new injection valve, whereupon the simulation result is matched with actual data.Applying venturi orifice gas lift valve has produced successful results: the liquid rate of the pilot wells has increased by about 40%, with 30% gas injection rate increment under the same conditions. Computer simulation also provided similar results to the actual well performance and met expectations. The pay-out time (POT) of this project was less than 2 days.

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