Gas Injection into the Tengiz oilfield began in January 2007 with sweet gas injection as Phase 1 of the SGI pilot project. Phase 2 of the project, sour gas injection, began in October 2007, but was interrupted due to initial injection start up problems. Continuous sour gas injection began in January 2008. As part of the SGI Surveillance Plan, a variety of reservoir measurements are routinely acquired to monitor the progress of the flood. The SGI project has four signposts for success: Compressor reliability. Injectivity, wellbore durability, and reservoir performance. The sour gas compressor at Tengiz was the first of its kind and has had greater than 90% availability when SGP has been operational. Injectivity has exceeded expectations and wellbore durability has also been excellent. The fourth area is reservoir performance and all indications are that the reservoir is performing as expected. The SGI project consists of seven inverted five-spot patterns. To expedite data acquisition, the SGI well patterns were designed to include one "super-spot" pattern (twin injectors 100 m apart providing dedicated injection support to different geologic layers) and three short-spaced producers (producer-injector spacing approximately 1/3 of the standard spacing). Tracers, pulse tests, multiphase meters, gas saturation logging, and production and injection are used to monitor and understand reservoir performance. A specialized simulation model (Monitoring Model) was constructed which uses local grid refinement in the SGI pattern area. On August 22, 2008, gas breakthrough occurred in Well T-318, the first of the short-spaced producers. This breakthrough was predicted with the Monitoring Model. Predictions from Monitoring Model suggest most likely breakthrough is near top of reservoir, and this was substantiated by logs. The gas injection project will continue to be closely monitored to improve our understanding and reservoir forecasts.