Abstract An extensive study was conducted to evaluate enhanced oil recovery (EOR) potential of a giant, offshore, Middle East oil field. The subject field contains a very large reservoir with light oil and has a long production life extending beyond 100 years. Primary recovery began in 1968. The field has been under water flooding with pattern injection since 1982. In order to sustain oil production and increase the ultimate recovery, EOR will likely be implemented at some future time. Miscible gas injection with a water-alternating-gas (WAG) injection scheme is considered to be the most suitable method among the currently available technologies. The WAG injection process is a well-established EOR technique with several successful field applications around the world. This paper describes Water Alternating Gas (WAG) Optimization through Tapered WAG technique (more gas is injected earlier and then reduced over time) and its impact in the subject reservoir. The primary focus of this study is to estimate the benefit of miscible WAG EOR for the subject reservoir. Finely-gridded, compositional, mechanistic, 2D cross-sectional models as well as 3D sector models were constructed and used for the evaluation. One non-hydrocarbon (CO2) and two hydrocarbon ("associated" and "lean') miscible gases were tested as injectants. The study encompasses different geological areas in the same reservoir and addresses key design parameters including well spacing, optimal WAG operating scheme, and timing of WAG application. In this paper, we will focus on formulation of optimal CO2 WAG operating scheme. As part of optimization of the miscible WAG application, a concept of Tapered WAG was tested. A typical WAG application consists of injection of water followed by gas injection. Each cycle of water and gas injection is of fixed duration. In Tapered WAG concept, the durations of gas injection varies with longer gas injection earlier and reducing with progressive WAG cycles. Such miscible WAG application, termed as Tapered WAG, was found to be more effective than the Uniform WAG application where the water and gas injection cycle durations are same. The Tapered WAG technique reduces response time i.e. oil bank arrives earlier. It also uses gas injection more efficiently, i.e. produces more incremental oil per unit of injected gas. Findings were applicable to both a homogeneous (low and relatively Uniform perm) and heterogeneous (high perm with high perm streaks dominant) areas of the reservoir.