A miscible C02 project was initiated in the Rangely Weber Sand Unit in northwest Colorado in 1986, Over the past seven years, the project has expanded areally to include 80% of the Unit, and substantial investments have been completed in injection, production and gas recompression facilities. As the project matures, emphasis has shifted from adding new C02 injection patterns to management of C02 injection, with goals of maximizing recovery and reducing operating costs. Optimization of the Water-Alternating-Gas (WAG) injection process is an important part of this effort.Our initial injection strategy was based on reservoir simulation studies. It became apparent as the project matured that we would need to make changes in WAG processes to adapt to operational constraints and maximize pattern economics. Flood performance was optimized through further simulation and field testing of changes to C02 slug size and WAG ratio tapering sequence. New methods for managing the WAG process based on injection pattern performance and economic triggers have been adopted. We have completed an initial comparison of predicted versus actual performance for injection patterns with tapered WAG ratios.References and illustrations at end of paper. 141

A miscible C02 project was initiated in the Rangely Weber Sand Unit in northwest Colorado in 1986, Over the past seven years, the project has expanded areally to include 80% of the Unit, and substantial investments have been completed in injection, production and gas recompression facilities. As the project matures, emphasis has shifted from adding new C02 injection patterns to management of C02 injection, with goals of maximizing recovery and reducing operating costs. Optimization of the Water-Alternating-Gas (WAG) injection process is an important part of this effort.Our initial injection strategy was based on reservoir simulation studies. It became apparent as the project matured that we would need to make changes in WAG processes to adapt to operational constraints and maximize pattern economics. Flood performance was optimized through further simulation and field testing of changes to C02 slug size and WAG ratio tapering sequence. New methods for managing the WAG process based on injection pattern performance and economic triggers have been adopted. We have completed an initial comparison of predicted versus actual performance for injection patterns with tapered WAG ratios.References and illustrations at end of paper. 141