WAG Process Optimization in the Rangely CO2 Miscible Flood

Abstract

Abstract A miscible CO2 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 CO2 injection patterns to management of CO2 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 CO2 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. Simulation studies and field tests have also been completed to determine an optimum half-cycle for the WAG process in Rangely. A half cycle is defined as the amount of CO2 or water that is injected, measured in pattern hydrocarbon pore volumes, before switching to the alternate fluid. Our simulation and pilot project results have shown that decreasing the half-cycle has favorable economics. Results of the pilot have also demonstrated that production and fluid handling costs can be reduced by controlling fluctuations in gas and water production. Future plans for WAG optimization include continuation of performance monitoring to help optimize tapering strategy, and extension of the reduced half-cycle application to include simultaneous injection of CO2 and water.