Because EOR oil response at Prudhoe Bay has been difficult to measure directly, a number of different types of field measurements have been made to evaluate the miscible-flood displacement efficiency. These measurements include water- and solvent-injection profiles, logging data from an observation well, and single-well tracer test (SWTT) data. Despite ambiguity in these data, the measurements support the simulation and laboratory data and generally indicate that the Prudhoe Bay Miscible Gas Project (PBMGP) is performing well. The most useful EOR surveillance data have been the separator-gas-sample database, with =4,000 compositional analyses. Separator flash analysis and allocation programs use this database to infer EOR performance on the basis of produced solvent. Reservoir mechanisms that adversely affect the EOR process efficiency have been identified. The project has exceeded initial expectations in terms of solvent retained within the reservoir, which has favorable implications for solvent sweep efficiency. Procedures have been developed to use the field and simulation data to determine how the solvent should be allocated to the existing patterns and when the project should be expanded into new areas. These procedures are designed to maximize the value of the PBMGP.
The paper describes the Prudhoe Bay field and the history of EOR operations there, then examines the various field measurements that have been made to evaluate the miscible-flood performance. The bulk of the paper examines compositional data gathered from separator-gas samples. A brief background of laboratory and compositional simulation data and how they are used to interpret the gas-sample data is provided. The paper then describes the programs that use the separator-gas samples and the monthly production database to infer EOR performance. The performance to date of the PBMGP is examined and reservoir mechanisms adversely affecting EOR are cited. The paper concludes with an explanation of procedures used to determine solvent allocation and when the project should be expanded into new areas.