The paper describes the development of an EOR project for a supergiant oil rim field, and how the optimal injected gas composition was determined. This includes the laboratory tests to develop a miscible gas equation of state (EOS), cryogenic plant design considerations, and the evaluation of gas EOR development strategies for the different Novoportovskoye reservoirs.
Facilities models were built to evaluate options to increase extraction of C2-C4 components from produced gas. This rich gas can then be mixed with lean gas to create miscible injectant (MI) for EOR. Because the gas caps are so lean (90+% methane), the best options were to chill the produced gas to either −55°C or −80°C with a turbo-expander unit. The lower temperature option requires special metallurgy, but it boosted extraction by 35%.
The first EOR PVT model was designed from the limited compositional data available. MI compositions for each reservoir were developed. A laboratory experimental program was undertaken to calibrate the EOS for Novoportovskoye EOR. The new EOS was used to determine the optimal composition of the MI. According to slimtube tests and simulation studies, the optimal composition of miscible injectant is 62-67% methane, with the rest being C2-C4.
The currently planned waterflood patterns and gravity drainage/gas cap expansion areas were studied to determine the best areal and cross-sectional location of the MI injectors. An integrated model was built to account for injected and produced gas volumes and compositions. This allowed the analysis of bottle-necks in the production network, cryogenic plant, and gas facilities.
WAG in most areas performed better in terms of gas utilization, which leads to increased incremental oil given the same amount of MI available. By choosing the most efficient patterns in terms of gas utilization it is possible to increase oil recovery in these areas by 10-15% OOIP.
