Lack of gas source and poor physical properties of oil are two key problems for the application limitation of CO2 miscible flooding in China. However, in Jilin oilfield, the discovery of natural gas reservoir with different CO2 content (5%-98%) make it possible, especially for some low permeability blocks. MMP determined by slim tube test is 23.5MPa. Reservoir pressures after prolonged waterflood development are maintained at 23 MPa to 24.5 MPa. Thus, CO2 near miscible flooding for these oilfields is of great potential. Advanced CO2 injection is proposed for the undeveloped blocks and WAG is proposed for the high water cut blocks.

Long core (~1m) test shows that CO2 injection before oil production can ease the fast decline of reservoir pressure for this low permeability reservoir. Oil and CO2 has more time to contact then miscibility can be fully developed, fingering improved and oil damage of depleted development can be avoided. Recovery incremental is about 6.7%.

In addition, injection rate, injection opportunity, injection pressure, water gas ratio (WGR), water cut, gas oil ratio and oil recovery were all considered to optimize the parameters of WAG in waterflooded reservoirs with different water cut (30%, 50%, 75%, and 90%). Results show that the optimum WGR is 1:2, and the optimum gas slug is 0.1 PV. Channeling and bypassing were concluded to be the main reason for the early breakthrough and low oil recovery considering the low transmissibility. However, slow gas injection rates, may fail to maintain pressures and oil rates. Thus, the optimum injection rate is 0.05 cc/min for both water and CO2. Also, it was found that the earlier the WAG conducted, the faster the water cut and oil production response, and the more obvious the water cut decline and oil recovery increase. Results show that more than ~30 % of the residual oil after water flooding could be recovered, which indicates that CO2 near miscible flooding, including advanced CO2 flooding and WAG, could be feasible in some blocks of this low permeability reservoir.

The results of this study should contribute to our understanding of the mechanisms and injection strategy of CO2 near miscible flooding in water drive reservoirs and support the ongoing research, development and application of CO2 tertiary oil recovery process in Jilin Oilfield.

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