Enhanced oil recovery by CO2–CH4 flooding in low permeability and rhythmic hydrocarbon reservoir

•CO2–CH4 flooding is suitable for the different rhythm hydrocarbon reservoirs.•CO2–CH4 flooding owns the highest oil recovery for the homogeneous reservoir.•CO2–CH4 flooding can mitigate the gravity override and delay breakthrough.•Homogeneous reservoir is given priority, followed by positive rhythm reservoir.

To promote CO2 utilization and protect the environment, CO2 is often employed to enhance oil recovery. Meanwhile, injection of CO2–CH4 mixtures can offer better mobility ratios and delay water breakthrough resulting in the favorable oil sweep efficiency. The gas-alternating-water (WAG) flooding in rhythmic hydrocarbon reservoirs has been studied by simulation software. The difference between water and CO2–CH4 flooding lies in the different density and viscosity among CO2, CH4, oil and water. Water tends to displace oil along the lower reservoir; inverted rhythmic reservoir may slow down the trend. CO2 and CH4 tend to displace oil along the middle-upper and upper reservoir, respectively; positive rhythmic reservoir may slow down the trend. Therefore, a homogeneous reservoir should be given priority in WAG flooding, followed by positive rhythmic reservoir.