Numerical Simulation of CO2-N2 Combination Flooding in Low Permeability-Ultra Low Permeability Homogeneous Reservoir

For low permeability, ultra-low permeability reservoir, the CO 2 preslug + N 2 flooding, using CMG numerical simulation software, the ideal homogeneous model which permeability is 3 × 10 −3 μm 2 has been established. The recovery of CO 2 flooding under the 11 length-thickness ratios was calculated; the effects of 7 kinds of flooding slug combinations and 4 kinds of permeability on recovery with input-output ratio were analyzed; factors such as length-thickness ratio, permeability and injection slug combination affecting flooding effective were studied deeply. The result shows that: when the length-thickness ratio under the 100, as the increasing of the length-thickness ratio of the model, the oil recovery increased. When the length-thickness ratio reaches to 100, the oil recovery tends to be stabled, that is, when the single oil layer thickness less than 1% of the well spacing, it is more benefit to improve the oil recovery. As the permeability increases, the compound flooding recovery will decrease when the CO 2 injection (PV) is a constant and the CO 2 slug requires for the combination flooding to achieve the ultimate recovery increases, that is, the lower the permeability is, the more favorable it is for the CO 2 -N 2 combination flooding. In actual oil reservoirs with a lengthen-thickness ratio of 30, the slug composition of CO 2 -N 2 compound flooding should adopt the way of 0.3 PV CO 2 slug + subsequent N 2 .