Fractured-cavity reservoirs have diverse storage spaces

strong heterogeneity

large differences in oil-water properties

high mobility ratio of immiscible nitrogen flooding in water flooding

serious water and gas channeling

limited sweeping range

and primary and secondary oil recovery stages. Oil recovery is much lower than that of clastic sandstone reservoirs. Taking the S91 fracture-cavity unit in Tahe Oilfield as the object

on the basis of geological modeling

history matching

remaining oil characterization and phase state experimental fitting

a numerical simulation study on enhanced oil recovery by hydrocarbon injection in fractured-cavity reservoirs is carri

ed out

so as the evaluation of the hydrocarbon-gas miscibility conditions in fractured-cavity reservoirs

and the optimization of EOR parameters of hydrocarbon injection. The results show that the minimum miscibility pressure of injected hydrocarbon gas and formation crude oil in S91 unit is 42.5 MPa

which can be miscible under the current formation pressure. The mechanism of miscibility is vaporization gas flooding. As the content of heavy hydrocarbon components in the injected gas increases

the recovery factor increases. The gas injection method is gas injection in fractured reservoirs

and liquid recovery in caved reservoirs. The gas injection parameters are preferably 168×10

4

m

3

gas injection rate of 4×10

4

m

3

/d

and injection-production ratio of 1∶0.9.