Laboratory study on optimization of CO_2 injection development ways of ultra-low permeability reservoirs

In order to seek effective CO2 injection development ways of ultra-low permeability sandstone reservoirs,CO2 injection simulation experiments under different CO2 injection ways are carried out with long core samples. The results show that: for those reservoirs in which natural fractures is not developed,and the development way of directly injecting CO2 is the best under the condition of CO2 miscible phase injection,its oil displacement efficiency can be increased by 48. 97% than that of water flooding; under the condition of CO2 immiscible phase flooding,the development way of CO2 injection after water flooding is the best,its oil displacement efficiency can be increased by 35. 41% than that of water flooding,moreover it will has the functions of water control and reducing the influence of the gas fingering. For those reservoirs in which natural fractures is well developed,the alternating flooding of water with gas is the best development way regardless of miscible or immiscible flooding,its oil displacement efficiency can be increased by 20. 39% and14. 34% respectively than that of water flooding under miscible and immiscible flooding. Under miscible flooding,optimal gas slug size and gas-to-water ratio are 0. 05 PV and 1: 1 separately; while under immiscible flooding,optimal gas slug size and gas-to-water ratio are0. 1 PV,and 1: 2 separately.