Abstract:In order to form miscible flooding after CO2 injection in extra-low permeability reservoir, the pressure maintaining design method of injection-production pressure system in CO2 miscible driving of extra-low permeability reservoir was studied through seepage mechanics and reservoir engineering methods. Firstly, the seepage resistance mathematical model of CO2 miscible flooding in extra-low permeability reservoirs was established by using equivalent seepage resistance method with considering the "four steps" characteristic of reservoir permeability distribution after fracturing in extra-low permeability reservoirs. Secondly, considering the displacement hysteresis effect caused by the dissolution of CO2 in crude oil after injection into the formation, the CO2 displacement hysteresis factor was introduced to modify the traditional front-edge displacement equation. On this basis, the pressure maintaining design method of injection-production pressure system in CO2 miscible driving of extra-low permeability reservoir was established. The influence degree of single factor was analyzed, including injection-production pressure difference, injection-production well spacing, near-well permeability of injection well and near-well permeability of production well. The results showed that the injection-production pressure difference had the greatest influence on the pressure maintaining design parameters. The ratio of seepage resistance in the pure oil zone and the CO2 miscible zone was only related to the injection-production pressure difference, and had no relationship with the other factors and the displacement front. The CO2 injection speed was the key parameter for the pressure maintaining design of CO2 miscible driving. And within the reasonable construction parameters, it was unable to achieve miscible driving in the whole distance. The limit position of displacement front is about 3/4 of the distance between oil and water wells. The relationships between the CO2 injection speed and displacement front position in M reservoir under different injection-production pressure differential conditions were drawn, which can provide theoretical basis and technical guidance for the pressure maintaining design of injection-production pressure system in CO2 miscible driving of extra-low permeability reservoir.