The degassing of crude oil in the formation will increase the seepage resistance,which seriously affects the development effect of low permeability reservoirs.This paper takes a low permeability reservoir in Weicheng as an example,using steady state water flooding test method to explore the effect of oil viscosity change on the oil water two⁃phase threshold pressure gradient,and study the effect of threshold pressure gradient on the effective displacement pressure difference in low permeability reservoirs.The results show that the threshold pressure gradient increases with the increase of oil viscosity after oil degassing,which results in the decrease of effective displacement pressure difference between wells.When the injection and production pressure difference is small,it is difficult to establish effective displacement.At the same time, some countermeasures are put forward to deal with this problem.The research results are of guiding significance for the design of injection⁃production well pattern and the later adjustment in low permeability reservoirs.

Field experiments of domestic and overseas studies show that WAG flooding can further improve the oil recovery by 5%~10%. It can be used as an economical and effective way to improve oil recovery in the tertiary phase.With the help of laboratory displacement experiments,improved simulated cores,and setting up the experimental control group, a new experimental idea is put forward, and the mechanism of WAG flooding for enhancing oil recovery is revealed and clarified. The experimental results show that WAG flooding can slow the gas channeling and change the unfavorable water⁃oil ratio. Injecting⁃gas enhances the exchange, diffusion and imbibition of three⁃phase molecules. After alternating injection of water and gas, water drive seals high permeability zone, gas drive sweeps tiny pores along with the effect of gravity differentiation. The displacement process is a dynamic process in which the state of water binding in pores is broken constantly, and the large pores are plugged alternately. The breakthrough effect of injection gas on fluid is weakened, and the water absorption profile is improved, which prolongs the water breakthrough time of oil well and improves the oil recovery. For the reservoir with serious heterogeneity, the dynamic plugging effect produced by alternating water injection can further improve the oil displacement effect of WAG flooding.