Experimental study on timing and pattern of gas injection in low permeability reservoir

Experiments were conducted to study the selection of development methods for low permeability reservoir exploitation after water drive, including the effects of different gas injection cycle times and different injection modes on oil displacement efficiency. Results showed that both gas injection cycle time and injection mode have significant effects on oil displacement efficiency. Based on the results of gas injection cycle time of the displacement with mode shifting to gas drive when water cut reaches 50% and 100%, it can be concluded that in the low permeability reservoir, which is exploited by water drive in the early time, the sooner the gas injection, the higher the oil displacement efficiency is. Because high water saturation can cause serious water-blocking effect, leading to a large amount of trapped oil and low displacement efficiency. Further, in view of the results of water drive, gas injection, and water alternating gas (WAG) flooding, it is obvious that WAG flooding is one of the best methods in enhanced oil recovery. Under the effect of gravity differentiation, WAG flooding can spread to the upper reservoir, which cannot be swept by water drive. Thus, the oil recovery factor can be improved effectively. However, it should be noted that gas injectivity should be considered in the field test.