Study Of Enhanced Oil Recovery By Nitrogen-low Salinity Water Alternate Injection In Low Permeability Reservoir

Nitrogen-low salinity water alternative injection in low permeability reservoirs combines the advantages of water-gas alternation and low salinity water flooding.Nitrogen can easily flow in formation to displace crude oil due to its low viscosity.The addition of low salinity water slug can not only inhibit the rapid advance of nitrogen to control gas channeling,effectively improve the sweep volume of nitrogen in formation,but also improve reservoir and oil-water properties and reduce residual oil saturation by using low salinity water.It is of great significance to improve oil recovery in low permeability reservoirs.In this paper,the effects of oil-producing gas production,permeability and heterogeneity on the oil displacement effect during nitrogen flow under low permeability conditions were studied by nitrogen flooding experiments.Different ion types were analyzed by interfacial tension and contact angle measurement and the influence of salinity change on the three-phase properties of reservoir rock-oil-water was analyzed by contact angle experiment.Through the low salinity water flooding,the nitrogen-low salinity water alternating experiment was carried out to study the effect of the gas-water alternate injection method on the enhanced oil recovery.The results show that there forms a sensitive interval when the core permeability is less than 2×10^-3?m²during nitrogen flooding.In this sensitive interval,the threshold pressure gradient of gas flooding rises rapidly,the gas production time at core outlet is late,and the gas production speed is obviously higher than that of gas injection when gas channeling occurs.The proportion of gas-free recovery in nitrogen drive increases and the final recovery of crude oil gradually stabilizes at about 40%with the increasing of permeability.Heterogeneity has a significant impact on nitrogen flooding,with the increase of permeability ratio,most of the nitrogen flows along the high-permeability core,and the recovery of the low-permeability core is gradually reduced.It has also been known that different ion types and the decrease of solution salinity result in different degrees of oil-water interfacial tension.5×10³ mg/L diluted formation water has the largest change in three-phase wetting angle during the process of reducing salinity,and the hydrophilicity is measured much stronger.In static imbibition experiment,the results of imbibition recovery test show that the increase of imbibition recovery by low salinity water is about 8.43%-12.83%.Increasing solution temperature and p H value increases the final recovery by increasing the later stage imbibition rate and the earlier stage imbibition rate respectively,while the change of pressure has no obvious effect on the improvement of imbibition recovery.In the low salinity water-nitrogen alternate injection process,nitrogen gas channeling is inhibited by water slug.The recovery of gas-water alternating flooding is 7.56%higher than that of low salinity water flooding and 12.74%higher than that of nitrogen gas flooding.When the permeability is lower than 1×10^-3?m²,it is not suitable for alternate driving of gas and water due to high injection pressure and small increase in recovery factor.Under heterogeneous conditions,gas-water alternating flooding can increase the liquid separation rate of low permeability pipe to a certain extent,but with the increase of permeability gradient,the effect of water-gas alternating flooding on improving the recovery rate of law permeable core is becoming more and more small.