The Effects And Mechanisms Of Low Salinity Waterflooding On Oil Recovery In Medium And Low Permeability Reservoirs

The medium and low permeability reservoirs in D oilfield are rich in oil reserves,and currently they are mainly developed by water injection.However,the salinity of the injected water is relatively high in some areas,while in other areas,the salinity of the injected water is relatively low,resulting in problems such as high injection pressure,serious loss of chemical agents,and formation damage.The performance of water flooding is not ideal in the medium and low permeability reservoirs.In view of the above mentioned problems of traditional water flooding,this paper proposes to improve the oil recovery of medium and low permeability reservoirs by the low salinity waterflooding technology.The oil displacement efficiency of low salinity waterflooding on medium and low permeability sandstones was clarified by the core flooding experiments.Ten sets of core flooding experiments were carried out by continuous displacement of formation water with different salinity brines,followed by the optimal salinity brine to simulate the secondary and tertiary oil recovery,respectively.The impacts of salinity,permeability,and other factors on the oil recovery of low salinity waterflooding were studied,and the changes in pressure drop,p H of the produced liquid,and relative permeability were analyzed.The experimental results showed that the recovery factor in medium and low permeability sandstones first increased and then decreased with the decrease of salinity,with the highest recovery factor reached at 5000 mg/L formation water;hence the salinity threshold was 5000 mg/L.In successive flooding with the 5000 mg/L formation water,the recovery factor increased whilst the injection pressure decreased.This indicated that the low salinity waterflooding(by 5000 mg/L)could improve the oil recovery and lower the injection pressure of the medium and low permeability sandstones.The oil-water relative permeability curves showed characteristics of: as the salinity decreased,the isotonic point shifts to the right,and the residual oil saturation decreased.At 5000 mg/L of formation water,the isotonic point was the most right,and the residual oil saturation was the lowest.In addition,the enhanced oil recovery by low salinity waterflooding in the low permeability reservoirs was significantly better than that in the medium permeability reservoirs.On the basis of the mechanisms of enhanced oil recovery by low salinity waterflooding,this paper conducted p H,contact angle,oil-water interfacial tension,and Zeta potential measurements.It was determined that the alkali-like flooding,changes in wettability,and double layer expansion were the main mechanisms for the enhancing oil recovery of low salinity waterflooding,however,the in-situ emulsification effect during low salinity waterflooding was not obvious.Using the geochemistry method,a surface complexation model for the reactions of low salinity water with sandstone and crude oil was established.The experimental Zeta potential results had been accurately fitted using the surface complexation model along with a double layer model.It was clarified that the types and content of clay minerals in the medium and low permeability reservoirs were crucial for improving the oil recovery by low salinity waterflooding.