Investigations Into Displacement Efficiency And Mechanism Of In-situ Emulsion Flooding

X reservoir in Dagang oilfield is a high temperature(104?)and high salinity(TDS 25571.53 mg/L)heterogeneous reservoir.The oil viscosity is 9.42 mPa s and the serious heterogeneity of formations resulted in the injection water channeling from higher permeability pathways,thus leading a poor water sweep efficiency,a high water cut of 96.7%.Conventional chemical flooding has great limitations for X reservoir.Therefore,taking X reservoir as an example,this paper innovated a new enhanced oil recovery method suitable for high temperature and high salinity heterogeneous reservoirs—in-situ emulsification flooding method.First,through the performance evaluation experiment,the emulsion type surfactant HC-2 was selected for X reservoir.After 90 days aging under reservoir conditions,HC-2 still had good interfacial tension(IFT)of 10-2mN/m with crude oil.The emulsifying properties of HC-2 under reservoir conditions were investigated.The results showed that the emulsification of oil-water happens and the W/O emulsion with high viscosity will form when the water-oil ratio was lower than 7:3.The emulsion viscosity firstly increased and then decreased with the increase of water-oil ratio.The maximum emulsion viscosity appeared when the water-oil ratio was 7:3,with the viscosity increasing rate of as high as 420%.Under different shear rates,the emulsification of oil-water happened and the W/O emulsion will form.The higher the shear rate,the higher the viscosity of emulsion.HC-2 was also good at wettability improvement.It can further enhance water-wet glass hydrophilicity and reverse the wettability of oil-wet glass,which make it from lipophilicity to hydrophilicity.Static adsorption experiments showed that the static adsorption capacity of HC-2 on formation sand firstly increased and then stabilized with the increase of concentration,and the saturated adsorption capacity was 1.78 mg/g.Experiment of surfactant in-situ emulsification flooding indicated that HC-2 could form W/O emulsion with high viscosity in core.The optimum injection rate was 0.3mL/min and the optimum injection amount was 0.5PV.The results of natural core in-situ emulsification flooding indicated that the recovery factor could increases 27.2%according to the optimum injection mode.On the other hand,the recovery factor firstly increased and then decreased with the increases of permeability differential by in-situ emulsification flooding.When permeability differential was lower than 7.5,in-situ emulsification flooding could achieve good mobility control and heterogeneity regulation.The three-layer heterogeneous core experiment indicates that the better in-situ emulsifying ability of surfactant system was,the higher displacement efficiency was,when the interfacial activity is similar.Experiments of in-situ emulsification micro-flooding and nuclear magnetic resonance flooding indicated that the mechanism of in-situ emulsification flooding included two aspects:(1)W/O emulsion with high viscosity can achieve mobility control and heterogeneity regulation;(2)Enhancing displacement efficiency by reducing oil-water interfacial tension and improving wettability.In this study,a surfactant system suitable for high temperature and high salinity environment of X reservoir was formed,and the displacement efficiency and mechanism of in-situ emulsification flooding method were studied in laboratory.The research results have significant theoretical significance for enriching the oil displacement mechanism of in-situ emulsification flooding.Simultaneously,it can provide reference for the field application of surfactant in-situ emulsification flooding method.