Research On The Mechanism Of Decompression And Augmented Injection Of Surfactant In Ultra-Low Permeability Reservoir

As the development of most of the conventional oil and gas fields is at the stage of high/ultra-high water cut in our country,the further improvement of the space for recovery is limited and the development is becoming more and more difficult.At present,the development degree of ultra-low permeability reservoir is very low and the development effect is bad,so the ultra-low permeability reservoir has great potential for development.The technology of decompression and augmented injection of surfactants which is an effective technique to improve the recovery of ultra-low permeability reservoir,can effectively improve the matters such as rapid water cut and water injection difficulty in the process of water flooding.Intensive study on the mechanism of decompression and augmented injection of surfactant in ultra-low permeability reservoir can make clear the effect of surfactant on the rock and the fluid in ultralow permeability reservoir and effect on the oil displacement;provide theoretical basis for highefficiency development of water injection in ultra-low permeability reservoir,so it has important practical significance and great value in theory.This paper firstly tested the performance of four kinds of surfactants provided by oilfield to reduce interfacial tension,change wettability and emulsified crude oil under reservoir conditions,then analyzed and screened out surfactants with good performance.Secondly,the effects of the types of surfactants,injection velocity,permeability range and concentration on the effect of decompression and augmented injection were compared and analyzed.Through the relative permeability curve,the influence of surfactant on fluid percolation capacity,rock surface properties and residual oil saturation was studied,and the mechanism of enhanced oil recovery and decompression and augmented injection was analyzed.Thirdly,through the surfactant imbibition and displacement experiment of ultra-low permeability core and capillary,we studied the imbibition displacement rule and microscopic imbibition displacement mechanism of surfactant without the action of external driving force.Finally,we use microscopic etching model to simulate surfactant flooding.We observed and analyzed the formation mechanism of residual oil after water flooding and the mechanism of surfactant to start residual oil,and analyzed the mechanism of surfactant decompression and augmented injection from microscopic perspective.The results showed that,compared with other surfactants,the ability to reduce the interfacial tension of oil and water of surfactant A was relatively strong,and the effect was obvious when the concentration was 0.5%-1.5%;surfactant A solution can change water wet surface into strong close water surface,but to the oil wet surface,both surfactant A and other molecular membrane agents produced a wetting reversal on its surface;the emulsifying effect of surfactant A was obvious but the stability of emulsification was weak,and when the concentration of surfactant A is 0.3%-0.5%,the effect of emulsified crude oil was most obvious.The indoor simulated reservoir condition stimulation experiments showed that surfactant A have better ability to increase the flow injection than other kinds of surfactants;when the injection rate was low,the effect of depressurization and increasing injection on low permeability core was better;high concentration of surfactant solution was beneficial to reservoir with relatively high permeability,while the effect of low concentration of surfactant solution on depressurization and increasing injection of low permeability reservoir was obvious;the relative permeability curve of water flooding and surfactant flooding showed that the point of overlap of relative permeability curves of surfactant flooding moved to the right,the relative permeability curve of oil phase moved to the right,the relative permeability curve of water phase moved down,two-phase regions increased,and residual oil saturation reduced,indicated that the hydrophilic property of rock and the fluidity of crude oil can be improved by adding surfactants solution,which improved the efficiency of the oil displacement and increased the effect of decompression and augmented injection.Through the ultra-low permeability core imbibition experiment and capillary self-priming experiment,it can be concluded that surfactant can affect on core porous media spontaneously,improve rock wettability and the interface properties of rocks and fluids,which is conducive to the separation of oil and the effect of augmented injection.The concentration of surfactant and diameter of the capillary have certain influence on the imbibition displacement effect.The higher the concentration,the bigger the capillary diameter is,the more favorable for the imbibition.Microscopic displacement experiments showed that surfactant can effectively eliminate Jia min effect,reduce capillary resistance in pores,and efficiently wash out residual oil in blind ends,which had the function of dredging pore paths,reducing seepage resistance,increasing the number of effective flow channel,in the end,decreasing pressure and increasing injection and enhancing oil recovery.