Interfacial Property Changes Induced By Displacement Agents And Their EOR Mechanisms

In order to ensure high production of oilfield,on one hand,the water flooding recovery needs to be improved;on the other,enhanced oil recovery methods are to be further developed.During water flooding or EOR process,the interfacial property among displacement agents,crude oil and rock surface is one of the important factors influencing oil recovery,and it is of great importance to study the effect of interfacial property changes on enhanced oil recovery.Quartz and calcite were used as typical sandstone and carbonate minerals,and the effect of crude oil on surface wettability was studied.Sessile drop method was used to measure the contact angle and to evaluate the surface wettability of mineral surface.The effect of aging time,asphaltene concentration,water film on mineral surface and ion composition in water film were studied,and the quartz surface property was characterized with ?-potential measurements and infrared(IR)spectroscopy to clarify the wettability alteration mechanism.The experimental results showed that the adsorption and precipitation of asphaltene on quartz and calcite surface is the main reason for wettability alteration.The surface wettability of quartz and calcite turns from water-wet to oil-wet after aged in asphaltene toluene solution or n-heptane crude oil mixture for 3d.The quartz with a pure water film shows less oil-wetting after soaking compared to the dry quartz surface,while the presence of metal cations in the water film can improve oil wettability of the surface.?-Potential measurements showed that Ca2+and Mg2+ compress diffused double layer of quartz powder more effectively than Na+,which makes asphaltene adsorption easier because of the reduced electrostatic repulsion.IR spectra analysis indicates that asphaltene molecules can interact with hydroxyls on the quartz surface by polar interactions when the water is absent and asphaltene can be absorbed on the quartz surface with ion binding of hydrated divalent metal cations with water present.A cationic polymer(RPM)was studied,and its static adsorption,surface tension,oil-water interfacial tension,effect on water and oil permeability and wettability alteration ability on oil wetsurface as well as effect on oil recovery were conducted.The results showed that RPM has good adsorption ability,and the static adsorption is 1110?g/g.RPM can reduce the surface tension and oil-water interfacial tension to a limited amount.RPM can also selectively reduce the water permeability more than oil permeability.The oil-wet quartz surface can be altered to weakly water-wet by RPM solution,while wettability of water-wet surface is unchanged after RPM treatment.Core flooding experiments showed that oil recovery increment(18.2%)in oil-wet core is larger than that in water-wet core(13.1%).Three kinds of surfactants,including cationic cetyl trimethylammonium bromide(CTAB),anionic sodium alcohol ether sulphate(AES)and nonionic alkyl polyglycoside(GD70)were tested for their ability of reducing oil-water interfacial tension and altering wettability of rock surface.Surfactant flooding and spontaneous imbibition tests were also conducted.For water-wet quartz,after soaked in various surfactant solutions,quartz in AES and GD70 solutions remains water-wet,while for CTAB,the surface turns water-wet at low concentration and then becomes water-wet again.For oil-wet quartz,CTAB and AES can alter the surface to water-wet,while wettability remains oil-wet after treated in GD70.For surfactant flooding,the highest recovery region is when interfacial tension is in the magnitude of 10-1mN/m,and contact angle is in the range of 40-60°after surfactant treatment for both water-wet and oil-wet cores.Reverse bond number(NB-1)is used to judge imbibition mechanism.When NB-1>5,capillary forces are driving the imbibition process,and the surface wettability should be modified to water-wet to ensure high recovery;When NB-1 falls in the range of 1< NB-1<5,the imbibition mechanism is the results of combination of capillary and gravity forces,if the rock is water-wet,the imbibition recovery is usually high;When NB-1<1,the imbibition is dominated by gravity,and the recovey is usually low due to lack of driving forces.The effects of salinity and ion compositions on wettability alteration and imbibition recovery of two carbonates,including chalk and dolomite were also studied.Chalk cores were put into sea water and modified sea water to test the recovery,and results showed that sea water without NaCl yields more oil recovery.Micro-CT imaging showed that water advances into the center of the chalk core during imbibition.SEM was used to observe the asphaltene distribution in the core,and it is found that the chalk surface is mixed-wet after aging,and the adsorption layer of asphaltene seems thinner after spontaneous imbibition,and the core becomes more water-wet.Imbibiton tests of dolomite plugs insea water(SW)and 100 times diluted sea water(0.01SW)showed that the recovery in 0.01 SW is much higher than that in SW.Zeta potential measurement showed that oil droplet is negatively charged in SW and 0.01 SW,while dolomite is positively charged in SW and negatively charged in 0.01 SW.Oil droplet is easier to desorb from dolomite surface in 0.01 SW due to electrostatic repulsion,and the oil recovery is high than that in SW.Micro-CT imaging showed that the wettability of dolomite in large pores is mixed-wet.Most of the water in the pore is in the form of water droplets,while other adsorbs onto the rock surface and alters the wettability to partially water-wet.