| H reservoir in Tahe oilfield is a high temperature(97 ?)and high salinity(TDS 204600.24 mg/L,divalent cation 11480.34 mg/L)clastic rock reservoir.The formation permeability is mainly between 5.00 and 95.00 mD,and the coefficient of permeability variation is between 1.09 and 1.45.The serious heterogeneity of formations resulted in the injection water channeling from higher permeability pathways,thus a poor water sweep efficiency,a high water cut of 90%and a low recovery percent were achieved.How to further improve oil recovery is the major issue facing the reservoir.Profile control and flooding technologies using chemical agents have been proved to be the effective methods for EOR.However,the harsh conditions limit the effective application of the conventional chemical flooding techniques.On the basis of the reservoir characteristics and the issues of water injection process,the micro-scale dispersion gel-surfactant flooding process was proposed for EOR.In this study,the new anionic-nonionic surfactant and dispersion gel formulations were developed.Then core flooding experiments,nuclear magnetic resonance(NMR)displacement experiment,microscopic oil displacement experiments and 3D data visualization displacement experiment were carried out for the following purposes:1)to investigate some important parameters that affect displacement efficiency;2)to analyze the remaining oil microscopic distribution after surfactant flooding;3)to study the microscopic oil displacement mechanism of the emulsification and reducing IFT and their effects on displacement efficiency;4)to establish a quantitative matching relationship between the particle size of dispersion gel and pore-throat size of cores;5)to investigate the distribution change of water and oil saturation during the process of dispersion gel-surfactant flooding and thus analyze the EOR mechanism of dispersion gel-surfactant flooding.Through the above research,the main results and understandings are as follows:(1)The obtained anionic-nonionic surfactant exhibited the performance that interfacial activity increased with the increase of salinity.0.2%AYF obtained a stable ultra-low interfacial tension(IFT)of 3.40×10-3 mN/m with crude oil after aging for 90d under reservoir temperature and salinity.2-Acrylamido-2-methylpropane sulfonic acid(AMPS),N-Vinyl-2-pyrrolidone(NVP),and a twin stern hydrophobic monomer were employed to synthesize gels by polymerization in surfactant solution.The dispersion gels was obtained by drying and multistage crush process.The synthesized dispersion gel still remained a high toughness index of 0.82 and a higher transition pressure of 73.80 KPa than the initial value of 70.80 KPa after aging for 90d under reservoir temperature and salinity,which indicates the dispersion gel has considerable strength and toughness.The surfactant and dispersion gels had a good compatibility under high temperature and high salinity.(2)The migration and plugging patterns of the dispersion gel under different permeability were studied through analyzing and fitting the pressure curves and comparing the changes in physical properties of produced dispersion gels during the injection process.The migration and plugging patterns were divided into five types:full blocking,deformation or smash through-blocking-deep migration(including strong blocking,ordinary blocking,and weak blocking),and smooth through-noneffective blocking-steady seepage.A quantitative matching relationship between the particle size of dispersion gel and pore-throat size of cores was established by defining the feature coefficients ?=d10/d,?=d50/d,?=d90/d and pressurization coefficient.(3)The single core experiments indicated that surfactant flooding could improve displacement efficiency under experimental permeability conditions(0.77?940.91 mD).There is an optimal injection rate range for surfactant flooding.A high or low velocity beyond the optimal velocity range will lead to the decrease of displacement efficiency.The core with an initial wettability of intermediate wet yielded the highest displacement efficiency.(4)The AYF surfactant with a constant concentration of 0.20 wt%was employed to obtain five surfactant formulations(No.1?No.5)by only changing water salinity.These five formulations from No.1 to No.5 could reduce the oil-water IFT to 10.41,2.77×10-1,4.89×10-2,2.41×10-3 and 1.44×10-4 mN/m,respectively.In addition,these five formulations have an unique characteristic that the emulsifying ability and stability successively decrease with the reduction of IFT.Therefore,it was verified that the lower the IFT,the higher the displacement efficiency after eliminating the effect of emulsification.Simultaneously,the results indicated that reducing IFT is dominant for surfactant flooding.(5)Parallel core displacement experiments and nuclear magnetic resonance(NMR)displacement experiments indicated that the surfactant formulation under low IFT could mobilize the remaining oil in low permeability layer that can not be mobilized by water flooding.The mechanism includes two aspects:First,reducing oil-water IFT results in the decrease of capillary resistance,consequently,the difference of the capillary resistance between high and low permeability layer is reduced.Second,the reduction of oil-water IFT and water-rock solid surface IFT makes the rock more hydrophilic and wetting contact angle smaller,and thus lower the adhesion work.(6)Microscopic oil displacement experiments indicated that the residual oil after water flooding was mobilized by forming a lot of small emulsion droplets under strong emulsification but high IFT.These emulsion droplets had poor deformation capacity and easily got stuck,thus the accumulation occurred,which could generate an instantaneous conformance control effect.However,the tadpole shaped oil droplets that were stripped from the residual oil were generated under ultralow IFT but weak emulsification.These tadpole shaped oil droplets were stretched,and finally the oil-water interface became fogged.It exhibited a state that the oil was "dissolved" into the water and moved with the water.The displacement efficiency was greatly improved under ultralow IFT.(7)3D data visualization displacement experiment showed that the EOR mechanism of the dispersion gels/surfactant process mainly includes four aspects:?Dispersion gel could effectively block the higher permeability pathways and reduce permeability heterogeneity.?The deep migration of dispersion gel generates a conformance control effect and makes the waterflood front move more evenly,thus improves the swept volume.?The surfactant under low IFT improve the producing degree of the low permeability zone.?The surfactant under low IFT improve displacement efficiency.The oil recovery was 24.00%,which shows a high displacement efficiency.This study provides a new EOR method for high temperature and high salinity reservoirs,which has practical significance for the high efficient development of these reservoirs.Simultaneously,the research results have significant theoretical significance for enriching the oil displacement mechanism of surfactant flooding and the migration and plugging mechanism of dispersion gel in porous media. |
PDF Full Text Request