Study On Foam Systems For Enhanced Oil Recovery

Surfactant foam flooding has been a new technology for enhanced oil recovery in recent years because foam systems behave high viscoelasticity and good plugging effects.However,the defoaming characteristics of the crude oils hinder applications of the foam systems in enhanced oil recovery seriously.Aiming at the conditions of Daqing reservoir and high temperature and high salt reservoirs,this paper carried out a systematic study with respect to the technical problems met by surfactant foam flooding.The main research contents and achievements are as follows:1.For the relatively mild Daqing reservoir conditions（45℃,total salinity 6,778 mg/L,p H=8.0～9.0）,the foams stabilized by the two most commonly used cheap foaming agents,fatty alcohol polyoxyethylene ether sodium sulfate（AES）and sodium dodecyl sulfate（SDS）,which are used as foaming agent in many daily chemical products,will be defoamed easily once contacting with crude oil during foam flooding.The composite foam index is only100～500 Ls,and the oil resistance index is only 0.1～0.2.Based on the theory of synergistic effects between different surfactants,this problem can be solved by addition of a foam stabilizer.Our researches indicate that among various conventional surfactants the（amphoteric）sulfobetaine surfactants,especially cetyl dimethylpropyl sulfobetaine(C₁₆SB)and cetyl dimethyl hydroxypropyl sulfobetaine(C₁₆HSB),are the best ones as foam stabilizer against crude oil for foams stabilized by either AES or SDS.With addition of the C₁₆SB or C₁₆HSB at a mole fraction of 0.2,the half-life of the foams stabilized by binary mixtures can be increased significantly,resulting in a composite foam index as high as 3000 Ls and 5000 Ls,respectively,and an oil resistance index as high as 1.0 and 4.0,respectively.Compared with the foams stabilized solely by AES and SDS,the foams stabilized by the binary mixtures show synergistic indices in oil resistance as high as 17 and 34,respectively.Study on the stabilization mechanism indicates that there are attractive interactions between sulfobetaines and anionic surfactant so that both form densely arranged mixed monolayers on both air/water and oil/water interfaces,which further reduces the surface tension and oil/water interfacial tension（IFT）,and endows negative E,S,and B coefficients.The oil droplets are therefore prevented to enter bubble surface and spread on it.On the other hand,during foaming process,the crude oil is emulsified into fine oil droplets,which are distributed in the continuous phase or liquid lamellae of the foams.The viscosity of the lamellae is therefore increased and the liquid drainage in the lamellae is slowed down,thereby ensuring a stable film between air bubble and oil droplets（pseudo-emulsion film）.2.For high temperature and high salt reservoirs,AES and SDS are easily hydrolyzed at high temperature due to their sulfate（C-O-SO₃Na）structure,and SDS is neither salt-tolerant.Even if theα-olefin sulfonate（AOS）which has excellent resistance to hard water is used,it precipitates out in solutions with low concentration of Ca²⁺.Obviously,these conventional anionic surfactants are not applicable.In this study,a novel anionic-nonionic composite surfactant,alcohol ether sulfonate（AEO₃-HS）was synthesized by using fatty alcohol polyoxyethylene（3）ether（AEO₃）with two-step reaction.The total conversion rate is 78%,and the purified product reaches a purity of 98.5%.Using the new surfactant to replace AES or SDS and mixed with C₁₆HSB,foam systems of both temperature-resistant and salt-resistant for foam flooding are constructed.The experimental results show that the composite foam performance of AEO₃-HS/C₁₆HSB binary mixture（the optimum mole fraction ratio=0.6/0.4）is much better than that of AES/C₁₆HSB and SDS/C₁₆HSB systems.Dissolved in Na Cl（20,000mg/L）or Ca Cl₂（1000 mg/L）solutions at 45℃,AEO₃-HS/C₁₆HSB（0.6/0.4）binary mixture can give a composite foam index as high as 1361 Ls and 2479 Ls,respectively.At 60℃ the binary mixture can be well dissolved in aqueous solution with 5000 mg/L Ca Cl₂ plus 100,000mg/L Na Cl.On the other hands,the composite foam index of the binary mixture solution in simulated Daqing connate water after being stored at 90℃ for 3 months remains unchanged.Similarly,AEO₃-HS has an attractive interaction with C₁₆HSB,which is beneficial to emulsify oil into small oil droplets and improve the stability of the foams against oil.3.Surfactant foam flooding mainly utilizes the increase in the viscosity of the displacement fluid to plug high-permeability reservoirs,thereby increasing the sweep coefficient of the displacement fluid.If ultralow oil/water IFT can be achieved at the same time,it will be beneficial to the deformation of the trapped oil droplets so that they can be driven out easily through the porous throats（main mechanism of surfactant flooding）,thereby greatly improving the efficiency of foam flooding.However,for simple surfactant systems,ultralow oil/water IFT and high foam performance are usually not achievable concurrently because both are achieved by different mechanisms.Aiming at this technical problem,this study proposes a novel formulation route based on the theory of synergistic effects of mixed surfactants:using small molecular anionic/cationic binary mixtures plus a nonionic surfactant as solubilizers.With respect the Daqing reservoir conditions a foam system,composed of anionic（SDS）/cationic（DTAB）/nonionic（APG）ternary surfactants,is obtained which can achieve both ultralow oil/water IFT and high foam performance concurrently.At the optimum composition（mole fraction ratio）0.36/0.24/0.4 and a wide total concentration range of 2～7 m M,the oil/water IFT can reach ultralow（<0.01 m N/m）and the minimum can be as low as 2.9×10^-5m N/m at which the composite foam index reaches 936 Ls.At a total concentration of 8 m M,the composite foam index reaches the maximum（1342 Ls）,whereas the oil/water IFT remains the order of 10^-2 m N/m.Study on the mechanism indicates that SDS/DTAB binary mixture dominates the mixed monolayer at the air/water and oil/water interfaces（mole fraction being as high as 0.94）due to strong electrostatic attraction.The dense monolayer formed by the binary mixture is the key to obtain ultralow oil/water IFT and low surface tension.The addition of APG further reduces the surface tension of the solution and the oil/water IFT,and improves the emulsifying ability of the mixed system to the simulated oil.During the foaming process,the oil is emulsified into small and stable oil droplets,which are dispersed in the continuous phase or lamellae of the foams,ensuring negative E,S,B coefficients and a stable pseudo-emulsion film.4.The introduction of nanoparticles into the oil displacement system is one of the current research hotspots.Although a lot of researches have been done,it is far from the stage of practical application.In view of the negatively charged surface of Daqing oil sands and the fact that anionic surfactants are widely used as oil displacement agents in practice,this paper investigates the synergistic effect of adding negatively charged Si O₂ nanoparticles into surfactant solution for foam flooding.The results show that adding a small amount of Si O₂nanoparticles（～0.01 wt.%）into the aqueous solution of SDS in pure water can improve the foam performance of the SDS solution at 25℃.However,the addition of Si O₂ nanoparticles into the SDS solution in simulated Daqing connate water at 45℃ shows no synergistic effect in enhancing foam performance of SDS.However,positive synergistic effects are shown in the presence of oil,as indicated by the positive synergistic index in oil resistance of binary dispersion（containing 0.06 wt.%Si O₂ nanoparticles）,average of 2.57 and maximum of 3.82,at a wide SDS concentration range between 1m M and 10 m M.But it is still far lower than that given by SDS/sulfobetaine binary mixture（the maximum is 34）.Since both the silica nanoparticles and SDS are negatively charged,they synergistically promote the emulsification of oil so that the oil can be emulsified into fine oil droplets.These oil droplets and particles are dispersed in the liquid film or lamellae of the bubbles,which increases the viscosity of the lamellae and slows down the drainage,enhancing foam performances and improving foam stability against oil.Nevertheless,further researches are necessary for finding more systems that can simultaneously achieve ultralow oil/water IFT and excellent foam performance,especially foam systems suitable for high temperature and high salt reservoirs.And more basic researches are needed to explore the application of nanoparticles in surfactant flooding systems.