| Large amount of wastewater has been produced in middle and later stage of water-flooding oilfield. At present, Shengli oilfield is characterized by fluid production rate86.7×104m3/d, composite water cut92%, and daily water production7.6×104m3. All of oilfield sewage will be reinjected underground reservoir for displacing oil and realizing the resource recycling.There are two main problems in the sewage reinjection process for oilfield sewage containing oil, suspended solids, bacteria and a large amount of inorganic salts. At first, the main characteristics of medium-low permeable reservoir are bad reservoir physical property, low permeability, narrow channel which oil and water pass by, very strong percolation flow resistance force, the notable interaction force between liquid-solid interface and liquid-liquid interface. That results in high water injection pressure, low water injection rate and Enhanced Oil Recovery. Enhancing sewage reinjection rate by surfactant is the new method of supplement the formation energy and improving well productivity. Surfactant can reduce sewage reinjection pressure and increase sewage injection rate by improving the oil-water and rock-water interface activity to improve oil-water percolation capacity in stratum. The study plays an important role in reducing sewage discharge, increasing sewage injection production cycle and oil displacement efficiency of mid-low permeability reservoir.On the other hand, sewage is the major cause of the polymer solution viscosity loss. In chemical flooding processes, increasing the viscosity of injected water is the main means of enhancing oil recovery by adding a lot of polymer. As a result of polyacrylamide molecular degradation for a variety of complex chemical composition in oilfield sewage, the polymer solution viscosity decreased in the preparation and injection process, the mobility control ability weaken, the oil displacement efficiency reduced. That has been hindering application of the chemical flooding technique in oilfield. On the basis of the analysis of a large number of literatures and documents, the seepage resistance and percolation characteristics of oil-water two-phase flow was analyzed starting with the interface mechanics research. The polymer molecular chain conformation was described by the scanning electron microscopy for analysis of degradation mechanism. The internal mechanism was explored to increase injection production and oil displacement efficiency by the surfactant. All these possess the theoretical guidance for oil field sewage reuse and efficient development of the mid-low permeability reservoir. The main research contents and results are as follows:1. Analysis of surfatant’s correlation with the capillary resistance, Jiamin effection, Interfacial friction and water flooding capillary number in the process of water flooding by physical simulation experimental technique and NMR techniques.The research result shows that the better interfacial activity of surfactant is not corresponding to the better water injection capacity in low permeability reservoir. Under Shishen100reservoir conditions, the injection pressure drop maximum up to75%as oil-water interfacial tension4x10-2～2×10"2mN/m.2. Synthesis and purification of different kinds of anionic surfactant, which physicochemical properties including surface tension (ycmc), the critical micelle concentration (cmc), interface adsorption quantity of PC20, surface pressure, Gibbs adsorption energy. We obtained the following relation:(1) As the single sulfonate surfactant, petroleum sulfonate (SHL-PS, KAQ-PS), C2024AOS, C1gAESO-3can rapidly reduce the dynamic interfacial tension to10-3mN/m or10-4mN/m, and equilibrium interfacial tension is maintained at10-2mN/m finally. While the oil-water interfacial tension of other surfactant is at10-1mN/m or higher due to their stronger hydrophilicity.(2) In the binary system mainly of C2024AOS, according to synergistic effect the sequence from small to large were C2024AOS/C1416AOS<C2024AOS/Ci6AESO-3<C2024AOS/Dow8390, which can be considered that anionic surfactants has better synergistic effect between the small hydrophilic group and the big hydrophilic group, but not between the small hydrophilic groups.(3) The binary system mainly of petroleum sulfonate has a strong synergistic effect at C1416AOS/SHL-PS mixed ratio1:19, which dynamic interfacial tension is stably maintained at10-3mN/m. all of different EO number C18AESO has synergistic effect with SHL-PS, which can reduce oil-water interfacial tension to10-3mN/m. The sequence of dynamic interfacial tension from small to large were C18AESO-3<C18AESO-5<C18AESO-7. In view of the significantly different between C18AESO and C1416AOS in hydrophilic group size and molecular structure, it is suggested that two complex systems are different in the mechanism of reducing the oil-water interfacial tension.3. The correlation was investigated between properties (ycmc, cmc) and performance (interfacial tension, emulsion performance, oil film adhesion work, wettability, oil cleaning efficiency) of a few groups of anionic surfactants and compound system. We obtained the following relation:(1) Overall, it is substantially positive correlation between the critical micelle concentration and the oil-water interfacial tension for different types of sulfonate surfactants.(2) There is no apparent association between the surface tension and the oil-water interfacial tension for the different sulfonate surfactants solution.(3) To reduce the critical micelle concentration is beneficial to improving the emulsifying properties, but there is no apparent relationship between CMC and emulsified index for different series of surfactants. To moderately increase alkyl chain length is beneficial to improving the emulsifying capacity. To increase the number of EO is not conducive to the emulsion.(4) There is no correlation between the surface tension and emulsion performance.(5) It is a basically positively correlated among the surface tension, the contact angle and the adhesion work for series of different surfactants solution, i.e. the contact angle decreases as the surface tension reduces on the same hydrophobicity solid surface, but the wettability reversal ability is changing with the molecular structure.(6) There is no correlation among critical micelle concentration, oil membrane adhesion work and wettability.(7) Wettability has greater influence than interfacial tension and emulsification on oil displacement efficiency for lipophilic surface. In other words, with the rock wettability transfered to middle wetting state, the oil displacement efficiency could get up to the highest value.(8) Lower interfacial tension, stronger emulsifying ability and medium wetting surface are beneficial to enhance oil displacement efficiency, which contributions have good additity.(9) With different molecular structure, surfactants have significantly different elution capacities for crude oil and paraffin. Aliphatic alcohol ether sulfonate can wash out the aromatic in the oil, but not waxy substance. Dow8390has high eluting capacity on paraffin. 4. Influence of quality parameters on polymer solution viscosity was investigated by the method of single factor analysis. The method of orthogonal testing was used to study the synergistic effect of water quality parameters on reducing the polymer solution viscosity, and to determine main water parameters for the viscosity of polymer solution. We obtained the following relation:(1) Large amounts of metal ions in oil field sewage have different effects on the polymer solution viscosity. The sequence of cationic from big to small effcting polymer solution viscosity were Fe2+> Fe3+> Ca2+/Mg2+> Na+/K+.(2) Fe2+/S2-possesses reducing property in oilfield sewage and more impact on the polymer solution viscosity than the other ions, that is closely related to dissolved oxygen in oilfield sewage. Under oxygen free conditions, Fe2+/S2-would rapidly reduce the polymer solution viscosity of. As the oxygen reach the best concentration, Fe2+/S2-would significantly decrease or disappear.5. By the method of infrared spectroscopy and scanning electron microscope analysis, the degradation mechanism of the polymer was discussed. We obtained the following relation:(1) Influence of Na+, K+, Ca2+, Mg2+on the polymer solution viscosity should mainly owe to their positive charge, which shield polyacrylamide molecule carboxyl negative charge, then make the polyacrylamide molecule to hydration, molecular chain to contraction, and polymer solution viscosity to reduction, in which Ca2+/Mg2+decreased more polymer solution viscosity because of more charge, firmly bonded to carboxylic acid moiety.(2) Fe2+affects on polymer solution viscosity by redox reaction between Fe2+and oxygen. Fe2+reacts with oxygen and generates oxygen free radicals, at the same time generates of peroxides in water, then induces a series of free radical chain reaction, thus makes the polyacrylamide molecular chain fractured and polymer solution viscosity reduced. The degradation mechanism of S2-is similar to Fe2+.For the stability of the polymer solution viscosity, it is need to improve the spatial effect, the charge density and salvation of polyacrylamide molecular. Surfactant design for viscosity stability can be considered from these three aspects.6. In view of influence of sewage on the polymer solution viscosity, three kinds of surfactants were designed for quality modification, that increased the polymer solution viscosity from12.5mPa·s to above30mPa·s, implemented resource utilization of oilfield sewage in chemical flooding. |
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