| Polymer flooding is a relatively mature method for enhancing oil recovery. Due to the influence of polymer properties, reservoir conditions and other objective factors, polymer flooding can achieve an ideal effect when reservoir heterogeneity is better. However, with the reservoir heterogeneity enhancement, the ability of improving the sweep efficiency of polymer flooding is greatly reduced. Therefore, the ability of improving displacement fluid sweep coefficient of polymer needs to be further reinforced to enhance oil recovery of strong heterogeneity reservoir.With the development of polymer flooding technology, the structure of polymer for enhancing oil recovery is gradually extended from linear to branched or hyperbranched direction. Hyperbranched polymer has better shear resistance, and it can obtain higher resistance factor and residual resistance factor in porous media, so hyperbranched polymer exhibits great potential in improving sweep coefficient of displacement fluid. At present, the kernel of hyperbranched polymer is mainly organism, and the chains of hyperbranched polymer are less. Increasing the reaction algebra of kernel is beneficial to increase the branch number of hyperbranched polymer, but at the same time it also increases the molecular size of hyperbranched polymer, so that the injection of hyperbranched polymer will get worse. Therefore, the organism as hyperbranched polymer kernel is difficulty in effectively increasing the number of polymer chains in the situation of injection meeting the requirements.In view of the above problems, the nano-SiO2is introduced to the polymer for enhancing oil recovery. The modified nano-SiIO2/AA/AM copolymers with multi branched chain, hyperbranched structure are synthesized through copolymerization of the modification nano-SiO2, acrylic acid (AA) and acrylamide (AM). According to the design of molecular structure, molecular chain of these copolymers will be more flexible, and they exhibit stronger deformation recovery ability and shear resistance ability. In the situation of injection meeting the requirements, these copolymers can obtain higher resistance factor and residual resistance factor in porous media, and they can effectively improve the plane heterogeneity and interlayer heterogeneity. Therefore, these copolymers can effectively improve the sweep efficiency of displacement fluid.The controllable modification method of nano-SiO2is studied, and it will provide the possibility for the introduction of nano-SiO2to oil displacement agent. In order to facilitate the determination of the nano-SiO2surface modification degree, the nano-SiO2is modified by the silane coupling agent3-(Trimethoxysilyl)-1-propanamine with an amino group. Through the determination of the surface hydroxyl content of nano-SiO2and the amino content of modified nano-SiO2surface. the modification degree can be obtained. Then the modified product is reacted with maleic anhydride which has a carbon to carbon double bond, so that the modified product can participate in the polymerization reaction. The controllable modification is implemented by controlling the reaction temperature, time, solvent and raw material amount. In addition, the structure of modified nano-Si02is characterized by thermogravimetric analysis, element analysis, infrared spectrum and nuclear magnetic resonance.Through the design of polymer molecular structure, the modified nano-SiO2is copolymerized with AA and AM, and then the nano-SiO2is introduced to the polymer for enhancing oil recovery. Orthogonal test and single factor test methods are used in combination to optimize the synthesis conditions of modified nano-SiO2/AA/AM copolymer. The obtained synthesis conditions are as follows:the total concentration of monomers is25.7%; the mass ratio of AM with AA is3.35; the initiator dosage is0.20%of total monomers mass; the temperature is40.4℃; pH is7.4. Two kinds of copolymers are synthesized using the modified nano-SiO2with28%and36%modified degree, respectively.The thickening property, temperature tolerance, and salt tolerance of modified nano-SiO2/AA/AM copolymer can meet the requirements of polymer flooding. The modified nano-SiO2/AA/AM copolymers have stronger shear resistance. With the increasing of shear strength, the decline of viscosity retention rate of modified nano-SiO2/AA/AM copolymers is lower than partially hydrolyzed polyacrylamide. The molecular chain of modified nano-SiO2/AA/AM copolymer exhibits better flexibility, viscoelasticity and deformation restoring capacity. The dissolution time of the copolymer is about2h. The filter factor is less than1.5. It can meet the injection requirements when the permeability of porous medium is higher than0.7D.The modified nano-SiO2/AA/AM copolymer can obtain higher resistance factor, residual resistance factor and flow resistance in porous media. The physical model experiments show that the copolymer can increase the injection pressure of displacement fluid, improve the plane and interlayer heterogeneity, and start moderate-to-low permeability layers effectively, so as to improve the sweep efficiency of displacement fluid.The mechanism of improving displacement fluid sweep efficiency of modified nano-SiO2/AA/AM copolymer is studied. Due to the particular molecular structure, the shear resistance, flexibility and viscoelasticity of the copolymer are strengthened. At the same time, the ability of improving the viscosity of displacing phase and reducing the water permeability is improved. Therefore, the modified nano-SiO2/AA/AM copolymer can obtain higher flow resistance in heterogeneous porous media, so as to effectively improve sweep efficiency of displacement fluid. |
PDF Full Text Request