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Weak gel is a gel system formed by the mixing and crosslinking of a low-concentration polymer and a slow-release crosslinker. It can be used for profile control in deep reservoir, but its effect is greatly affected by mechanical shearing. Currently, the shearing effect on weak gel is mainly studied by way of mechanical stirring, while the effect of porous media shear on weak gel molecules and properties has been rarely discussed. In this paper, polymer solution, aluminum gel and phenolic gel were prepared. The molecular coil size, viscoelastic modulus and microscopic aggregation morphology in water solution of three systems before and after core shearing were investigated, and the injection performance of the three systems in cores with different permeabilities was tested by physical simulation experiments. The study results show that at equivalent permeability, the system with a larger equivalent sphere diameter of molecular coil is more seriously sheared and suffers greater viscosity loss. In the core with permeability of 1.0 D, polymer solution remains as the aggregation, while phenolic gel and aluminum gel cannot form network aggregations and they are inferior to polymer solution in migration capacity in the mid-deep part of the core. In the core with permeability of 1–5.8 D, the polymer solution remains as a Newtonian fluid, while phenolic gel and aluminum gel become purely viscous non-Newtonian fluids. The elastic modulus of aluminum gel and phenolic gel is four times more than that of a polymer. In the core with permeability higher than 8.5 D, aluminum gel and phenolic gel migrate with less effect by core shearing, and their profile control capacity in deep reservoir is higher than that of the polymer. In the core with permeability lower than 8.5 D, because the monomolecular activity of weak gels becomes poor, they migrate in porous media with more effect by core shearing, and their profile control and oil displacement capacity in deep reservoir is lower than that of the polymer. [ABSTRACT FROM AUTHOR]

Weak gel is a gel system formed by the mixing and crosslinking of a low-concentration polymer and a slow-release crosslinker. It can be used for profile control in deep reservoir, but its effect is greatly affected by mechanical shearing. Currently, the shearing effect on weak gel is mainly studied by way of mechanical stirring, while the effect of porous media shear on weak gel molecules and properties has been rarely discussed. In this paper, polymer solution, aluminum gel and phenolic gel were prepared. The molecular coil size, viscoelastic modulus and microscopic aggregation morphology in water solution of three systems before and after core shearing were investigated, and the injection performance of the three systems in cores with different permeabilities was tested by physical simulation experiments. The study results show that at equivalent permeability, the system with a larger equivalent sphere diameter of molecular coil is more seriously sheared and suffers greater viscosity loss. In the core with permeability of 1.0 D, polymer solution remains as the aggregation, while phenolic gel and aluminum gel cannot form network aggregations and they are inferior to polymer solution in migration capacity in the mid-deep part of the core. In the core with permeability of 1–5.8 D, the polymer solution remains as a Newtonian fluid, while phenolic gel and aluminum gel become purely viscous non-Newtonian fluids. The elastic modulus of aluminum gel and phenolic gel is four times more than that of a polymer. In the core with permeability higher than 8.5 D, aluminum gel and phenolic gel migrate with less effect by core shearing, and their profile control capacity in deep reservoir is higher than that of the polymer. In the core with permeability lower than 8.5 D, because the monomolecular activity of weak gels becomes poor, they migrate in porous media with more effect by core shearing, and their profile control and oil displacement capacity in deep reservoir is lower than that of the polymer.

The relevance. Polymer flooding is widely used in practice in the development of oil fields. The high-concentration polymer solution has strong viscoelasticity, but with the increase in mass concentration of the polymer, the compatibility problem occurs between polymer molecules aggregate size and reservoir rock pore size during its practical application. Cr3 + polymer gel based on intramolecular cross-linking has a smaller aggregate size and good compatibility with the reservoir, so it is relevant to study the effect of gel formation of Cr3 + polymer gel and its influence factors. The main aim of the research is to study filtration and rheological properties of polyacrylamide-based aqueous gel with chromium acetate to enhance oil recovery. Methods: physical simulation of oil displacement and fluid filtration under reservoir conditions on a filtration unit, determination of viscosity of cross-linked polymer gel using the Brookfield DV-II viscometer, measuring the size of the molecular coil Dh in a Brookhaven BI-200SM light scattering system (Brookhaven Instruments Cop., USA), determination of viscoelasticity using the Harke10 rheometer. Results. Regulating the concentration of polymer, water solution salinity and the ratio of the polymer content and chromium (w(П/Cr3+)), one can obtain the Cr3+ polymer gel system based on intramolecular cross-linking with a smaller molecular aggregate and higher resistance confidence. After the reaction of intramolecular cross-linking of the polymer solution, on the one hand the viscoelasticity of the system is significantly improved and the oil displacement coefficient increased. On the other hand, the intramolecular crosslinking reaction reduces the flexibility of the polymer molecule aggregates, increases their rigidity, weaken their deformability in the process of passing through the pores, and enhance the retention ability in the high permeability layer. Cr3 + polymer gel system not only has good injectability, it can also gradually reach a higher injection pressure, so it enlarges sweep volume effectively. In total, it is shown that the oil recovery coefficient of the polymer gel system is higher than the polymer solution.

Relevance. In recent years, polymer flooding technologies have been widely used in exploitation of oil and gas fields, especially in the later stages of field development. However, when operating reservoirs with elevated temperature of more than 70–80 °C and a high degree of salinity of formation water, many polymeric oil-displacing agents undergo rapid degradation, which leads to decrease in the level of hydrocarbon production. In this regard, one of the important problems is the creation and development of thermo- and salt-resistant polymeric materials and compositions based on them to increase the production of hydrocarbons in the oil and gas fields. The main aim of the research is to investigate the effect of formation water salinity on the physicochemical and filtration characteristics of polymer solutions and gels to enhance oil recovery. Methods. Viscosity of the polymer solutions and polymer gel were determined on Brookfield DV-II viscometer; shape and size of the polymer particles and polymer gel were studied on Hitachi S-400N scanning electron microscope; polymer size of molecular tangle Dh was measured on a wide-angle dynamic/static light scattering system Brookhaven BI-200SM (Brookhaven Instruments Cop., USA); physical modeling of fluid filtration at reservoir conditions was carried out on the filtration unit; determination of viscoelasticity and rheological properties of polymer solutions and gels was studied using a rheometer Harke 10. Results. The degree of mineralization of formation water has a significant effect on the viscosity of polymer solutions. Due to the fact that calcium and magnesium ions were preliminarily removed in partially demineralized water, the polymers have good solubility and the ability to increase the viscosity of the solutions. With increasing concentration of displacing agent, the viscosity of the solutions increases. Destructive effects of salts of formation water on polymers cause a significant decrease in viscosity of solutions of displacing agents. Due to the high degree of mineralization of partially demineralized water, a large amount of sodium chloride ions surround the molecular skeleton of the polymer of the displacing agent. The macromolecules of polymer P-1 have a predominantly two-dimensional network structure. The macromolecules of polymers P-2 and P-3 have a predominantly spatial three-dimensional network structure, in which some polymer molecular chains are broken and the network structure of polymers is defective. Compared with polymer P-3, the three-dimensional network structure of the polymer molecular aggregate P-4 has a clearer spatial structure. Comparing the physico-chemical properties of polymers P-1, P-2 and P-3, it follows that the coefficient of resistance and the coefficient of residual resistance for polymer P-1 are greater than for polymers P-2 and P-3. This is due to the fact that the salt-resistant polymer P-1 forms aggregates of the network structure due to intermolecular association, which leads to poor compatibility with core pores, and the resistance coefficient and residual resistance coefficient for polymer P-4 are the greatest the polymer mixture underwent reactions of intramolecular crosslinking of polymer molecules and Cr3+ cations, which led to a significant increase in the retention of the polymer solution, filtration resistance, injection pressure and to enhance oil recovery.

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