The seepage flow characteristics of hydrophobically associated polymers with different aggregation behaviors’ in porous media
By Saja Haider Mohammed
The mechanism of action of polymers is to increase the solution viscosity of the displacement phase and reduce the permeability of the displacement phase to achieve the fluidity control effect, thereby improving the displacement efficiency and sweep efficiency . Thus, polymer solution is widely used and is one of the main methods used to improve oil recovery in oilfields. The use of polymers to control the stability and flocculation behaviour of some dispersions has great technological importance.
However, the process of polymer injection into the formation induces strong shear action (for example, the polymer injection pump, borehole, perforation hole, etc.) . Mechanical shear will inevitably occur in the process of polymer flooding, which will lead to a significant reduction in the performance of polymer solutions and subsequently affect polymer flooding . Therefore, improving the shear resistance of polymer solutions for oil displacement is the main direction of scientific researchers at present. Among them, hydrophobically associated polymer is a typical polymer flooding agent with better shear resistance . Given its unique association, polymer molecular chains can form a stronger three-dimensional structure, making the apparent viscosity of polymer solution change from a single bulk viscosity to a comprehensive presentation of bulk viscosity and structural viscosity. Especially after the critical association concentration, the structural viscosity of the solution increases greatly, and the apparent viscosity of the solution increases greatly. After mechanical shearing, the molecular chain of the polymer is broken, the solution structure is damaged and the solution viscosity is reduced. Although bulk viscosity is irreversibly destroyed by mechanical shear, hydrophobic association can restore part of the structural viscosity, thus restoring the solution viscosity.
Hydrophobically associated polymers exhibit good shear resistance mainly due to the restoration of space network structure and structural viscosity under association. The size of association depends on the spatial distribution of hydrophobic groups in polymer solutions. Therefore, the influence of polymer aggregation behaviour on its shear resistance is particularly important . Based on the general linear hydrophobic associative polymers, the topological structure theory to divide hydrophobic associative polymers into two structural models: grafted and far-out polymers (figure 2).
The former includes the random and continuous block distribution of hydrophobic groups in the main chain and the end of hydrophilic barrier. Hydrophobic modified polyacrylamide polymer (HMPAM) is a typical representative of this type of polymer .The latter mainly includes ‘star’ polymers with hydrophobic groups attached to one or both ends of the hydrophilic main chain and one hydrophobic group connected to multiple hydrophilic main chains . Research demonstrates that the spatial structure of the polymer molecule is strengthened, the distribution of hydrophobic groups is more homogeneous after shearing, and the association effect can be better exerted such that the polymer molecule exhibits stronger shearing resistance.
To further improve the field application of polymer flooding, it is necessary to recognize the percolation characteristics of hydrophobically associated polymers in porous media after mechanical shearing. It is necessary to study the porous media percolation characteristics of hydrophobic associating polymers with two different molecular structure models by considering the actual mechanical shear effect in the field and the mechanical degradation in the application of polymer flooding. One is a hydrophobically modified partially hydrolysed polyacrylamide, and the other is a branched hydrophobically associated polymer. The main research contents include three aspects: (i) the aggregation behaviour characteristics of two types of polymers; (ii) the influence of shearing on the viscosity enhancement, aggregation behaviour, microstructure and static adsorption properties of polymer solutions; and (iii) the percolation characteristics of polymers before and after mechanical shearing in porous media at different velocities.