Файл: Магистерская диссертация тема работы Оценка эффективности применения полимерного заводнения на месторождении х удк 622. 276. 43 678. 7 Студент.pdf

Stability of polymer solutions
One of the disadvantages of flexible-chain synthetic acrylamide polymers is their susceptibility to mechanical, thermal-oxidative, and biological degradation.
Destruction results in breaking of macromolecular chains, the decrease of molecular weight of the polymer, and consequently, reduction of the thickening ability of the polymer reagent. Mechanical degradation occurs when mechanical loads exceeding a certain critical value are applied to polymer molecules. For example, the shear rate can be a measure of the load.
For fluid flow in circular channels the shear rate is calculated by formula (9): where j - shear rate, s
Q - volumetric flow rate, m3/s, v - linear velocity, m/s, r - channel radius, m, d - channel diameter, m.
At flow in cracks the shear rate is also proportional to the linear filtration rate and inversely proportional to the characteristic size (formula 10): where b is the width (opening) of the cracks, m.
Here the characteristic size of pore channels is the root of permeability. As can be seen from the above formulas, the shear rate is proportional to the linear velocity of the fluid and inversely proportional to the size of the conductive channels. The maximum mechanical stress occurs when both features are combined. Polymer destruction as a result of mechanical destruction is seen as a result of the so-called throttle effect - fluid flowing through narrow holes from the high-pressure line.

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When polymer flooding technology is implemented, mechanical destruction of polymers takes place in the pump assemblies (for this reason it is inadmissible to use centrifugal pumps for pumping polymer systems), in the narrowing of pipelines. Mechanical destruction in porous medium takes place only in the near bottomhole zone, at a small distance from the wellbore. As the filtration radius increases, the filtration rate hyperbolically decreases and the probability of mechanical destruction approaches zero.
Thus, with the long duration of polymer flooding, usually measured in years, the process of mechanical destruction of polymers as a result of destruction is very short and amounts to several hours. Another peculiarity of mechanical destruction of polymers is that during the long period of research on this process, no polymer grades distinguished by increased resistance to mechanical destruction or additives that increase this resistance have been identified. Undoubtedly, polymer grades that are promising for polymer flooding technology should be tested for resistance to mechanical degradation, but this test is purely experimental.
At the stage of polymer market analysis, it should be taken into account that susceptibility to mechanical degradation increases with increasing molecular weight of the polymer.
As an illustration, Figure 7 shows how the molecular weight of the polymer decreases when a polymer solution is stirred with a mechanical stirrer.

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Figure 7 - Kinetics of mechanical degradation of polyacrylamide in solution at a stirring speed of 4000 rpm
This character of the influence of the molecular weight on the resistance to degradation once again shows that one should be quite careful in selecting a polymer with an ultra-high molecular weight. This applies both to the dissolution kinetics of the polymer and the resistance to mechanical degradation.
The increase in molecular weight of PAA also decreases the resistance to thermo-oxidative degradation and leads to deterioration of technological properties of solutions under the influence of temperature and additives which have redox properties. Unlike mechanical destruction, thermal-oxidative degradation under the effect of temperature and active additives contained in water, rock and polymer itself lasts for a long time - the whole period of polymer flooding. Moreover, not only the macromolecules present in the solution and creating the resistivity factor are destroyed, but also the molecules adsorbed on the rock, which gives rise to the residual resistivity factor.
Unlike mechanical degradation, thermo-oxidative degradation is more of a controllable process, both increasing and decreasing its rate. There are quite a lot of ways to increase the resistance of polymers to thermo-oxidative degradation,

114 including at the stage of polymer synthesis. Such methods include, in particular, the removal of active micro-impurities from the polymer. The process of thermo- oxidative degradation is a chain radical process.
Traces of active inorganic and organic substances can initiate this process.
For example, copper additives at a negligible concentration (less than 0.5 mg/L) can increase the PAA thermo-oxidative degradation rate by several times. For a long time, the initial reagent for polymer (acrylamide) was produced on copper catalysts. Acrylamide and polyacrylamide synthesized from it contained traces of copper compounds, which resulted in an inflated rate of polymer degradation. At present, advanced companies use a biosynthesis-based method to produce acrylamide. The polymers synthesized from it are correspondingly more heat resistant due to the absence of copper.
The most common method of increasing the resistance of polymers to degradation is the use of degradation stabilizers. The most common stabilizers are chemicals containing sulfur (so-called deactivators) - mercaptobenzthiazole, mercaptobenzimidazole, thiourea. The concentration of deactivators (both individual and synergistic mixtures) is usually 0.5-1.5% of the polymer weight.
The listed methods slightly, but still increase the cost of the polymer. At the same time not all PAA manufacturers produce products, which are highly resistant to thermal oxidative degradation. The reason is not only the desire of some companies to save money on polymer production, but also the fact that the selection of effective stabilizers requires a large amount of research, both physicochemical and filtration.
For this criterion - resistance to thermal-oxidative degradation, as well as for resistance to mechanical degradation, the choice of high-molecular polymers should be made very carefully for the same reason that the rate of degradation increases with the increase of the molecular weight of the polymer (Figure 8).

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Figure 8 - Kinetics of thermooxidative degradation of polyacrylamides in aqueous solution at 70°C

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