Mechanism of Hydrolyzed Polyacrylamide Viscosity Loss; Oxidative Degradation vs Chelation

摘要

Stability of polymer is one of the major concerns for polymer flooding projects. For polymer flooding job to recover optimal volume of oil and to be profitable economically, the viscosity of polymer solution should be retained, if not forever, then for a certain period of time and transit through the oil reservoir. Optimal recovery is achieved when the mobility ratio of flood swivels around unity. Polymer reacts with forces of nature, which could be mechanical, physical, chemical or biological in nature, and these reactions sometimes manifest in loss of viscosity of the polymer solution, which is a problem.;Polymers could be natural as Xanthan Gum, or synthetic as Polyacrylamide, also known as PAM. In industry, PAM is widely accepted and used. However, there is a cost involved in this whole polymer flooding process. When partially hydrolyzed polyacrylamide (HPAM) solution contains iron along with dissolved oxygen, for more than a threshold, it leads to loss of viscosity of polymer solution and loss of viscosity is a loss of money. Conventional wisdom says this degradation is because of the free radicals produced during the oxidation of iron, while a Norwegian scientist, Svein Viggo Aanesen, claims it's because of chelation of HPAM molecules by Fe3+ ions.;Dissolved oxygen in the polymer solution system is the oxidizing agent for Iron, either in its natural form or in the reduced (Fe2+ ) state, present in the polymer solution. Whenever the redox reaction between iron and oxygen occurs in a hydrolyzed Polyacrylamide solution, there is no sign of gelation but solution changes its colour from colourless to yellow signifying the presence of ferric iron accompanied with a loss in viscosity of the solution. Yellow colour and viscosity loss were concomitant. Reducing Fe3+ in the degraded polymer solution did reverse the colour change from yellow to original, colourless, but did not reverse the degradation and it aids to the traditional theory that redox reaction is the reason behind this viscosity loss.;Several methods have been proposed to mitigate this degradation such as the addition of free radical scavenger, the addition of chemicals to scavenge dissolved oxygen by chemical means, removal of iron, adjustment of pH and Eh, removing dissolved oxygen mechanically or no action. No action does not help with prevention of viscosity loss while mechanical removal of oxygen with some precautions is the best option as it stops the iron in the system from being reduced, which is the reason behind polymer molecule's chain breakage.