Dilute solutions of polymers used to provide mobility control for EOR often lose viscosity. especially at higher temperatures. This loss of viscosity with time brings into question the feasibility of using polymers as mobility-control agents. A literature study of the many possible reaction mechanisms indicated that oxidation/reduction (redox) reactions involving free radicals probably caused polymer degradation and concomitant viscosity loss. A preliminary search for antioxidants known to retard free-radical reactions located several types and positive synergistic formulations that significantly retarded biopolymer solution viscosity loss during accelerated tests at high temperature. The most effective type formulation found contained (1) a radical transfer agent; (2) a sacrificial, easily oxidizable alcohol; (3) a compatible oxygen scavenger; and (4) sufficient brine concentration. Samples prepared with this technology have not lost viscosity after 1-year storage at 207 deg. F [97 deg. C]. A high-surface-area effect (so-called "wall effect") known to retard radical propagation, was also found to operate in the presence of sandpacks; this should be beneficial in porous media. The variables and beneficial antioxidant formulations identified in this study allow tentative conclusions and recommendations regarding biopolymer mixing and handling procedures prior to injection.
Two commercially available polymers are currently considered suitable for mobility control. They are (1) the synthetically prepared polyacrylamides, and (2) the biopolymer (xanthan gum) prepared by fermenting the bacterium Xanthomonas campestris and collecting the exude gum. The major advantages of the biopolymer over polyacrylamides are the good shear stability and the good thickening power at high salinity. The major disadvantages of the biopolymer have been the high cost, the difficulty of preparing solutions that do not plug core material, and the prevention of viscosity loss from biochemical or chemical reactions. The requirement of shear stability and tolerance to salts, especially multivalent cations, significantly reduces the number of reservoirs where polyacrylamides can be used. Biopolymer solution-preparation problems have been overcome by development of solution processes including proper mixing equipment, chemical addition-both caustic and enzymes-and filtration techniques. Biopolymer broths, which preclude the need to wet and disperse a dry powder, are also available. Chemical stability of the polymer is the subject of this report. A literature study of the various reaction mechanisms indicates that redox reactions involving free radicals probably cause polymer degradation and concomitant viscosity losses. This is undoubtedly the type of reaction responsible for polyacrylamide decomposition. Removal of oxygen with excess sodium dithionite appears to provide sufficient chemical treatment to prevent autoxidation of polyacrylamide. This straightforward chemical treatment did not prevent biopolymer-solution viscosity loss, and further antioxidant addition was required to stabilize the solution viscosity.
A literature review of polysaccharide chemistry suggested thermal, biological, mechanical, radiation, and chemical as the most important degradation mechanisms. Thermal- and radiation-induced degradation mechanisms were discounted since biopolymers should not be used if the reservoir is too hot or too radioactive.