Stability of polymers used for enhanced oil recovery



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The purpose of this work was to study polymer degradation mechanisms as well as ways to mitigate it. In the area of chemical stability, defined as divalent cation tolerance of acrylic polymers as hydrolysis increases, use of the n-vinyl pyrrolidone (NVP) monomer helps to preserve viscosity and tolerate higher calcium concentrations over those polymers without NVP. Also, ethylenediaminetetraacetate tetrasodium salt (EDTA-Na+4) is shown to sequester calcium ions at alkaline conditions (pH>10) and, in the case of lab-aged post-hydrolyzed poly(AM-co-AMPS), helps to retain full viscosity at all calcium concentrations when EDTA is present at a stoichiometric equivalence of calcium.
Many discrepancies exist in the literature concerning the presence or absence of degradation under various field or laboratory conditions. Carbonate and bicarbonate, which are typically present in natural waters but often neglected in lab-prepared brines, prove to be a hidden variable in resolving why Shupe (1981) saw no loss in viscosity when sodium dithionite was added to polymer in the presence of oxygen (with bicarbonates) but others (Knight, 1973 and Levitt and Pope, 2008) observed severe degradation under similar conditions (but without bicarbonates). A commercial HPAM polymer (Flopaam 3630S) has been shown to be stable in the presence of ferrous iron in the absence of oxygen, clarifying an apparent discrepancy in the literature between the results of Yang and Treiber (1985) and Kheradmand (1987).
Dissolved oxygen (DO) levels, and not redox potential (ORP) measurements, are often reported in polymer stability research on oxidative degradation. ORP is shown to be a better measure of the onset of degradation because oxygen is initially being consumed and may not appear until substantial degradation has occurred. Although generally believed to be a detriment to polymer stability in the field, aeration of iron-laden source water prior to hydration of polymer may be beneficial in certain cases where exposure to air in unavoidable. Also, a novel process of safely producing sodium dithionite in the field proves to perform better in terms of long-term polymer stability in anaerobic conditions than the traditional method of using a solution made from powder dithionite. Finally, a pre-sheared 5 million Dalton HPAM is successfully injected into a 3 mD carbonate reservoir core plug. Remarkably, permeability reduction factors remain at values close to unity. However, pressure data from ASP tertiary corefloods suggest that polymer is not feasible for field injections.