Polymer flooding is a mature EOR technique successfully applied in both sandstone and carbonate reservoirs and it is expected to have much wider applications in the future. However, it is currently limited mainly by a combination of reservoir temperature and make-up brine salinity and hardness, as synthetic polymers can chemically and thermally degrade at high temperature and salinity.
This paper reports the results of an ongoing study aimed at finding polymers that can enlarge the application envelope of polymer flooding to, for example, high-temperature and high-salinity Middle Eastern carbonate reservoirs. In this study thermal stability of a number of commercially available polymers was tested at 120°C in various brines. The polymers investigated are partially hydrolyzed polyacrylamide (HPAM) based co- and ter-polymers functionalized with 2-Acrylamido-2-Methylpropane Sulfonate (AMPS) and/or n-Vinyl Pyrrolidone (n-VP) monomers. These monomers provide tolerance to divalent ions and protection to hydrolysis at higher temperatures.
The thermal stability tests showed a clear relationship between the polymer stability and the concentration of the functionalizing monomer. Incorporation of the AMPS monomer alone does not stabilize the polymer in brines which contain divalent ions. The additional incorporation of n-VP demonstrates a stabilizing effect which is optimal at a concentration of 35-50%. The optimal ter-polymer is stable at 120°C in brines with salinity up to 200,000 ppm TDS and divalent ion concentration up to 18,000 ppm for more than 180 days.
Another challenge to polymer flooding is the adsorption of polymers onto the reservoir rock. The adsorption of the terpolymers on carbonate core material was investigated in this study as a function of the polymer type and pH.
This is the first work to prove the feasibility of polymer flooding at temperatures up to 120°C in high-salinity reservoirs in the presence of a high concentration of divalent ions. These results will significantly enlarge the application envelope for polymer flooding.