Is Polymer Retention Different Under Anaerobic vs. Aerobic Conditions?

This paper examines whether retention of partially hydrolyzed polyacrylamide (HPAM) is different under anaerobic vs. aerobic conditions. Both static (mixing with loose sand) and dynamic methods (corefloods) were used to determine HPAM retention. There are both advantages and disadvantages associated with determining polymer retention with static tests vs. dynamic tests and with aerobic vs. anaerobic conditions. From static-retention measurements, polymer-adsorption values on pure silica sand or Berea sandstone were small, and they showed little difference between experiments conducted aerobically or anaerobically. For both aerobic and anaerobic conditions, HPAM retention increased significantly with increased pyrite or siderite content. Static retention under anaerobic conditions ranged from 45 to 75 mu g/g with 1% of either pyrite or siderite to 137-174 mu g/g for 10% pyrite or siderite to 1161-1249 mu g/g for 100% pyrite or siderite. If iron minerals are present, the most representative polymer-retention results are obtained (for both static and dynamic tests) if conditions are anaerobic. Retention values (from static measurements) under aerobic conditions were commonly twice those determined under anaerobic conditions. If iron minerals are present and retention tests are performed under aerobic conditions, total organic carbon (TOC) or some similar method should be used for polymer detection. Viscosity detection of polymer may provide retention values that are too high (because oxidative degradation can be misinterpreted as polymer retention). For a broad range of siderite content, retention from static tests did not depend on whether dissolved oxygen was present. However, for a broad range of pyrite content, HPAM retention was significantly lower in the absence of dissolved oxygen than under aerobic conditions. These results may be tied to iron solubility. When polymer solutions were mixed with 100% pyrite over the course of 12 hours, 360-480 ppm of iron dissolved into polymer solutions under both aerobic and anaerobic conditions, whereas with 100% siderite, only 0.0-0.6 ppm of iron dissolved. If dynamic methods (i.e., corefloods) are used to determine polymer retention under aerobic conditions, flow rates should be representative of the field application. Rates that are too high lead to underestimation of polymer retention. With 10% pyrite, dynamic retention was 211 mu g/g at 6 ft/D vs. 43.2 mu g/g at 30 ft/D. In contrast, retention values were fairly consistent (40.6-47.8 mu g/g) between 6 and 33 ft/D under anaerobic conditions.