Polymer retention in porous media can significantly delay the arrival of the oil bank. This delay erodes value and affects the economics of any potential polymer project. In this paper, the effect of the presence of residual oil on polymer (partially hydrolysed polyacrylamide [HPAM]) retention is investigated. Various corefloods have been carried out using different sandstone reservoir rock material, both in single phase and at residual oil saturation. Corefloods without oil showed high polymer retention values, i.e. > 100 µg/g. Experiments at residual oil saturation led to polymer retention values that were approximately 10 times lower for medium to low permeable cores, i.e. 271 vs. 28 µg/g for respectively single-phase and at residual oil saturation. For the high permeable cores, the polymer retention in the presence of oil was very low, i.e. between 0-11 µg/g. This was significantly lower compared to the value that was measured in the absence of oil (38 µg/g). Polymer retention consists of physical surface adsorption and mechanical entrapment. Physical adsorption is dependent on the adsorbing surface area. The presence of oil can reduce the available surface area for the polymer to adsorb onto, especially in an oil-wet or mixed-wet core.

The results of the coreflood experiments demonstrate the effect of the presence of oil on the retention of polymer in porous media. Corefloods without oil show polymer retention values that are much higher than values in the presence of oil.

This suggests that injecting polymer slugs below the oil-water contact could potentially result in a delay in the breakthrough of polymer and the arrival of the associated oil bank, which in turn could have significant detrimental effects on the economics of a commercial polymer project. In the experimental program to de-risk polymer flooding for full field commercial projects comprising polymer injection in oil legs, corefloods to assess polymer retention should always be executed with (residual) oil present.

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