In her doctoral work, Siv Marie Åsen has taken a closer look at what happens to different types of polymers when they are injected in the reservoir. Are the polymers degraded when transported through pipes or through the sandstone reservoir?
Polymers are used to make the injection water more viscous. When you inject a viscous liquid to the reservoir, you delay water breakthrough, accelerate oil production and reduce the amount of produced water.
Polymer molecules are long-chain molecules consisting of up to half a million carbon atoms. When polymers for use in oil recovery are dissolved in water, they increase the viscosity of the water. A concentration of 1 per mille (0.1%) may be enough to increase the viscosity 10-100 times, depending on the polymer size and the amount of salt in the water. When the polymer solution is subjected to strong forces, some of the bonds that hold the long chain may break. This is called mechanical degradation. The viscosity of the polymer solution decreases and the positive effects are reduced. Therefore, it is important to understand where the degradation occurs and what can be done to prevent this from happening.
The flow behavior and mechanical degradation have been studied in porous media, tubes, and equipment across scales. In porous media it was found that linear flow degradation increases with length but it will not be a problem at in-depth propagation rates. Likewise, it was found in pipe-flow, that degradation happens at the inlet of a flow constriction and that polymer solutions can travel long distances, even at turbulent flow, without further degradation. Methods for mitigating degradation during choking was identified.
Thermo thickening associative polymers demonstrated great potential as their performance is enhanced by salt and temperature, and they are less prone to mechanical degradation and injectivity issues.
Åsen's PhD was done at The National IOR Centre of Norway, a research collaboration between UiS, IFE and NORCE, funded by the Research Council of Norway and nine industry partners.
Download thesis here: Synthetic polymers for Enhanced Oil Recovery; Mechanical degradation, and alleviation thereof