Effects of Mechanical Degradation on the Molecular Weight Distribution of High Molecular Weight Polymers
Research output: Research › Master's Thesis
Partially hydrolyzed polyacrylamides are viscosity enhancing polymers commonly used in enhanced oil recovery treatments as in the specific case of an OMV polymer project in the Matzen field, Lower Austria. The thesis at hand characterizes an analogue reservoir system, comprising 356 to 443 mD Berea Sandstones and 10 MDa 250 ppm FLOPAAM 3330 polymer solution. The two experimental methods applied here were core flooding setups for the evaluation of apparent viscosity and size exclusion chromatography for the examination of according molecular weight distributions. At low far field flow rates of 1 to 20 m/day shear thickening apparent viscosity effects were notable with peak values of 35 mPas. A critical offset rate was recognized below which no significant mechanical degradation occurred. The offset flow rate lies at 60 m/day and leads to a flattening trend in shear thickening which transitions to shear thinning behavior and a drop of apparent viscosity at elevated near wellbore rates. Mechanical degradation is proven to occur exclusively at this level in a significant extent. Reinjecting previously degraded polymer solution into another core, leads to a similar viscosity profile but shifted to lower viscosities and higher flow rates. The averaged absolute viscosity loss due to mechanical degradation is as high as 58.1% with peak values of 84%. The average modal molecular weight of a 10 MDa HPAM solution is diminished to 8.4 MDa in the first place and further to 7.7 MDa after the second flood. In terms of average percentages, this equals a reduction of 16.4% in the first core flood and another 6.5% during a subsequent core experiment, adding up to 22.7% total loss. A combination of more elaborate core flooding experiments was designed to investigate time and temperature effects on degraded HPAM probes. Larger amounts of sample liquids were taken after a first experiment and stored for 2 months at temperatures of 8, 22, 50 and 80°C. The lowest temperature scenario yielded an identical result as a normal reinjection experiment would have. The 80°C configuration ended in a dramatically deteriorated viscosity profile in the range of 2 mPas, implying massive thermic degradation. The two temperature scenarios at 22 and 50°C exhibited small amounts of viscosity healing and molecular size restoration. Further analysis of these data sets allowed the conclusion that a share of 15.8% of lost apparent viscosity is due to temporary effects and can be restored at certain conditions. The overall permanent damage must be considered not as grave as first measurements would suggest and can be stated to equal 48.7 instead of 58.1%. In terms of molecular size, this effect is much smaller. Only 1-3% of lost molecular weight is re-established over the same time span. This means that 1% of size growth is responsible for 15.8% of viscosity restoration. Permanent damage is considered to be the consequence of frequent chain scission processes. Temporary viscosity loss occurs most likely due to chain unfolding at higher temperatures and proved to be possibly undone given enough time.