Consequences of degradation forces on molecular weight distribution of high- weight hydrolyzed polyacrylamide (HPAM) solutions
Research output: Research › Master's Thesis
Water- soluble hydrolysed polyacrylamides (HPAM) are well established viscosity modifiers. Aqueous HPAM solutions show a complex shear rate dependence of viscosity. For viscometer experiments (e.g, cone- plate viscometer) a pseudo plastic and for core flood experiments a viscoelastic rheology has been reported. The flow of HPAM solutions in porous media (PM) is influenced by shear forces and elongation effects of macromolecules. The forces lead to an alternation of the molecular weight distribution (MWD). These changes are manifest also in an alteration of the apparent viscosity, µapp, from µapp= 170 mPa∙s to 10 mPa∙s at a Darcy velocity of 16.9 m/day for core floods experiments. Rheometer viscosity changes, µ, from µ= 12.5 to 8 mPa∙s at a shear rate of 10 s-1. Polymer flooding was simulated with core floods (Nordhorn sandstone cores, Φ= 22% and k= 2D) at flow rates from 1 to 63 m/day. Additionally a capillary tube experiment was conducted, where HPAM was forced through several capillaries(r= 0.286 mm, L= 18 m). The MWD of the effluent from the performed experiment (e.g. MMModal= 20 MDa, MDa=106 g/mol, Half height of peak MM= 2- 100 MDa) were determined by aqueous size exclusion chromatography (SEC) for high weight macromolecules (calibration range between 1 to 20 MDa, projected exclusion limit of the column 500 MDa). Results of the conducted experiments showed small changes in MWD of polymer standard with nominal 10 MDa and measureable impact for standards with a modal molecular mass (MMModal) of 20 MDa. Consequently a nominal 10 MDa polymer solution is an attractive alternative to a higher MM solution. A nominal 20 MDa polymer standard degraded strongest at the capillary experiment in MM range between 60 to 80 MDa with a maximum reduction of the modal MM to a value of 7.5 MDa. The degradation shows a more systematic behaviour than degradation during a core flood. Slight changes in MWD were noticed when the effluent of the experiment was forced through the same capillary for a second time. The degradation of a nominal 20 MDa standard during a core flood is characterised by a reduction of MMModal at low velocities (MMModal= 15 MDa, vDarcy= 1.1- 4 m/day) and a slight build up of MMModal at high velocities (MMModal= 19.5 MDa, vDarcy= 4- 33 m/day). A growth in MMModal is the consequence of a progressive degradation of higher weight molecules from 70 to 120 MDa, which degraded to higher weight molecules with increasing velocities. The velocity dependence of maximum losses of MM occurs in three stages. A degraded (vDarcy= 33 m/day) HPAM solution was forced through a second core to simulate reservoir flow, where further changes in MWD were observed and a reduction of MMModal up to a value of 12.5 MDa was identified.
|Award date||26 Jun 2015|
|State||Published - 2015|