Polymer flooding enhances oil recovery due to mobility reduction of the injected water, mainly because of the viscosifying capability of the polymer. However, polymer degradation causes viscosity loss. Thus, determining the degradation magnitude is key to a successful polymer flooding. This paper focuses on mechanical and chemical (due to iron presence) degradation mechanisms. The objective is to investigate the impact of these degradation mechanisms on polymer solution viscosity, in both coupled and individual fashions, during a dynamic experiment.
A polymer solution was stored in a stainless steel bottle (at 23°C) and injected in a porous medium at different instants (during three weeks) and flow rates. The flow rates varied from 1.0 cm3/min to 0.25cm3/min and then back to 1.0 cm3/min, in ten steps (duplicates for each flow rate), during the experiment duration. The solution was composed by a partially hydrolyzed polyacrylamide (HPAM) copolymerized with acrylamide tert-butyl sulfonate (ATBS) in a brine of 10.5% TDS containing both mono and divalent ions. Oxygen was dissolved in the solution, and iron was available from the bottle internal wall surface, which resulted in chemical degradation. The flow at different flow rates resulted in distinct levels of mechanical degradation. Rheological data from samples collected before and after injection were compared to evaluate both degradation mechanisms. The polymer concentration in the solutions was confirmed by a UV-spectrophotometer analysis. The iron content was evaluated through inductively coupled plasma optic emission spectroscopy.
The level of degradation in the presence of iron is consistent with other literature for the same polymer. However, the mechanical degradation data is new for this specific HPAM. We divided the chemical and mechanical components of the degradation and quantified the viscosity loss due to each of these phenomena. Additionally, we saw that the mechanical degradation was more relevant at the beginning of the experiment. Although, as time passed, the scenario shifted to a more relevant chemical degradation. We also observed that, for a given flow rate, the mechanical degradation varied depending on the level of chemical degradation of the injected solution. The observed viscosity reduction ranged from 6.1% to 74.2%.
The main contribution of this work is the experimental quantification of two polymer degradation mechanisms as both individual and coupled phenomena.