Synergetic activation of persulfate by heat and Fe(II)-complexes for hydrolyzed polyacrylamide degradation at high pH condition: Kinetics, mechanism, and application potential for filter cake removal during cementing in CO2 storage wells

The long-term integrity of the interface between cement and formation rock in CO2-capture and storage wells is crucial to avoid leakage of CO2 in/along wells. However, the interface can be easily damaged by the filter cake, which is a compressed composite of bentonite, polymers such as hydrolyzed polyacrylamide (HPAM), and barite, on the wellbore rock. Therefore, removing the filter cake during the cementing process by degrading HPAM in an efficient way is essential. In this study, chelated-Fe2+ activated potassium persulfate (KPS) was used for HPAM degradation and filter-cake removal. Ethylenediaminetetraacetic acid disodium salt dihydrate (EDTA-2Na) and diethylenetriaminepentaacetic acid (DTPA) were adopted to control the precipitation of Fe2+/Fe3+. A mixture of 0.4 mM Fe2+, 0.8 mM DTPA, and 4 mM KPS at a pH of 10.0 at 70 °C reduced the molecular weight of HPAM significantly from 3.0 × 106 to (3.6–10) × 104 Da. Electron paramagnetic resonance (EPR) analysis suggested that HO was the dominant radical and that SO4− and O2− were responsible for the degradation. The reactions conformed to continuous distribution kinetics with an activation energy of 38.36 kJ mol−1. A possible degradation pathway was proposed based on analyses via infrared spectroscopy (IR) and time-of-flight liquid chromatography-mass spectrometry (TOF-LC/MS). >90 wt% of the filter cake was removed by the system. The results suggest that the proposed DTPA-Fe2+ activated KPS system exhibits promising potential for in situ degradation of high molecular weight HPAM and for the removal of filter cake in downhole wells.