Influence of Hydrolyzed Polyacrylamide (HPAM) Molecular Weight on the Cross-Linking Reaction of the HPAM/Cr3+ System and Transportation of the HPAM/Cr3+ System in Microfractures

The influence of the molecular weight (Mw) of hydrolyzed polyacrylamide (HPAM) on the cross-linking reaction of HPAM/Cr3+ and the transportation of HPAM/Cr3+ in microfractures is systematically studied using viscometry, ultraviolet–visible absorption spectrophotometry, and displacement experiment with a visual microfractured model. The results show that a high-Mw HPAM is advantageous to the intramolecular cross-linking reaction of the HPAM/Cr3+ system but disadvantageous to the transportation of the HPAM/Cr3+ system in microfractures. At the intramolecular cross-linking stage, the injection pressure of the HPAM/Cr3+ system in microfractures is almost equal to that of the HPAM solution, which undergoes no change with the degree of the cross-linking reaction. The higher the HPAM Mw, the earlier the intramolecular cross-linking ends (thus, the intermolecular cross-linking reaction of HPAM/Cr3+ occurs earlier, which leads to an earlier increase in the injection pressure of the HPAM/Cr3+ system). Moreover, there is a matching relationship between the fracture aperture and the HPAM/Cr3+ system to minimize the chromatographic separation when the HPAM/Cr3+ system transports in the microfracture. For the conformance control of a fractured tight oil reservoir, we conclude that an HPAM/Cr3+ system with a low-Mw HPAM can more easily enter the deep reservoir to expand the swept volume on a larger scale. However, the system with a high-Mw HPAM can form a gel with a higher viscosity to produce a higher plugging strength.