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 (M_w) 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-M_w 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 M_w, 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-M_w HPAM can more easily enter the deep reservoir to expand the swept volume on a larger scale. However, the system with a high-M_w HPAM can form a gel with a higher viscosity to produce a higher plugging strength.