Insights into the key aspects influencing the rheological properties of polymer gel for water shutoff in fractured reservoirs

Polymer based gels have been widely applied in the fractured reservoir for water shutoff and enhancing oil recovery. In this study, a polymer gel system with long-gelation time and high strength was prepared for water shutoff in fractured reservoir, and the key effects on its rheological properties are systematically studied. The gel involves crosslinking of the hydrolyzed polyacrylamide (HPAM) with water-soluble phenolic resin (WSPR), in the presence of an activator. The effects of nanoparticles, polymer, activator, and crosslinker concentrations on the gelation time, strength and stability of the gel are experimentally studied. The rheological measurements, Cryo-scanning electron microscopy (Cryo-SEM), and differential scanning calorimetry (DSC) are utilized to explore the underlying mechanism. It is found that the steric effect of the crosslinker may contribute to a longer gelation time of the WSPR gel system. Moreover, the results show that the gelation time and gel strength can be controlled by adjusting the concentrations of the polymer, crosslinker and activator. However, excessive crosslinker, superfluous activator and low polymer concentration can unfavorably affect the thermal stability of the gel, which may lead to a severe syneresis problem. Besides, the rheological tests show that the gel with excessive crosslinker or activator becomes more sensitive to the strain and its critical strain may be decreased to a low strain amplitude. The creep tests also show that the excess crosslinker and/or activator may result in a low failure stress of the gel, which would considerably decrease the blocking ability of the gel in the fractures. Additionally, the microstructures of the gels were studied, which demonstrate that the performance of the gel is closely related to its microstructures. These findings allow us to delimit a range of concentrations of components for gel conformance in fractures, which also provide a systematic approach to evaluate the gel performance for conformance control in fractured reservoirs.